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BM Audio Power Amplifier

Before I begin, I would like to thank the person who helped me on the amplifier, despite his other commitments. He is none other than the designer himself, Bert Molenkamp.

Over the past year, I have been struggling to get my power amps working. As you can see, I spent quite a lot of effort on the casework. Note: Pictures have already been scanned.
 

DESCRIPTION

First have a look at the schematic. It is fairly straightforward and parts are easily accessible to the amateur constructor. Here is a close-up and a description of the circuit board. This is my first major project and due to my lack of experience, I did a really bad job out of it. All parts are of the cheapest quality (sad to say) except for the casework and power supply. For a taste - All carbon resistors 5%; Mospec (Taiwanese) transistors and; the cheapest capacitors you could find. Bert Molenkamp, the designer of the amps says that the power supply is the most important. I took his advice and spent a large part of my budget on the power supply. It consists of a medium-sized transformer (Avel Linberg 330VA), and six 12,000uF Mallory capacitors per monobloc channel. I used 25A bridge rectifiers and a 7824 voltage regulator (for the input stage). If you are curious, here is a picture and a small description of the power supply. And here is a schematic of it. Wiring was done with 16A electrical wiring (solid core) throughout, more for aesthetic reasons than anything else.
 

The casework is my pride and joy. In my case, it is the one I spent most time on (drilling, mounting etc.). Each monobloc consists of four 0.8 C/W heatsinks. The base is super heavy-duty - it is machined from 6mm aluminum milled with adequate ventilation holes. The top panel is 3mm sheet aluminum. The rest of the casework is simply 1.6mm sheet aluminum chosen for easy workability. Although it doesn't look like a commercial amp, I'd say I did a fairly decent job out of it. After all, don't most (if not all) DIY amplifiers look like this?
 

PROBLEMS ENCOUNTERED

Constructing the amplifiers was no easy task, even for seasoned constructors. I would classify the circuitry as intermediate. But this was actually the part which took the least time. If you are not particular about quality or brand, searching for parts should not be too difficult, and it should be easily available anywhere. The major "find" for me were the heatsinks and the filter capacitors (Mallory). I had properly estimated the cost to be around US$600, but if you are really interested in building the amps, a stereo one should cost about US$600 total, but for my monobloc version, around US$1000. This figure is not too exaggerating, I assure you. As you read on, you will understand why. Soldering the board was pretty easy, just look out for orientation and short-circuits. When wiring the power supply to the amps, it is safety first.

Though simple enough for amateur constructors, proper wiring ensures that nothing goes wrong (or explodes!). All switches and sockets should be rated at least 10A. Although this is an overkill, it will go a long way to protect the user in case of a fault (common to most DIY amps). I cannot determine a proper value for the fuses, these should be the smallest value usable (for safety!).
 

TESTING

Due to the typical amateurish syndrome, I was extremely impatient to get the amps up and running. The minimum equipment you need are a good digital multimeter and proper safety rules. No, I do not have a variac, but I strongly urge you to get one if you do not already have one. OK, so I plugged in the amps. I did, however, follow the procedures given by Bert Molenkamp. The first time went well with good results (see PERFORMANCE) until thermal runaway occurred. This is the part I want to warn all fresh amateurs out there - use the smallest fuses possible, and ALWAYS use an output fuse! (e.g. 1A) A thermal switch is also a good idea. Impatience on my part led to blowing not only my output transistors, but also my precious Chario speaker! Worse still, the amps were plagued with problems thereafter. One channel is working perfectly right now, with class AB bias current of about 150mA. However, offset is pretty high (0.1v standby, 0.25v when in use). This causes audible distortions, especially on high musical peaks. I suspect this has more to do with parts tolerance than anything else. If anyone has a solution for this, please e-mail me.
 

PERFORMANCE

I would say I was really dumbstruck the first time I listened to the amps. I used Chario Hyper 1mk2 speakers (5 ohms nominal and not very easy to drive). I used only 400mA bias, probably only about 10 watts in class A, the rest in class AB. With a supply voltage of +/-36vdc, I probably got around 60 watts class AB into 8 ohms. You can change the power output to your needs by varying the supply voltage. Generally, the sound was clean but never dry. Particularly to note was the bass. Bert Molenkamp states his amps at a damping factor of 2000 at 100Hz for an 8 ohm resistive load. In my version, I would say around 500 (note the lousy parts used and the relatively small filter capacitors and transformer). This poses no problem, however. The amps grip the speakers with amazing control and the result is - music. Detail was good with adequate depth and width of soundstage. If you ask me, I am amazed how such lousy parts could put out such wonderful sound. In fact, it sounds similar to the YBA2. Those of you who are interested in building the amps should get good parts (e.g. Metal film resistors, Motorola transistors, and of particular note, the right capacitors). I can only guess how good the amps will sound with such parts. Also, since it is quite a rugged design, high bias currents can be used (up to 5 Amps). This makes the amps run in full class A, with even lower crossover distortion and better audible performance. However, the power supply and heatsinking should be adequate. The construction details can be obtained from Bert Molenkamp. I am, of course, willing to help anyone if you would e-mail me, but I am also a beginner, so don't ask me advanced questions.
 

Better still, construct the Bride of Zen from Pass Labs (see article).

ENDING NOTE

At this point, you may like to have the PCB traces and additional construction notes. But due to a lack of space, I am only making this available through e-mail. In short, this is a wonderful amp to have if built with patience and reasonable parts (not like me!). Think about it - if I can do it, so can you! I have also made a few additions to the amps to make them more stable. Due to limited space, I cannot put all data on my homepage. If you would like to know more, do e-mail me. I will be glad to reply.