Technical Article :
Public Address System
By : Malik Rashid Ahmad. CEO :
Relic Technologies.
Amplifiers Anatomy -– Part
1
What do power amplifiers do?
Power
amplifiers drive loudspeakers. After
an audio signal has been mixed, equalized and otherwise processed at a
standardized line level, it is sent to the power amplifier. Its job is to increase the power of the signal until we get the
desired sound level from the loudspeakers, without otherwise altering the
waveform of the signal. We need to talk about loudspeakers for a
moment. A loudspeaker is an
electromagnetic device that converts electric current into motion at audio
frequencies. Because of the weight og the cone and the unavoidable resistive losses in the
voice coil, it takes a lot of power to produce a high sound level. We know (especially if we read last
month’s article, “Mr. Ohm and His
Talking Electrons”) that electric power is a product of voltage and current. Loudspeaker cone movement is
proportional to the current in the voice coil. The amount of heat in the amplifier components is also
proportional to current. However, it
takes voltage to make the current flow, so a power amplifier must deliver high
voltage and current simultaneously. Loudspeakers are generally made with a voice
coil resistance of about 8 ohms, so the amplifier must produce 8V across the
loudspeaker terminals to cause a 1A current to flow. This means that ideally the amplifier works into an impedance of
8 ohms.
In the real world, the impedance of loudspeakers is more complex. As the cone moves, it generates electrical
back pressure, which can increase or decrease the current flow from the
amplifier. Because of interactions with air pressure
in the loudspeaker cabinet, the cone motion varies greatly, especially in the
bass region. Therefore, the
loudspeaker’s impedance varies at different frequencies, and may range from 4
ohms to 20 ohms, averaging about 8 ohms.
In addition, more than one 8 ohm loudspeaker may be connected to the amplifier. For these reasons, most professional amplifiers
are built to work into impedances as low as 2 ohms, which will draw up to four times
the normal current. The amplifier
must also withstand very high impedances in case the load is disconnected. This requirement is normally not a problem
because the current flow with no load is zero.
AMPLIFIER POWER
We all
know that the amplifier’s power rating tells us how loud the amplifier will get. A “200W at 8 ohm” amplifier is
designed to deliver 40V to an 8 ohm loudspeaker, resulting in 5A of current
(40V divided by 8 ohms). Of course,
40V times 5A yields the amplifier rating of 200W. If we want to double the
cone motion, we have to double the current, from 5A to 10A. Because the loudspeaker
impedance is still 8 ohms, it will take 80V to get 10A. Therefore, the power rating must increase from 200W the 800W (80V
multiplied by 10A). You can see why
the power ratings escalate pretty quickly in high-output systems.
HOW DO POWER AMPLIFIERS WORK?
An
amplifier is basically an ac-to-dc power converter. It takes ac power from the wall outlet (at fixed frequency and
voltage) and converts it to audio power at the loudspeaker terminals (with
variable frequency and voltage). The
audio output is supposed to be a faithful replica of the line-level audio
input, only larger. Let’s look a
little further into the block diagram, at some of the major sub systems inside
a power amplifier. We will explain more about each sub-system
later in the article. First we need
a power supply. This subsystem
accepts the ac power from the wall, isolates the audio circuitry from shock
hazard, raises or lowers the ac voltage to suit the needs of the amplifier
power rating, converts the ac power to dc, and stores it in an energy reservoir. The other major subsystem is the
output section. This is the
electronic circuitry that accepts the linelevel audio input and uses this
information to control high-power transistors. These convert the energy contained in the dc reservoir to a
high-power audio waveform that is a magnified replica of the input signal.
AMPLIFIER PERFORMANCE LIMITATIONS.
All
amplifiers have a maximum power limit.
The voltage at the amplifier output can only go as high as the voltage in the
dc power supply. If the signal tries
to exceed this limit, it ”hits the ceiling,” and the
waveform becomes flattened. This
problem, called clipping because it looks like the top of the waveform has been
clipped off, results in the familiar ”blatting” sound
of an overdriven amplifier. Increasing the supply voltage adds cost
and weight to the amplifier, so amplifier power has a big effect on price. Amplifiers
have a minimum rated output impedance, which should be
equal or less than the impedance of the loudspeaker load. As the impedance of the loudspeaker gets lower, more current will
be drawn from the amplifier. This is
why, up to a point, the amplifier power rating increases into lower impedances. However, the increased current puts a
greater strain on the amplifier components and the power supply. At some minimum impedance, the strain
will get so high that the power-supply voltage sags or the transistors overheat. Any further decrease in impedance
will cause the amplifier circuitry to collapse, resulting in less power, or it
could even cause amplifier failure. Amplifiers also must reproduce all audio
frequencies, from the highest to the lowest, at equal volume. This ability is called flat frequency
response because the graph of amplifier gain vs frequency is a flat line. If
the gain at low frequencies falls off, the sound will be thin or lacking in
impact. If the high frequency gain
rolls off, the sound will be dull or muffled. Most modern direct-coupled amplifiers are capable of very flat
response, but sometimes the frequency response is intentionally limited to protect
the loudspeakers from excessive power at frequencies we can’t hear.
MORE ABOUT THE POWER SUPPLY.
Why do we
have to convert the ac power from the wall into dc power, and then back to ac? The
ac power from the wall is at a fixed voltage and frequency, which are
completely different from the audio voltages and frequencies. If we tried to use the ac voltage “as
is” for a power supply, we would only be able to reproduce small parts of the
audio waveform. We have to convert the ac power into a
fixed dc source, and provide enough energy storage to carry us through the
periods where the ac voltage is passing through zero. This way, the audio output section has the power available to
respond at anytime as required by the input signal. So, let’s take a more
detailed look at the amplifier’s power supply. The ac power comes into
the amplifier through the ac cord, is controlled by the on/off switch, and usually
goes through a fuse or circuit breaker, which cuts off ac power in case of
massive overload. It then reaches the power transformer,
which is in the heart of the power supply.
A transformer consists of two coils of
wire around a common magnetic core. The ac power is connected to the first or
primary winding, which converts electric energy into magnetic energy. This magnetism flows through the iron
core to the secondary coil, which converts it back to electricity. Why
do we do all of this? The two coils are insulated from each other, so that the
secondary coil is isolated from any shock hazard in the primary coil. The ac voltage and current can also
be scaled up or down by changing the number of turns in the secondary coil. Transformers are so useful that they
are the major reason we use ac power distribution instead of dc (transformers
only work on ac). The simplest and least expensive
transformer is the E-I type, which is generally cubic-shaped (roughly equal
height, length, and width). This
type is widely used, but it has a tendency to give off hum, which might be
picked up by nearby circuitry. The U-I type is more expensive, but it is
easier to make in a flatter shape that can fit into low-profile amplifiers. It also reduces the hum emissions. The toroidal type is built on a
donut-shaped core, which has the best magnetic properties. It can be made quite flat, it weighs somewhat less and is has low
hum emissions, but it is the most expensive.
At QSC, we use the U-I transformer for most of our low-profile amplifiers
because it offers most of the advantages of the toroidal at a lower cost. Once
we have scaled and isolated the ac power through a transformer, we need to
convert it to dc. This is the job of the rectifier and dc
filter capacitors. The rectifier is a one-way ratchet that takes
the back-and-forth flow of ac current and redirects it so that is always flows
in the same direction. The rectifier
uses diodes, which permit current flow in one direction and block flow in the
reverse direction. A full-wave bridge rectifier circuit uses
four diodes. Each diode passes
current only in the direction of the arrow.
If you follow the current flow around the circuit, you will see that, no matter
which way the ac current is flowing into the rectifier, it always emerges in
the same direction. Now we have the dc
instead of ac (the current only flows in one direction), but it still has big
valleys in it. We need to smooth out
this ripple voltage. This is the job
of the filter capacitors. Capacitors are like tanks that hold
electricity. Once you fill them, it
takes a while to drain them out. Therefore, we connect a large capacitor
to the output of the rectifier. The
capacitor fills, or charges up, to the peak voltage of the rectified wave-form. If the capacitor is large enough, it
stays pretty full between the peaks, and we get an almost perfectly smooth dc
voltage. The electronic value of the
filter capacitors determines how well the ripple voltage is removed. In
the last 20 years, high-value capacitors have been considerably reduced in size. The high-density filter capacitors
are easier to mount on the circuit board right next to the power transistors,
which helps improve the high-frequency performance of the amplifier. The length of wiring between the old-style
capacitors and the transistors can introduce a slight inductance or electronic
lag, which prevents the transistors from instantly drawing power from the
supply.
POWER-SUPPLY REGULATION.
Another
concern with power supplies is regulation: the ability to hold the dc supply
voltage constant, despite changes in amplifier loading or ac voltage. The first characteristic is called
load regulation, or sometimes power-supply stiffness, and basically depends on
the resistance in the transformer.
An ideal transformer would have zero resistance and would be able to maintain a
constant voltage (perfect regulation) no matter how little or how much current
the amplifier needs. Real-world
transformers have resistance in the wire coils, which causes the supply voltage
to drop when current flow increases.
To minimize this voltage drop, thicker wire must be used, which increases the
size and weight of the transformer. Earlier
solid-state amplifiers used rather large transformers to keep the no-load
voltage (at rated power). The
designers needed to minimize no-load voltage because high-voltage transistors
were expensive, if not impossible to get.
In recent years, the cost of high-voltage transistors has come down, so the
trend has been toward somewhat smaller transformers to reduce the weight of
amplifiers, even though the no-load voltage rebounds to a higher level. A
side effect of the voltage drop is that, because the filter capacitors charge
up to higher voltage during periods of low demand, the amplifiers can deliver a
momentary burst of power above its normal rating. This feature, called dynamic headroom, can add 2dB or 3dB of peak
undistorted power, which is equivalent to having up to 100% more wattage. The
ability to hold a constant voltage despite ac voltage fluctuation is called
line regulation. Ordinary passive
power supplies, such as the transfomer-rectifier-capacitor system discussed
earlier, do not offer line regulation.
The dc supply voltage changes along with any change in the ac voltage. The power company usually tries to
maintain a constant voltage, but heavy loads or long ac cables can cause
voltage drops, which result in loss of amplifier power. Until switching power supplies
become more practical, the correction to this problem would unfortunately add
cost and weight to the amplifier, so the tendency has been to spend the same
money making bigger amplifiers. You
get the desired minimum power under worst-case conditions, and you come out
ahead when ac service is normal.
SWITCHING POWER SUPPLIES.
The size
and weight of power-supply components has been somewhat reduced over the last
20 years, but progress has been slow because we are only refining the same
basic technology. Meanwhile, other
industries, such as the computer industry, have been perfecting light-weight
switching supplies reduce the size and weight of the power transformer by
operating it at a much higher frequency.
For reasons beyond the scope of this article, high-frequency transformers are
much smaller that low-frequency transformers. However, we are stuck with the 50Hz or 60Hz ac power supplied by
the power company, so if we want to use high-frequency transformers, we must
generate our own high-frequency power, which results in a fairly complicated
block diagram. First, we rectify the incoming ac and smooth
it with capacitors, just as we did with the passive supply described above, but
without an ac transformer. Then we
use a high-speed switching transistors to convert the
dc power to a high-frequency ac waveform, usually 50kHz to 100kHz (about 1,000
times higher than normal ac power).
This high-frequency ac is fed to a small high-frequency transformer, which
isolates the secondary from ac shock hazard and scales the voltages, just as
the large ac transformer did in the passive supply. This high-frequency ac voltage is the rectifier and filtered
again, resulting in the final dc supply for the amplifier. The active supply is much more complicated than the passive
supply, but the weight of the components is much less. Although active supplies
are more expensive, costs are slowly coming down, and there are important advantages. In addition to the primary benefit of
greatly reduced weight, we can control the operation of the high-frequency
transistors to compensate for variations in ac voltage and load currents, thus
improving both kinds of power-supply regulation. The ultimate result will be more consistent amplifier
performance, but the audio industry must solve problems of cost, reliability
and radio/TV interference caused by the high-frequency switching. This will undoubtedly be an active area
of progress in the decade of the 1990s.
MORE ABOUT THE AUDIO OUTPUT CIRCUIT.
The story
of the actual power amplifier circuit begins with the input connectors. These humble components are crucial
to getting a high-quality signal into the amplifier because garbage in is garbage
out. In addition to corrosion-proof
plating and strong mounting, it’s a big help to have balanced inputs, which are
now fairly standard. Balanced inputs
permit the amplifier to ignore most forms of interference that occur in the
cabling between electronic units. Most amplifiers also have a gain control. It is usually operated full up, but
it is handy to be able to reduce gain for testing or to lower the noise floor
when you know you have input volume to spare. After the balanced-input
and gain-control circuitry, we enter the actual power amplifier circuit. The
main function of this circuit section is to increase the input signal from
about 1V to about 100V, and to increase the current from about 0.1mA to about 30A. This is a power gain of about 30 million! To understand how this
occurs, we need to discuss how transistors work. Transistors (and tubes in
earlier years) are variable-resistance elements that are connected between the
dc supply and the load (the loud-speaker).
The transistor acts as a valve. A small input signal causes a much larger
amount of current to flow from the dc supply to the load. The device controls a load current about 50 to 100 times greater
that the input current, so the device has a gain of 50 to 100. To
increase the gain, we can cascade devices by driving a second transistor with
the output of the first transistor, and so on. This way, we can build up the tremendous gains we need. The exact method of cascading is on
of the major differences among amplifier designs, and it would double the
length of this article to fully review these methods, However,
we can give some basic terminology. The last, or highest-power, set of transistors are called output transistors. These are high-power devices mounted
on large heat sinks. The outputs are
driven at a much lower power by driver transistors. Sometimes there are
pre-drivers before the drivers, though in some cases it is possible to go to
small-signal devices. QSC amplifiers
use a very-high-gain integrated circuit (called an opamp), followed by a
relatively simple 2-stage set of driver and output transistors. The
large load current is ideally a magnified replica of the small input current. However, for a number of reasons, the
load current might no be an exact replica; it might be distorted. The most obvious kind of distortion
is clipping, which occurs when the voltage across the load comes so close to the
dc voltage that the transistor saturates, or bottoms out, and can’t go any
further. A lesser form of distortion
occurs because the transistor’s gain is not uniform: It varies because of
temperature and current differences. All of these effects are called
non-linearities because the transistor deviates from the ideal of uniform magnification,
much like wavy glass makes straight lines look crooked. We will explain how distortion is minimized later in the article. Another
major problem with transistors is that they are one-way devices: They only
handle positive or negative currents.
Therefore, we need a way to connect positive and negative devices together to deliver
a complete audio waveform. This
method is called push-pull operation and has been the key to high power
performance since early tube amplifiers.
There are a number of ways to combine push-pull currents. Yet another problem is
that of heat loss. Let’s say we
connect a transistor to a 100V supply, but for the moment we only ask it to
deliver 40V to an 8 ohm load. We
know (from the example used earlier) that 5A of current will flow in the load,
resulting in 200W of output power.
That same 5A must flow through the transistor to get to the load. At the same time, the “unused” 60V
appears across the transistor. We have a combination of 5A and 60V in the
transistor, which results in 300W
(5A x 60V) of power in the transistor.
A basic low
of physics states that energy cannot be destroyed,
only changed. Because we aren’t letting
this unused power go to the load, it has to go somewhere, and the result is
waste heat. This example
show that it is very easy for the power wasted in transistors to exceed
the power delivered to the load.
This waste heat is the reason powerful amplifiers need large heat sinks, which
dissipate the unwanted heat.
Otherwise, the transistors would get so hot that they would fail. Although
I won’t burden the reader with excess detail about different ways to cascade
transistors in amplifiers, the final push-pull output transistors can be
combined in general ways that affect distortion and heat loss. These categories, called classes of
operation, were defined many years ago to allow discussion of these trade-offs. You have probably heard of class A, class B, class AB and other lettered designations for
amplifiers. Here’s a brief
explanation of their meaning (and it has nothing to do with USDA meat grades).
CLASS – A.
This is
the easiest class to understand, so it leads the list. The positive and negative output transistors each handle 100% of
the audio signal- they are biased so their zero-signal output current idles halfway
between zero and maximum. When the
audio current in one transistor increases, the current in one transistor
increases, the current in the other decreases; as a result, their voltage move
together. Each transistor can,
therefore, deliver a faithful replica of the signal all by itself, except for
the large idle current. If we were to connect just one transistor
to the loudspeaker, we would hear fair-quality sound, but that would force the
cone way off center and would probably overheat the voice coil. When we connect both transistors to
the load, the idle current from one transistor is absorbed by the other (rather
than going through the load), but the audio currents reinforce each other and
appear nicely in the load. The primary advantage of class-A operation is inherent lack of distortion. The full waveform is preserved in the
positive and negative transistors, so there is no trick to combining their
currents. However, a serious flaw is the extreme heat
loss at idle. The transistors
actually run hottest while “standing still”- sort of like controlling a car’s
speed with the brakes while keeping the throttle mashed down. Naturally, amplifier designers have
looked for other ways to ensure low distortion without such wasteful operation.
CLASS – B.
If we are
careful, we can let each transistor control only its half of the waveform. When the waveforms are combined
properly, we still get the complete output waveform, but we have eliminated the
large idle current. The amplifier
runs much cooler because no power is used until it’s needed. The trick, of course, is
to get seamless combining. If the
waveforms don’t joined together perfectly, we get
zero-crossing distortion (frequently called crossover distortion). This kind of distortion is quite objectionable
because it results in a slight gargling or rattling sound during quiet parts of
the program, where the signal is near zero. Fortunately, there are a number of ways
to eliminate this problem. One
popular method is to compromise between class A and B and operate
the amplifier in class AB. Bu
permitting a small idle current to flow, we get a small amount of idle heat,
but we eliminate any chance of “dead space” between the positive and the
negative waveforms.
CLASS – C.
When each
transistor controls less than 50% of the waveform, we call this mode class C. This mode is not usable for audio
because of the large gap between the waveforms, which causes severe zero crossing
distortion. Class C is used where
such distortion is unimportant or can be tuned out by other circuitry. In some amplifiers, the output
transistors are run in class C for less idle heat, with the driver transistors
filling in the gap. This method is
called class ABC.
CLASSES D, E, & F.
These
classes apply to switching amplifiers, which will be explained in the second
half of this article.
CLASS – G.
This mode
uses two or more sets of output transistors connected to different supply
voltages. The goal is to reduce the
heat loss in class A or B amplifiers.
Remember the example in which we had a 100V power supply but we only needed 40V
into the load? We had a lot of waste heat because there was
60 “unused” volts wasted in the output transistors. In a class G amplifier, we
have on set of transistors connected to a lower voltage supply, say 60V, which
supplies all output voltages up to this value. We then transfer to a second set of transistors connected to the
100V supply. With this method, the ”unused” voltage for a 40V output is cut from 60V to
20V, dramatically reducing the waste heat.
Because an amplifier spends most of its time supplying only a fraction of its
power, the average losses can be cut by 50% or more. The main problem is to
ensure seamless transfer from the low-voltage to the high-voltage transistors to
avoid any small glitches similar to zero-crossing distortion. QSC Series Three and the original MX
series amplifiers used this technique quite successfully.
CLASS H.
This
class uses a single bank or output transistors connected to a low-voltage
supply, along with some means of switching them to a higher-voltage supply when
required. This method has the same
thermal benefits as class G, but it avoids the second bank of output
transistors, thus reducing the size and cost of the amplifier. The
QSC EX series uses this technique to pack more power in the same chassis. (The EX4000 has twice the power of
the old MX2000). The new MXa series uses the same technique to
simplify the construction and to improve reliability. Most of these methods
require some alteration of the audio signal as it is broken apart and
reassembled. Not surprisingly, these alterations result
in errors in the reassembled waveform.
In Part II, we’ll discuss how error-correction circuitry, protection circuitry
and other design features allow the amplifier to perform its function without
altering the waveform.
Amplifier Anatomy - Part 2
Power
amplifiers, those “unsung heroes” of the sound system,
have traditionally been the old reliable that consultants and contractors
learned to count on as everything else in the sound system became more complex. While the rest of the system demanded
more and more attention, the power amps were something you just didn’t have to
worry about. Although power amplifier technology has more
to offer and requires more thought, there are some basic principles that every
leading amplifier manufacturer follows.
Understanding how equipment designers are solving amplifier problems can give
you a new appreciation for this basic piece of equipment. In Part I, we described
how an amplifier works, how power supplies affect amplifier function and which
amplifier classes work best for various situations. In Part II, we will go into more detail about amplifier circuitry
and design.
NEGATIVE FEEDBACK AND DISTORTION.
In Part
I, we mentioned that transistors are not inherently perfect magnifiers. Most of the advanced circuit techniques we
described involve re-assembly of the audio waveform in various ways. If we had no way of correcting
errors, the result would typically be a rather garbled and harsh reproduction. Fortunately,
we have a powerful error-reduction technique called negative feedback.
You probably associate negative feedback with criticism. Actually, this perception is not so
farfetched, but in electronics there is nothing bad about “negative” feedback. This technique is basically the same
process we use daily when observing one’s progress and making mid-course
corrections. Let’s use driving as an example. Imagine if you tried to steer around
a corner with your eyes closed. Even if you turned the wheel about the
right amount at about the right place before you closed your eyes, the car
would soon leave the road because of uncorrected small errors. In the real world, you drive with
your eyes open. You turn the wheel,
observer how the car is turning, and then make small corrections to maintain
the desired course. This is a basic
example of how we use feedback to correct for errors and produce the desired
result.
We can also use feedback in amplifiers. The output of real-world circuitry is distorted: The output is higher
or lower than desired. We usually
don’t know just what the errors are, or we could correct for them. We can correct for unknown errors by
comparing the output to the input and telling the circuit to increase or
decrease the output until they match. To describe the actual process, let’s
assume we want an amplifier with a gain of 10. The actual, imperfect amplifier has a gain that varies
unpredictably from 8 to 12, resulting in up to 20% of output error. We attenuate, or reduce, the actual
output of the amplifier by a factor of 10.
If the amplifier were error-free, this reduced feedback signal would be a
perfect match to the input signal, but the actual feedback signal varies around
the input signal by 20%. We have an accurate picture of just the error
because we have compensated for the desired gain by the 10-to-1 attenuation. Now, here’s the tricky part. If we magnify the apparent error by amplifying
the mismatch between the input signal and the feedback signal, then combine
this information with the original input signal, we can get the main amplifier
to reduce its own errors automatically. We have a crucial choice, however. The error signal can be added or
subtracted from the input. If we add the error signal, it just makes the
error worse. This is called positive
feedback, and it turns transistors into oscillators. It turns sound systems into oscillators, also. In an amplifier it leads to wild, runaway operation. If, on the other hand, we subtract
the error signal, the error is always diminished. Thus, we have used negative feedback. In practice, we reduce
error by putting much more gain than we need into the amplifier. It we increase the gain of the
amplifier by a factor of 10, the gain, even with errors, will range from 80 to 120. When we “close the feedback loop,”
using the 10-to-1 attenuation, the error signals always a large positive value. The amplifier quickly reduces its own
output until the feedback and input signals match up. In effect, because the
amplifier has extra gain, it is in a constant state of “holding back,” which makes
it easier to hit the desired target.
With enough extra gain, it is ultimately the accuracy of the feedback circuit
itself, not the amplifier, that determines the
accuracy of the final output. There is a limit, however. All circuitry has a slight lag between input and output. If you increase the gain of the
circuit too much, it will become too sensitive, and the combination of lag and
feedback will cause hunting or oscillation around the desired value. This problem, called instability,
limits the amount of feedback that can be used. The only fundamental cure
is to reduce the circuit lag by using high-speed components. There has been a lot of progress in this area in the last 20
years, and today’s transistors are about 10 times faster. This progress probably explains how solid-state amplifiers have
gradually eliminated the harshness that some listeners heard in early
amplifiers, which used relatively slow power transistors. Feedback can be applied
around all of the cascaded elements in the amplifier. This process, called global feedback, is popular because it
corrects all internal errors in one swoop.
Some designers prefer a process in which they sub-divide the amplifier into
several cascaded feedback loops, called local feedback. Certain forms of
instability are easier to eliminate with the local feedback technique, but
internal signal levels must be higher when one feedback loop feeds the next. QSC amplifiers use global feedback
because it is less expensive and is easier to assure accuracy with only one set
of critical feedback components to worry about. One notable effect of high
feedback is on amplifier clipping characteristics. Without feedback, transistors approach saturation gradually,
giving the effect of cushioning the impact.
This type of clipping is called soft clipping. With feedback, the output signal is forced to stay on track until
the last possible moment, resulting in an abrupt impact, called hard clipping,
which sounds more fuzzy than soft clipping. Some amplifiers feature low-feedback
designs to smooth out the clipping. Because it is so hard to reduce
distortion without high feedback, the trade-off is often between clean
amplifiers with harder clipping and slightly mushier amplifiers with smooth
clipping. The choice depends on
personal preference and on how much the amplifier will be overdriven. In
any case, it is important to avoid sticky clipping. Poor feedback circuit design can make the amplifier track its
input too far, and then snap back and ring without damping. The amplifier should enter and leave clipping cleanly with no
snapping or chattering.
PROTECTION CIRCUITRY.
The lower
the impedance of the load, the greater the current drawn from the amplifier,
and the greater the heat generated in the output transistors. If too many loudspeakers are
connected to the amplifier, or if the ends of the loudspeaker wire touch
together by accident, the load impedance goes very low, and the current flow
becomes dangerously high. If the
flow is not limited, the output transistors will burn out. Therefore, amplifiers need some kind of short-circuit protection. There are many ways to protect
against short circuit, but the trick is that you can’t prematurely limit
performance into normal loads. QSC
amplifiers continuously monitor the load impedance. Loads above 2 ohms draw safe currents, and only normal voltage
clipping occurs. Below 2 ohms, the
maximum amplifier current is reduced if it exceeds the safe limit for more than
a fraction of a second. This way, short
peaks are permitted even into marginal loads, yet the amplifier is still
protected against gross, sustained overloads. Other common protective
circuits include turn-on and turn-off muting, shut-down or muting in case of
excessive temperature, protection against radio pickup (RFI), and dc fault
protection, which shuts down the amplifier is a
transistor loses control. The design
of these circuits is just as important as the actual audio circuitry because
they can make the difference between surviving an accident or winding up with
burnt rubble.
SWITCHING AMPLIFIERS.
Remember
those classes D, E and F that we promised we would talk about in Part I? As you have seen, heat in the output transistors is a
major problem, and it is inherent in the way linear amplifiers operate. Whenever the amplifier is delivering
only part of its power to the load, waste heat is created. There is another way of
converting dc power into audio power that reduces the inherent heat losses. Because
the losses occur when the output transistors are partially on, we avoid this
state. We turn them fully on and
send all of the dc power to the load, or we leave them fully off so that no
power flows. In both cases, little
or no power is wasted in the transistor. To get the desired average power in the
load, we rapidly switch the transistors on and off for the desired percentage
of the time, with the time the transistors are on varying from 0% to 100%. The switching must occur much faster
than the highest audio frequency if the averaging is to work correctly. This is the switching amplifier or class-D
amplifier. (Classes E and F are
special cases, as is class C, that apply mainly to
non-audio uses). How does on-off switching drive a
loud-speaker? The magnetic field in the voice coil does not collapse instantly
when the amplifier switches off. The
loudspeaker continues interpolating a fraction of the waveform while the
amplifier is switching. Class-D amplifiers are only now becoming
practical. The speed requirements
for the switching transistors are 50 to 100 times greater than for linear audio
amplifiers. The high-frequency
switching causes radio interference, and many
practical problems must be solved to attain the same audio fidelity that we
expect with linear amplifiers. When
the switching causes radio interference, and many practical problems must be
solved to attain the same audio fidelity that we expect with linear amplifiers. When the switching is perfected, we
can expect the heat sinks to be about one-fourth the size, reducing amplifier
size and cooling requirements. When
combined with the switching power supply, we will have a size and weight
breakthrough comparable to the transition from tubes to solid-state. It will take a great deal of
experience to overcome the reliability problems associated with complex new
circuitry. This, too, should be an
active area for development in the 1990’s.
MECHANICAL
DESIGN.
We have
treated amplifier design as an electronic problem, but the physical size,
location and ruggedness of the parts is at least as important. You can inspect these features by eye
at trade shows or on other occasions when the insides are on display. The power transformer is the single
heaviest element and must be mounted securely. Also, look for adequate clearance around it to allow for cooling
and shifting in case the amplifier is dropped. The power transformer is the single heaviest element and must be
mounted securely. Also, look for adequate
clearance around it to allow for cooling and shifting in case the amplifier is
dropped.
The height of the chassis is another important variable. Low-profile chassis allow you to
mount a lot more amplifier power in a given amount of rack space, but such a
design pots much more strain on the rack ears. The rack ears should be well-connected to the chassis in such a
way that overstress does not damage or loosen some other critical element, such
as the faceplate. Rear support is strongly
recommended. Standard-height chassis
provide a greater mounting surface and allow for more internal space around
components, such as the transformer. Fan cooling creates noise in the chassis,
but it dramatically reduces the size of the heat sink. Because of the increased power ratings of modern amplifiers, it
is harder to find convection-cooled units.
Class-G and -H amplifiers reduce heat sink requirements (as will class-D
designs eventually) and may help bring back convection-cooled amplifiers. Meanwhile, high-quality,
variable-speed fans can minimize the cooling noise. You should always check fan noise carefully if people will be
sitting near the amplifier. There are a number of details you should
check. For example, external
connectors, controls and displays should be recessed to protect them from
external damage, and lockout covers are sometimes available for extra security. All connectors should be high-quality. Make sure input and output connectors
are firmly mounted and will resist the strain of somebody tripping over the
cabling.
Internal connectors also need to be rugged. Sub-assemblies and circuit
boards might shift as the amplifier bounces around, so excessive rigidity can
be a problem. Card-edge connectors
are frequently troublesome.
Cable-type or shock-mounted connectors with some “give” are preferred.
Gold plating is frequently preferred on input connectors, but it might
wear through after a few insertions. The quality of the underlying plating is
probably more important. Internal connectors should also have
corrosion-proof plating. Gold
plating is best for small signal connections, but it can burn through at high
currents. Heavy-duty connectors
should use precious metal alloys or conductive oxide platings in which current
flow actually improves the integrity of the contact. You’ll find these
principles used in QSC amplifiers and in amplifiers from other leading audio
companies. Various engineers have their own opinions
on how each problem is best solved, but, in many cases, it’s basically a
question of continuously refining a chosen approach. The basic principles here should help users appreciate what goes
into modern amplifiers, often the unsung heroes of sound systems.
The term amplifier is very generic. In general, the purpose of an
amplifier is to take an input signal and make it stronger (or in more
technically correct terms, increase its amplitude). Amplifiers find application in all
kinds of electronic devices designed to perform any number of functions. There are many different types of
amplifiers, each with a specific purpose in mind. For example, a radio transmitter uses an RF Amplifier (RF stands
for Radio Frequency); such an amplifier is designed to amplify a signal so that
it may drive an antenna. This
article will focus on audio power
amplifiers. Audio power
amplifiers are those amplifiers which are designed to drive loudspeakers. Specifically, this discussion will
focus on audio power amplifiers intended for DJ and sound reinforcement use. Much of the material presented also
applies to amplifiers intended for home stereo system use.
The purpose of a
power amplifier, in very simple terms, is to take a signal from a source device
(in a DJ system the signal typically comes from a preamplifier or signal
processor) and make it suitable for driving a loudspeaker. Ideally, the ONLY thing different between the input signal and
the output signal is the strength
of the signal. In mathematical
terms, if the input signal is denoted as S,
the output of a perfect
amplifier is X*S, where
X is a constant (a fixed
number). The "*" symbol
means” multiplied by".
This being the
real world, no amplifier does exactly the ideal, but many do a very good job if
they are operated within their advertised power ratings. The output of all amplifiers
contain additional signal components that are not present in the input signal;
these additional (and unwanted)characteristics may be
lumped together and are generally known as distortion. There are many types of distortion;
however the two most common types are known as harmonic distortion and inter
modulation distortion.
In addition to the "garbage" traditionally known as distortion, all
amplifiers generate a certain amount of noise
(this can be heard as a background "hiss" when no music is playing). More on these later.
All power
amplifiers have a power rating, the units of power are
called watts. The power rating of an amplifier may
be stated for various load impedances;
the units for load impedance are ohms. The most common load impedances are 8
ohms, 4 ohms, and 2 ohms (if you have an old vacuum tube amplifier the load
impedances are more likely to be32 ohms, 16 ohms, 8 ohms, and maybe 4 ohms). The power output of a modern
amplifier is usually higher
when lower impedance loads
(speakers) are used (but as we shall see later this is not necessarily better).
In the early
days, power amplifiers used devices called vacuum
tubes (referred to simply as "tubes" from here on). Tubes are seldom used in amplifiers
intended for DJ use (however tube amplifiers have a loyal following with
musicians and hi-fi enthusiasts).
Modern amplifiers almost always use transistors (instead of tubes); in the late
60's and early 70's, the term "solid state" was used (and often
engraved on the front panel as a "buzz word"). The signal path in a tube amplifier undergoes similar processing
as the signal in a transistor amp, however the devices
and voltages are quite different.
Tubes are generally "high voltage low current" devices, where
transistors are the opposite ("low voltage high current"). Tube amplifiers are generally not
very efficient and tend to generate a lot of heat. One of the biggest differences between a tube amplifier and a
transistor amplifier is that an audio
output transformer is almost always required in a tube amplifier
(this is because the output impedance of a tube circuit is far too high to
properly interface directly to a loudspeaker). High quality audio output transformers are difficult to design,
and tend to be large, heavy, and expensive.
Transistor amplifiers have numerous practical advantages as compared with tube
amplifiers: they tend to be more efficient, smaller, more rugged (physically),
no audio output transformer is required, and transistors do not require
periodic replacement (unless you continually abuse them). Contrary to what many people believe, a well designed tube
amplifier can have excellent sound (many high end hi-fi enthusiasts swear by
them). Some people claim that tube
amplifiers have their own particular "sound". This "sound" is a result of the way tubes behave when
approaching their output limits (clipping).
The onset of output overload in a tube amplifier tends to be much more gradual
than that of a transistor amplifier.
A few big advantages that tube amplifiers have were necessarily given up when
amplifiers went to transistors.
First, tubes can withstand electrical abuse that would leave even the most
robust transistor completely blown.
Also, tube amplifiers use an output transformer to interface to the speaker;
such a device provides an excellent buffer (protection to the speaker) in the
case of internal malfunction. Modern
amplifiers (with no output transformer) occasionally fail in a way that
connects the full DC supply voltage to the speaker. If the amplifier does not have adequate protection circuitry
built in, the result is often a melted woofer voice coil.
Power amplifiers
get the necessary energy for amplification of input signals from the AC wall
outlet to which they are plugged into.
If you had a perfect amplifier,
all of the energy it took from the AC outlet would be converted to useful
output (to the speakers). However,
in the real world no amplifier is 100% efficient, so some of the energy from
the wall outlet is wasted. The vast
majority of energy wasted by an amplifier shows up in the form of heat.
Heat is one of the biggest enemies to electronic equipment, so it is important to
ensure adequate air flow around equipment (especially so for those units using
convection cooling). Most amplifiers
in the 200 watt per channel range (and up) have forced air cooling (via fans)
in order to prevent excessive heat buildup.
Many amplifiers
have a number of features to help monitor the status of the amplifier and also
to protect speakers (and the amplifier itself) in the event of an overload
condition. Some features include
power meters, clipping indicators, thermal overload shutdown, over current
protection, etc. Features vary from
manufacturer to manufacturer. In
addition, there are many variations in how protection circuits are implemented
and how much "safety margin" they allow. For example, I tested the clipping indicator on one particular
amplifier. The clipping indicator
did not come on until there was a substantial amount of clipping actually
occurring (as viewed on an oscilloscope).
In this case, I did not notice a significant degradation of the sound quality
despite the clipping. The
manufacturer in this case chose to "allow a little more volume"
before actually lighting up the warning light.
Power amplifiers
intended for DJ use have power output ratings starting from around 75 watts per
channel to over 1000 watts per channel.
However, keep in mind that MORE POWER DOES
NOTNECESSARILY MEAN A
All power
amplifiers have a power supply,
aninput stage, and an output stage. Many amplifiers have various protection features which fall into
a category I refer to as housekeeping.
Power Supply : The primary purpose of a power supply in a power
amplifier is to take the 120 VAC power from the outlet and convert it to a DC
voltage (VAC is an abbreviation for Volts
Alternating Current, and DC is an abbreviation for Direct Current). Conversion from AC to DC is necessary because the semiconductor
devices (transistors, FETs, MOSFETs, etc.)
used inside the equipment require this type of voltage. (By the way, FET stands for Field Effect Transistor, and MOSFET
stands for Metal Oxide Semiconductor Field Effect Transistor).Many different types of power supplies
are used in power amplifiers, but in the end they all basically aim to generate
DC voltage for the transistor circuits of the unit. The very best of amplifiers have two totally independent power
supplies (they do share a common AC power cord though). Such amplifiers are really just two monaural amplifiers mounted
in a single case.
Input stage : The general purpose of the input stage of a power
amplifier (sometimes called the "front end") is to receive and
prepare the input signals for "amplification" by the output stage. Most professional quality amplifiers
have various input connectors; typically they will have XLR inputs, “quarter
inch" inputs, and sometimes a simple terminal strip input (although these
tend to e found on amplifiers intended primarily for public address systems). XLR and most quarter inch inputs are balanced inputs (as compared to single ended inputs. Balanced inputs are much preferred over single ended inputs when
interconnection cables are long and/or subject to noisy electrical environments
because they provide very good noise
rejection. The input
stage also contains things like input level controls. Some amplifiers have facilities for "plug in" modules
(such as filters); these too are grouped into the input stage.
Output stage : The output stage of an amplifier is the portion
which actually converts the weak input signal into a much more powerful
"replica" which is capable of driving high power to a speaker. This portion of the amplifier
typically uses a number of "power transistors" (or MOSFETs) and is
also responsible for generating the most heat in the unit(unless
the amplifier happens to have a very bad power supply design). The output stage of an amplifier
interfaces to the speakers.
The Class
of an amplifier refers to the design of the circuitry within the amp. There are many classes used for audio
amps. The following is brief
description of some of the more common amplifier classes you may have heard of.
Power amplifiers
are typically rated for "8 ohm" and "4 ohm” loads, and some also
give ratings for "2 ohm" loads.
If you have ever looked at a spec sheet, you probably noticed that the power
output of an amplifier is higher when the load impedance (number of ohms) is
lower. Important: a load with a low number of ohms is a more
difficult load than one with a higher number of ohms! (that
is, a 4 ohm speaker is harder for an amplifier to drive than an 8 ohm speaker). The performance of an amplifier with
low impedance loads is closely related to the capabilities of its power supply.
If we had a perfect amplifier (and it was plugged into an outlet that had unlimited
current capability), its output power rating would double each time the load impedance was halved. For example, let's say the amplifier puts out 200 watts per
channel at 8 ohms. At 4 ohms, it
would put out 400 watts per channel, at 2 ohms it would put out 800 watts per
channel, and at 1 ohm it would put out 1600 watts per channel. For the perfect amplifier, one could keep going with this until the
load impedance approached zero, at which time the amplifier output would
approach infinity! On the other side, if the load impedance was 16 ohms, the
amplifier would put out only 100 watts per channel. In this direction, one could keep raising the load impedance, and
the power output would grow smaller and smaller.
The power supply
of the perfect amplifier generates a DC voltage that does not change no matter how much
current is demanded from it. This
means that the perfect amplifier can drive an unlimited number of speakers. In the real world, amplifiers have
real power supplies which do
have limits as to how much current they deliver. For such typical amplifiers, the 4 ohm power rating is usually
about 50% more than the 8 ohm rating (and if a 2 ohm rating is given, this is
maybe 50% more than the 4 ohm rating).
Amplifiers with exceptional power supply designs will do better than this, but
eventually a limit will be reached (if by nothing else the AC outlet can only
deliver so much current!). Lesser
designs will "run out of juice” when driving the heavier loads. Stay away from amplifiers that have a
4 ohm rating that is less than 25% greater than the 8 ohm rating!
Amplifiers
utilizing exceptional power supply designs will invariably be the more
expensive units available, and possibly the (physically) heavier designs. This is because good power supply designs
usually require heavier and better (low loss) " magnetics
". All power supplies utilize
some combination of transformers,
rectifiers, capacitors, and in the case of so called
"digital" amplifiers, switching
components.
"Analog" Amplifiers : ALL amplifiers
in use by DJ's today process analog
input (music)signals. An analog
signal is a continuous wave signal, a digital
signal is an analog signal which has been converted to a sequence of numbers. Analog when
spoken in terms of power amplifiers typically refers to the design of the power
supply, and most analog amps are those with a straight
"Digital" Amplifiers : When the term
digital is associated with a power amplifier, it often refers to
the design of the power supply and may that the power supply is of the switching type (sometimes referred to as
a DC - DC converter). Also, digital amps are often of one
of the more exotic classes (class G, H, S, etc). These classes of amplifiers use special switching circuits that
change the power supply voltage to the output stage on the fly such that higher
efficiency is maintained. NOTE: A
digital amp It in no way means
that it is inherently better at producing sound from "digital"
sources such as CD's and DAT's!!! I don't recall any manufacturers calling
their amplifiers "digital", but I have heard salespeople use this
term. What advantages does a
switching power supply offer? They are able to use much smaller transformers
and capacitors, and are therefore considerably smaller and lighter than an
equivalent analog power supply. The
concepts behind switching power supplies have been known for many years. However, until fairly recently the
components necessary for switching power supplies were unable to be produced
cheaply enough for consumer use.
Advances in transistor technology have made the necessary devices available at
a cost which permits their widespread use.
(Note : ALL
of the "super systems" heard in many automobiles today are powered by
amplifiers using switching power supplies).
On the minus
side, switching power supplies are a great deal more complicated than their
analog counterparts. They work
basically by first creating a "crude" DC voltage. This crude voltage is applied to a circuit which uses a specially
designed high frequency transformer.
A control circuit monitors the output voltage of this stage and makes
adjustments "on the fly” in order to keep the final DC output voltage as
close to the design value as possible.
So, the advantages of lighter weight and smaller size come at the expense of
increased parts count(which ultimately might translate
to less reliability if the parts are of lesser quality). Also, switching power supplies are harder to repair if they fail.
Many
"digital" amplifiers also use a "multi-rail" power supply
system. Such systems are more
complicated than conventional amplifier designs,
however they offer considerable improvements in amplifier efficiency. The amplifier selects a "rail
voltage” based on the output demands of the amplifier. Higher efficiency is achieved by minimizing the voltage drop
across the amplifier’s output transistors.
Since less of the amplifier's power is wasted as heat, the power supply and
transistor heat sinks do not have to be as large as those in a
"conventional" design. As
before, the theory behind "digital" designs has been known for
decades, but until recently components necessary to make unaffordable design
were unavailable.
Many of the
amplifiers on the market today are of the” digital" type, using switching
power supplies and/or special power supplies that maintain high efficiency at
high outputs. Some people believe
that "digital" amplifiers are not so good at producing powerful bass
notes. While it is true that there
probably some marginally designed "digital" amplifiers which do have less than ideal bass response,
weak bass response is not a necessity
of digital designs. The dominating
factor in performance comes back to the ability
of the power supply to provide adequate current; a solid design
means adequate current is available for loud bass notes and/or difficult
speaker loads. In addition, a second
important factor is the adequacy of the AC
power outlet. Two well
designed amplifiers (one of each type) operated on an AC outlet which doesn't
"sag" (see my article on AC Power)should
both provide excellent sound quality.
Many of the higher power amplifiers available today are of the” digital"
(switching power supply) design. But
keep in mind that this does not necessarily make them better or worse. Stay with vendors that have proven
track records of reliability and you should have few problems with either type
of design.
Two amplifiers
with the same power rating put out the same power, right? Not necessarily. Manufacturers vary as to how
conservatively they rate their amplifiers.
As an example, I measured one particular amplifier, rated at 350 watts/channel,
and found it actually was able to put out 450 watts/channel! Manufacturers
often understate what their units will actually putout. It would be a bad idea to publish the "absolute maximum
power" that the unit could put out, since a margin needs to be allowed to
insure that all production units will meet published specs. In addition, a manufacturer may publish a very conservative 8 ohm
rating in order to make the 4 ohm rating look better (a really terrible
amplifier will put out LESS power into a 4 ohm load!).
Amplifiers are
generally rated in watts per channel , at several load
impedances, with both channels driven, over a frequency range of usually 20 Hz
- 20,000 Hz, at some amount of total harmoonic distortion. Most amplifiers will put out slightly more (but not a tremendous amount more) power when only a single channel is driven. This occurs because the power supply
only has to provide power for a single channel, and its DC voltage doesn't sag
as much. The exception
are amplifiers which use dual independent power supplies (since each of
their supplies only has to supply power for one channel anyway).
Many of the
amplifiers on the market today are touting excellent performance with 2 ohm
load impedances. Some also state
that "continuous operation" with 2 ohm loads is possible. While such statements are probably
true, it is not really a good idea to run under such conditions!
First, a word on
speakers is in order (for much more detail see my
article on speakers). All speakers have
a characteristic known as impedance (measured in ohms), with most speakers being either 8 ohms or 4 ohms. Lower
impedances represent more difficult loads for amplifiers to drive. Two 8 ohm speakers connected in parallel will result in a 4 ohm load at the
amplifier. And, two 4 ohm speakers(wired in parallel) result in a 2 ohm load. In actuality, speaker impedance can
vary by a factor of 10 or more
over the audio frequency range. When
a speaker is said to be 8 ohms, it is understood that this is a nominal or approximate rating (the same goes for 4 ohm speakers). An 8 ohm speaker could have an impedance as low as 2 or 3 ohms and as high as 50 ohms
(impedance is frequency dependent)! Further, a speaker load is not the same as
a resistive load, speakers are reactive
loads. A reactive load is a load
that has inductive or capacitive properties. Depending upon the input signal frequency, speaker loads may be
resistive or resistive with an inductive or capacitive component. Without going into a ton of technical
explanation, what this means is that speakers are often difficult loads for
amplifiers to drive. Driving difficult speaker loads is where better
amplifiers are separated from lesser designs.
Even though an
amplifier may be rated for continuous use at 2ohms, there are several reasons
why this is not the best thing to do:
So, just because
an amplifier has a super powerful 2 ohm rating, don't look for ways to wire up
multiple speakers in order to "use" this power! Treat the 2 ohm
rating as "headroom" and know that your amp has the ability to more
easily handle the most difficult "normal" speaker loads that you are
likely to ever encounter. If you
need more power, get a second amp.
Two medium powered amps are better than one monster (what if your one big amp
dies? With two smaller amps at least you can still run!).
All amplifiers
generate a certain amount of electrical noise. Generally, the more powerful the amplifier, the
more noise. If you turn on an
amplifier (with the input jacks disconnected) and listen to a speaker you can
clearly hear a hissing sound. This
pretty much represents the noise floor of the amplifier. For a powerful system, the noise might seem pretty obvious;
however when actual music is playing the noise will be totally masked.
All electrical
circuits generate a certain amount of noise.
Better designs minimize the amount of noise, however no matter how good the
design there will always be some.
The noise is generated by the movement of electrons in the system and cannot be
eliminated (unless you chill your equipment to absolute zero!). The noise floor of an amplifier by
itself is usually not obviously audible in a typical room (unless you are
standing right next to a speaker).
However, the remaining components in a system (preamp, equalizer, processor,
etc.) each add in some noise. So, the total system noise (when no
music is playing) might be
objectionable. If this is a serious
problem, a device called a noise gate
can be used. Such a device is
essentially a "squelch" which is wired in just before the power amps
(or electronic crossover in multi-way systems). The device is basically cuts noise from upstream components when
no music is playing. Most noise
gates have adjustable controls to set the threshold at which noise cut begins
and also to set the amount of desired noise cut. Most DJ systems probably do not need noise gates unless they are
very high powered systems with along signal chain (or noisy components).
The noise floor
of an amplifier is relatively constant, meaning it does not increase with
increasing output signal (unless the amplifier has a poorly regulated power
supply). In other words, the
amplifier's noise floor is pretty much the same whether or not music is playing
loudly or softly. So, when music is
playing softly, the noise will be proportionally larger. When music is playing loudly, the noise is essentially
"buried" or masked.
As stated, an
amplifier with a poorly regulated power supply can create some additional noise. If the filtering of the power supply
is marginal, the "smoothness" of the DC power supply voltage will be
degraded when the amplifier is playing loudly. This will result in additional noise being added to the system(generally in the form of 60 Hz products). This type of noise isn’t really part
of the noise floor. Such noise is
often inaudible when music is playing loudly. It can be clearly heard however when playing test tones at levels
near the output limit of the amplifier (don't try this unless you are
thoroughly familiar with testing practices...
blown speakers will otherwise be the result!).
ALL amplifiers alter input signals, generally in two
ways: they make them stronger (amplify)
them, and they add characteristics which
did not exist in the original signal. These undesirable characteristics are lumped together and called distortion. Noise can be considered a type of distortion and was discussed in
the above section.
Everyone is
familiar with gross distortion, the sound quality that results when turning up
a radio or boom box to "full blast”.
An excessive amount of amplifier clipping
(see section below) results in hideous distortion that would be totally
unsatisfactory for a DJ sound system (as well as a listener’s ears). However, not all distortion is
blatant. In addition, there are
several types, two of which will be discussed. Knowing what causes distortion will help you to prevent it from
occurring. Knowing how to control
distortion is important because excessive distortion can be detrimental to
speaker systems (and your reputation).
Harmonic distortion : One common type of distortion is harmonic distortion. Harmonics of a
signal are signals which are related to the original or fundamental by an integer (non decimal)
number. A pure tone signal has no harmonics; it
consists of only one single frequency.
If 100 Hz pure tone signal was applied to the input of an amplifier, we would
(upon measurement with special test equipment) find that the output signal of
the amplifier was no longer pure.
Careful measurements would likely show that several "new" frequencies
have appeared. These new frequencies
are almost certainly to be integer multiples of the original tone; they are the
harmonics of the original signal. In
the case of a 100 Hz input tone, we might expect to find tones at200, 300, 400,
500 (etc.) Hz. We would also probably notice that the odd harmonics are much stronger than the even harmonics (we will not go into the
reasons why in this article). In a
good amplifier, the harmonics will be much
weaker than the original tone. By
much weaker, we mean on the order of a thousand times for decent amplifiers.
All amplifiers
are generally rated for Total Harmonic
Distortion (or THD),
usually at full power output over a given frequency band with a particular load. Good values are anything less than 0.5 % THD. When an amplifier is measured for THD, a pure tone is applied to
the input and the output is measured with special test equipment. The energy of the pure tone is
measured, and the energy of the harmonics is measured. Those two values are compared, and a THD rating is calculated. ATHD rating of 1% means that the total energy of all the harmonics combined is one one-hundredth of the
energy in the fundamental.
Harmonic
distortion (although certainly undesirable) is one of the more tolerable types
of distortion as long as it is kept reasonably low. Distortion levels of 10% may be very tolerable with music so long as the 10% level is only
"occasional" (10% THD on a pure
tone can easily be
heard by the human ear... but who
listens to pure tones?). The reason
that a seemingly high value of THD is acceptable for music is partially because
many sounds in nature are rich in harmonics.
Also, most decent cassette decks (which most people agree sound pretty good)
have THD (off the tape that is) of several
percent. Worse, even good speakers can have THD up to 10%,
especially at low frequencies! All in all, the human ear can tolerate a fair
amount of THD before it becomes objectionable.
Do two amplifiers with identical THD ratings sound the same,
everything else being equal? Not necessarily (but differences will be
subtle). The reason is that the THD
specification states nothing about where
the harmonics are in the frequency band.
For example one amplifier could have a dominant harmonic at one frequency and a
second amplifier could have a dominant harmonic at a very different frequency. Or, one amplifier could have a few
"big" harmonics while a second has many weak ones. These situations could easily result in identical THD ratings. The variations could be easily
measured with laboratory equipment.
However do not be overly concerned.
Minor variations in THD ratings will not cause major differences in sound when listening to music.
With pure tones as input signals it might be fairly easy to discern which of
two amplifiers was used (but again, who listens to tones?).
Inter modulation distortion : Inter modulation
distortion is the second "major" type of distortion that is often
specified for amplifiers. Inter modulation
distortion is much more objectionable to the human ear because it generates
non-harmonically related "extra" signals which were not present in
the original. It is analogous to
someone singing way off key in a choral group
Inter modulation
distortion (sometimes abbreviated IM) is more complicated to test for and
specify. Basically, two pure tones
are simultaneously applied to the input of the amplifier. If the amplifier were perfect, the two tones (and only the two
tones)would be present at the amplifier output. In the real world, the amplifier
would have some harmonic distortion (as described above), but careful observation
of the output signal (using laboratory equipment) would reveal that there are a
number of new tones present which cannot be accounted for as a result of
harmonic distortion. These
"new" tones are called "beat products" or "sum and
difference" frequencies, and are a result of the interaction of the two
pure tones within the amplifier. No
amplifier is perfect, all have some non linear characteristics. Whenever two signals are applied to a
nonlinear system, new signals (in addition to the original two) are generated. For a good amplifier, the new signals
are very small in relation to
the two original tones. This is fortunate, since the ear can detect much lower levels of
inter modulation distortion as compared to harmonic distortion.
It should be noted
that distortion measurements on amplifiers are made with test tones. These tones are usually sine waves(pure tones), which represent the simplest possible
test signal to measure and quantify.
A music signal is an extremely complicated
waveform consisting of many constantly changing sine waves. Since music has so many harmonics and frequencies present,
quantifying how two different amplifiers will sound by using simple THD and IM
specifications is extremely difficult.
In other words, just because two amplifiers have the same published specs for
THD and IM does not mean that they are equivalent. Fully and completely quantifying the technical performance of an
amplifier would be extremely complicated and costly (and would probably have
little benefit in the end). Most
amplifiers available today (from reputable manufacturers) have THD and IM
levels low enough to yield excellent performance (so long as they are not
overdriven). This leads nicely into
our next topic...
Clipping is a term which
many people have probably heard, but may not fully understand. Very simply, clipping of an amplifier
occurs when one tries to get a larger output signal out of an amplifier than it
was designed to provide.
As stated before,
all power amplifiers have a DC power supply which provides power to (among
other things) the output stage of the amplifier. For most amplifiers, the power supply consists of a
"plus" supply and a "minus" supply. The two voltages are often referred to as "rail
voltages" or simply "rails".
As an example, a 200 amplifier (at 8 ohms) might have a power supply voltage
(rails) of +/- 60 volts DC. This
means that the output voltage which drives the speaker can never exceed + 60 or - 60 volts. If the amplifier is playing at near
full volume, and someone” cranks up the volume",
the amplifier will attempt to put out more power. However, the power required to meet the sudden new demand for
more volume cannot be met by the power supply voltage, which has limits of
+/-60 volts in this example. The
result is a waveform with the top portion (or peak) "clipped" off
(hence the term "clipping").
Such clipping represents a distortion which is added to the waveform (and if it
is severe enough it will be clearly audible). If a signal is severely clipped, the waveform takes on the shape
of a "square wave", and the resulting sound will be absolutely
hideous. Clipping can be easily
observed using an oscilloscope attached to the amplifier output.
Clipping is not
usually a major problem for amplifiers (unless it is extreme), but it can be
very detrimental to speaker systems.
Whenever clipping occurs, two things happen: (1) the spectral content of the
music signal is altered (high frequency components are generated), and (2)
signal compression occurs. If excessive
clipping occurs, tweeters will be the first to blow followed by midrange
drivers. Woofers are best equipped
to survive clipping (unless the abuse is blatant).
In general,
clipping of an amplifier should be avoided.
Use an amplifier that has clipping indicators, and pay attention to them!
Occasional clipping is OK and probably not very audible. However if you find yourself clipping the amp most of the time,
you should consider obtaining stronger (or additional) amplifier.
The Damping Factor of an amplifier in general
refers to the ratio of the amplifier's output load impedance (the speaker,
nominally 8 ohms) to the output impedance of the amplifier. Ideally, the damping factor would be infinity (in other words,
the ideal output impedance for an audio amplifier is zero ohms). Damping factor, like many amplifier
specifications, is a function of many factors and is thus difficult to quantify
with a single number. As such,
"low end" manufacturers can have a "field day" with this
spec, publishing fantastic numbers (however with no information as to how the
measurement was made).
The damping
factor if an amplifier depends greatly upon the speaker to which it is
connected, the wire connecting the speaker to the amplifier, the signal
frequency that the amplifier is sending to the speaker, and the power level at
which the amplifier is operating, among other things. Damping factor is most critical at low frequencies, generally 100
Hz and below (i.e. frequencies that a woofer reproduces). At such frequencies, a high damping
factor is desirable in order to maintain a "tight" sound. If an amplifier/speaker pair has a
low damping factor, the bass response is likely to be "boomy",
"uncontrolled", and "loose" sounding.
Specifying
damping factor as a simple single number does not really tell the whole story. Damping factor is a ratio of two
numbers, one of which (the speaker impedance) varies by a large amount
depending upon frequency. This being
the case, the damping factor will also vary considerably as a function of
frequency. Most of the variation in
damping factor is due to the characteristics of the speaker connected to the
amplifier. The wire which connects
the speaker to the amplifier has finite resistance which must be accounted for;
basically it is lumped in with the impedance of the speaker. So, it is wise to use heavy speaker wire in order to minimize
degradation of the damping factor.
As mentioned, the
output impedance of an amplifier is ideally zero. In the real world, this is never the case. The next best thing would be a very low constant (non changing) impedance.
Again, the real world does not allow this either. The output impedance of most amplifiers is relatively constant
except for when they approach the last 10% or so of their voltage output. This is due to the nature of the
waveform from which most power supplies obtain their energy (especially analog
supplies) .
What this means is that the output impedance of an amplifier tends to rise
considerably as it approached its output limit. As the amplifier's output impedance increases, the damping factor
must decrease proportionally. In my
opinion, if manufacturers specified the output
impedance of their amplifiers, there would be a lot less ambiguity
among the numbers.
High damping
factor numbers go hand-in-hand with amplifiers that can drive very low
impedance loads (these are amplifiers with power supplies capable of delivering
tremendous current). If you want to
"artificially" degrade the damping factor of your system (to hear the
effects), a simple test can be done.
Listen to your system at a "healthy" volume (use a CD with lots of
low, tight percussion type sounds); be sure to use a heavy gauge short length
speaker wire. If you have a sound
level meter, note the sound level you listened at. Then, connect your speaker up through a 100 foot (give or take)
wire with much smaller gauge (use #20 or higher). Play the same music as before, but make sure the volume (to your ears, not the volume control!)
is the same (this is where the sound level meter comes in handy). The volume control on the amp will
have to be turned up a bit to overcome the power loss in the smaller wire. You should be able to tell that the
sound has changed (for the worst, in most people's opinion).
Do not be terribly concerned with damping factor
when choosing quality equipment.
Most of the good amplifiers and speakers available today will yield excellent
sound when used together. To avoid
degrading the damping factor of your system, simply follow these (easy) steps:
YES! Amplifiers
in the 400 plus watt per channel range are not uncommon today. Such an amplifier will put out about
50 to 60 volts RMS to a speaker.
While this is only about half the amount that comes out of a wall socket, it's
definitely enough to be unpleasant if you are holding on to it! Note: The US
Military defines any voltage in excess of 30 volts as hazardous. Such a voltage can be generated by
any amplifier in the 100 + watt per channel range.
As a side note,
it's not a good idea to plug or unplug speakers when the amplifier is playing
at high volume. The "make and
break" of connectors can cause momentary short circuits, as well as
voltage and current transients (none of which is healthy for the amp). The preferable procedure is to make
all speaker connections (and disconnects) with the amp turned OFF.
Bridging an amplifier refers to configuring a two channel
(stereo) amplifier to drive a single
load with more power than the sum of the two original channels combined. For an example, a 100 watt per
channel amp may put out 300 watts(one channel) after
bridging.
There are
important things to know about running an amplifier in the bridged mode:
Bridged
amplifiers work basically as follows: A single input signal is applied to the
amplifier. Internal to the amp, the
input signal is split into two signals.
One is identical to the original, and the second is also identical except it is
inverted (sometimes called phase-flipped). The original signal is sent to one channel of the amp, and the
inverted signal is applied to the second channel. Amplification of these two signals occurs just like for any other
signal. The output results in two
channels which are identical except
one channel is the inverse of
the other. The speaker is connected
between the two positive
amplifier speaker output terminals.
In words, one channel "pulls" one way while the second channel
"pulls" in the opposite direction.
This allows considerably more power to be delivered to a single load.
If we had our perfect amplifier, upon bridging it we
would have a single channel amplifier with exactly four times as much power as any one channel of the amplifier in "normal" stereo
mode, assuming an 8 ohm speaker load.
This is because the effective output voltage
available to drive the speaker has doubled as a result of bridging. A doubling of voltage
on a given load results in a fourfold
increase of power delivered to
that load. If we used a 4 ohm load
on the perfect bridged amplifier,
the output power would be a
very substantial eight times
the normal stereo single channel 8 ohm output! These numbers should give some
clues as to why real world amplifiers cannot meet such expectations. Once again, we are back to
limitations of the power supply. In reality, most amplifiers in
bridged mode will put out about 3 times the power as any one channel of the amp
in normal stereo mode. The fourfold
increase cannot be achieved because the power supply is unable to provide the
current required for such performance.
With 4 ohm loads, the situation is compounded. The amount of current required to drive a 4 ohm load when in
bridged mode will tax the amplifier’s power supply to its absolute limits. Not to mention, the output stage may
not be able to safely handle the extra heat that will be dissipated. Bottom line: stay away from 4 ohm
loads if you are running an amplifier in bridged mode!
Note: This
section is intended primarily for engineering students or those with a deeper
technical interest. The purpose is
to provide a "real world" explanation of the Maximum Power Transfer
theory and why it is NOT used in amplifiers designed for stereo systems!
Second year
electrical engineering students have most likely covered the theory that
basically states "maximum power is transferred to a load when the output
impedance of the source is identical ("matched") to that of the
load". The connection that some
people fail to make is that maximum power transfer doesn’t mean maximum efficiency! At best, if the maximum
power transfer theory is used, efficiency will be only 50% (not such a good
figure for an audio amplifier). In
other words, if an amplifier is designed for maximum power transfer to a load,
fully one half of the energy required by the amplifier's output stage will be
dissipated (i.e. wasted) in the source impedance!
For amplifiers
used in stereo systems (audio amplifiers), the goal is to have the amplifier
output impedance be as low as possible
(ideally zero, but this is never achieved).
If an amplifier were to have an output impedance of 8 ohms (a common value for
speakers), maximum power transfer would occur. However two other bad things result. First, the efficiency of the amplifier is at best only 50%, meaning
that the amplifier will generate a lot of heat. Second, the amplifier/speaker system will have a terrible damping factor. Damping factor basically refers to the ratio of speaker impedance
to amplifier output impedance; high numbers are better. A low damping factor will not damage anything but it will tend
louse up the sound considerably. To
maintain a "tight" sound, it is important to have the output
impedance of the amplifier be as low as possible with respect to the speaker. Otherwise, the amplifier will not
have as much control over the speaker.
Speakers, being highly complicated electromechanical devices with reactive
impedance properties, behave better when they are connected to an amplifier
with an extremely low output impedance. Speakers tend to electrically
"buck and kick" an amplifier when in operation; the best way to tame
this behavior is to put a heavy "load" (i.e. an amp with a very
low output impedance) on the speaker! An amplifier/speaker combination with a
low damping factor will tend to have a "boomier" sound and poorer
transient response, (such a sound is not always bad, some
people actually prefer it!).
There is a quick
test anyone can do to get a feel for what affect the damping factor has on a
speaker system. Disconnect your
speaker system from the amplifier, remove the grille, and gently tap on the woofer cone. You will hear a low frequency sound, this is the "resonance frequency" of the
system. Note the characteristic if
the sound as you tap the cone. Now,
connect the speaker up to the amplifier, and turn the amplifier ON (but leave
the volume at zero). Now tap on the
speaker cone as before. You will
observe that the sound has changed considerably. The sound will be much "tighter", and the cone will
seem harder to move. This is because
the amplifier has in effect "loaded" the speaker system. The case where the speaker was
disconnected from the amplifier represents the worse possible damping factor
(zero).
Anyway, back to
the topic of this section. Although
there are many applications where maximum power transfer is desired, audio
amplifiers are not one of them.
Audio amplifiers generally deal with a considerable amount of power, so high
efficiency is a more important design consideration.. In addition, to maintain high
quality audio, an audio amplifier ideally has an output
impedance which is VERY small compared to the impedance of the speaker it will
be driving. Note that using 4 ohm
speakers on an amplifier will degrade the damping factor as compared to using 8
ohm speakers.