NOTE: THE AUTHOR SHALL NOT BE HELD RESPONSIBLE IN ANY WAY FOR
USE OF THE INFORMATIONAL CONTENT OF THIS ARTICLE, OR DAMAGES OR
INJURIES ARISING FROM THE USE OF SAID CONTENT. THE AUTHOR URGES
ALL POSSIBLE SAFETY PRACTICES AND THAT GOOD CONSTRUCTION TECHNIQUE
BE USED SHOULD THE READER DECIDE TO CONSTRUCT A DEVICE FROM THE
INFORMATION CONTAINED HEREIN. CONSULT YOUR LOCAL LIBRARY FOR
INFORMATION ON AC LINE SAFETY PRACTICES, AND GOOD ELECTRICAL
Assembling a DIY balanced AC power unit is not a trivial or inexpensive task. However, you will be able to save money over a retail unit. Do not skimp on or substitute lesser parts for those specified, and only make substitutions of supposedly equal parts upon consultation with the proper specialist. The transformer source is located in the mainland USA, so any European hobbyists will have to find an equivalent unit from local sources.
The transformers called out in this note come from Signal Transformer
Company, a transformer company that has an excellent reputation for producing
a quality product at reasonable prices, and a company that will sell single
quantities to individuals. These transformers are 50/60 Hz rated,
and are good to a Hi-pot voltage of over 2500 volts, and use Class A insulation
(105C) or better. They have an electrostatic shield between the primary
and secondary windings.
No load to rated load regulation is around 5% or better. There are many fine transformer companies, but most will not sell single items to individuals. If you wish to use another source, you are on your own, I can not act as a free consultant for each and every desire to deviate from this recipe. I strongly suggest that you copy the technical support staff of the other transformer company this note, and try to fully explain what you wish to do, in order to get the most appropriate unit from them with the proper safety margins. For those wishing to find another source, such as European hobbyists, the Signal transformer information is located at: http://www.signaltransformer.com/
Some might wish to use toroidal transformers instead of the recommended E-I core style transformers. DON'T! Toroidal transformers tightly couple the AC line to the secondary in a wideband manner, This means that more noise, not less, will get through to the secondary windings. Also, toroids tend to saturate much more abruptly than E-I core transformers, and need to be over rated by a much larger factor to avoid compressing the dynamic range of a power amp or other electronics.
- Specific Projects -
Just Isolated Power
It is not absolutely necessary to use balanced AC power (where the signal is deliberately set to be +/- 60VAC referenced around ground) in order to gain benefit of AC line noise reduction and isolation.
In many cases, use of an isolation transformer will provide at least 6 dB of noise reduction, and will often provide more than that. The trick is to use a unit large enough not to limit the available current at the wall outlet, so that a large power amp can be plugged into the isolation transformer and gain benefit of the isolation too. Since most US wall outlets can theoretically do 15 amps continuously, the transformer needs to handle at least this amount of current on a continuous basis.
For this application I have selected the Signal DU-2 power isolation transformer, rated at 2 kVA and 18 amps of current. It has dual 120 volt primaries and secondaries, with taps at 110 and 104 on all of the windings. These transformers are marked as to the windings as PRIMARY or SECONDARY. Then, the individual terminals are marked by voltage numbers. I will provide specific wiring instructions for each project in this note.
For the purposes of isolation, the primaries are wired in parallel, and the secondaries are wired in parallel. If you have a low line (or a perennially high one) you can adjust the output voltage via the taps. If your line typically runs low and is centered around 110 VAC, then you could use the 110 taps on the two paralleled primaries to up the output to a nominal 120 VAC on the output. Only do this if the AC line is consistently low, and never rises to the full 120 VAC. For a line that is always high, use the 110 taps on the secondary to get a lower output voltage. This unit can also be used in Japan, where the AC voltage is typically 100 VAC by using the appropriate taps.
Specific wiring instructions for the DU-2 used as an isolation transformer: Connect the AC input power to the primary windings. There are a set of two rows of four terminals, marked "0", "104", "110", and "120". Place the "0" terminals in parallel (connected together) and the "120" terminals in parallel. These are the input terminals for the AC power from the line cord, which should run through a safety device such as a fuse or circuit breaker. See the common section at the end for details.
The terminals use large brass screw type terminals, so a heavy gauge wire (I recommend 14 gauge minimum) can be firmly wrapped around the terminal underneath the nut, and the nut tightened down.
The secondaries get connected to the high grade AC outlets you obtained, and they get connected in a similar manner, the two "0" terminals connected togehter, and the two "120" (or whatever) terminals connected together. These are then the output terminals for the AC outlets. Do not swap the primaries and secondaries, as Signal has allowed for the losses through the transformer, and doing it backwards will reduce the output voltage a bit from the input.
Amazingly enough, it does not matter whether the "0" or the "120" are connected to the hot or to the neutral, as this is an isolation transformer. For the sake of convention and consistency, use the "0" terminals for the neutral, and the "120" terminals for the hot on both the primary and secondary sides. This will insure that the output AC line phase is the same as it was going in.
The electrostatic shield is connected to the AC power ground, via the linecord, as is the AC outlet ground. If there is no discernable shield terminal, it is connected to the transformer frame, and the frame should be grounded.
Make sure none of the wires stick out from the terminal posts/nuts, and that they are all tight and firmly wrapped around the proper terminal. Double check the wiring, to make sure it is correct.
This particular transformer can be used as a balanced power unit, but it must be de-rated to 9 amps of total draw on the output, due to the wiring needed to get the center tap on the secondaries.
Specific instructions for use of the DU-2 as a balanced AC line unit:
For this use, the dual primaries and secondaries are connected in series. Connect the AC line input through a safety device to the primaries by connecting the neutral line to one of the "0" terminals, then connect the "120" terminal of that row to the "0" terminal of the other primary side row of terminals, and connect the hot AC line to the remaining "120" terminal. The secondary is connected identically, using the secondary side same row "0" terminal for the output neutral.
The grounding is where it gets tricky. If use of a technical ground
is contemplated, then see:
and the general Equi=tech web site for further info.
As near as I can tell, if the technical ground is done correctly, then the line cord ground from the original AC outlet powering the transformer primary should only get connected to the chassis/component box, and the transformer electrostatic shield can get connected to either the technical ground or the original AC wiring ground, and the secondary center tap (which is the "0" terminal and the "120" terminal that are connected together should be connected to the technical ground, as should the balanced power AC outlet ground.
If a technical ground as described in the equi=tech article is not constructed according to the proper method and connections, then the original AC ground should be used for all the grounds.
BTW, if you go to the trouble to run the technical ground properly, then you might as well run a dedicated AC line just for the audio and the balanced power unit. Balance it at the audio system, and run a conventional AC line along with the extra technical ground wiring that is required.
This balanced power transformer should provide at least 10 to 12 dB
of noise reduction, and about that much freedom from ground loop hum as
well. In some cases, the improvement might be as high as 20 dB.
That is one tenth the noise voltage, and one quarter as loud.
Balanced AC Power only
For this, we use the Signal SU-2 model, which will provide balanced AC power at up to 16 amps of current draw (it is also rated at 2 kVA, but the wiring places more stress on the core). This model can not be readily converted to simple isolation use, unless you were taking 240 VAC down to 120 VAC.
Specific instructions for use as a balanced AC power unit:
Connect the AC input power to the primary windings. There are a set of two rows of four terminals, marked "0", "208", "220", and "240". Place the "0" terminals in parallel (connected together) and the "240" terminals in parallel. These are the input terminals for the AC power from the line cord, which should run through a safety device such as a fuse or circuit breaker. See the common section at the end for details. For the secondary, connect the AC outlet neutrals to one of the "0" terminals, then connect the "120" terminal of that row to the "0" terminal of the other secondary side row of terminals, and connect the AC outlet hot to the remaining "120" terminal.
Grounding options are the same as the balanced unit #1 version.
Smaller isolation or balanced unit
The Signal model MPI-900-230 can provide either 7.8 amps of isolation, or 3.9 amps of balanced power. This unit also has dual primaries and secondaries. If the primaries are wired in parallel, and the secondaries also, then it will be wired as an isolation transformer and be capable of handling 7.8 amps (900 kVA rated). This unit also has an electrostatic shield. The primaries are marked "0", "100" and "115", and the secondary is just "0" and "115".
Specific wiring instructions for the MPI-900-230 used as an isolation transformer: Connect the AC input power to the primary windings. There is a single set of insulated screw terminals, marked "0", "100", "115", and "0", "100", "115". Place the "0" terminals in parallel (connected together) and the "115" terminals in parallel. These are the input terminals for the AC power from the line cord, which should run through a safety device such as a fuse or circuit breaker. See the common section at the end for details.
The secondaries get connected to the high grade AC outlets you obtained, and they get connected in a similar manner, the two "0" terminals connected together, and the two "115" (or whatever) terminals connected together. These are then the output terminals for the AC outlets. Do not swap the primaries and secondaries, as Signal has allowed for the losses through the transformer, and doing it backwards will reduce the output voltage a bit from the input. For the sake of convention and consistency, use the "0" terminals for the neutral, and the "115" terminals for the hot on both the primary and secondary sides. This will insure that the output AC line phase is the same as it was going in.
Specific instructions for use of the MPI-900-230 as a balanced AC line
unit: For this use, the dual primaries and secondaries are connected
in series. Connect the AC line input through a safety device to the
primaries by connecting the neutral line to one of the "0" terminals, then
connect the "115" terminal in the middle to the "0" terminal in the middle
of the strip of screw terminals. Connect the hot line to the remaining
"115" terminal on the end. The secondary is connected similarly,
the output to the AC outlet neutral is conected to the "0" terminal on
the secondary, the "115" and "0" terminals in the middle are connected
together (and form the center tap), and the "115'
terminal at the edge is connected to the AC outlet hot.
Grounding options are the same as for the unit #1 in balanced mode.
It is suggested that a suitably heavy duty enclosure be used to house the unit in a safe manner from both an electrical standpoint, and a thermal standpoint.
These units will have a temperature rise of 60 C above ambient at full rated power, so they must not be allowed to come into contact with the wiring or anything flammable for safety reasons.
The enclosure should have adequate ventilation, and not be a sealed cabinet or box.
As for AC cords, connections, outlets, etc. I leave that to the DIYer, as there are many different variations possible, and most will have their favorite approach. I would recommend that premium components be used, such as Hubel outlets, and heavy duty AC cords, etc, as the cost and effort that this project involves makes anything less a waste. Just be sure to provide the proper safety margin for al aspects of the project.
I am not affiliated with Signal transformer in any way, nor do I
receive any compensation from them. There are many fine transformer companies out there, I am merely familiar with Signal's quality and their willingness to sell to individuals.
Signal Transformer, 500 Bayview Ave., Inwood NY 11096
Signal has an online catalog which can be downloaded, and the
transformer specs and schematics showing the primaries and secondaries as I call them out. The ratings in the catalog are for more conventional wiring, I consulted with a Signal applications engineer to determine the ratings for this type of use.
Costs for transformer only:
MPI-900-230: $162 plus S&H, weight 20 lbs.
Dimensions: L=5.25", W (terminal to terminal)=5.2", H=4.8"
Mounting Dims=Mounting hole length=4.5" ctrs. mhw=4.2"
DU-2 OR SU-2: $315 plus S&H, 56 lbs.
Dimensions: L=7.56", W=8.25", H=7.37", mhl=6.75", mhw=6.0"
Delivery stock to 6 weeks, and they accept Visa, MC.
Freight is FOB.
Newark carries Stancor Transformers, and they have a line of
Isolation transformers with a 3 wire cord, and a AC outlet receptacle. They have a electrostatic screen/shield connected to the third wire ground. No box necessary, although there is always the issue of pets and children.
(It will still get hot under load)
Stancor Model # GIS-1000 Newark# 01F1004 $251.86 1000
watts, 8 amps
Stancor Model # GIS-500 Newark# 01F1004 $128.45 500 watts, 4 amps
A suitable metal box with ventilation louvres will run about $60-80, also available from Newark.
I would look at the units from Bud Industries, the infamous Bud Box. They have some industrial, but reasonably priced units, and some nicer looking cabinets.
For smaller transformer: Bud # CU-1099, Newark#91F644, $23.07
For the larger transformers: Bud # CU-880, Newark #91F645 $44.98
These would need ventilation holes, slots, whatever, drilled in them. They are gray painted 20 gauge steel utility cabinets. With some work, these are the minimum that would be suitable for the transformers, and ONLY if they were not going to be moved or handled a lot.
For smaller transformer: Hammond# 1401A, Newark#94F4836, $41.22
Bud # WA-1540, Newark#91F615, $61.90
For the larger transformers: Hammond#1401C, Newark #94F9447 $67.76
For the larger transformers: Bud # WA-1541, Newark #91F616 $85.39
The Hammond enclosures are 20 gauge satin white steel, with rear vented panel, and hardware and feet. This is the sturdiest readily available cabinet that is not a rack mount box costing hundreds.
The Bud enclosures are 0.050 aluminum, with plastic feet, a handle, and louvered sides. Metallic grey finish. These might be marginal as far as the weight of the transformers, but might be OK if just sitting there on the floor.
One could always add on wooden end panels for a nice touch, or some other way of dressing the enclosures up. I would not recommend a wholly wooden box, due to the possiblity of the transformer getting quite warm under load.
For suitable AC Line cords, see the DIY note link at the home page, and for IEC connectors, etc. see the AC Filter & SS note link on the home page. (home page link at bottom)
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