Pass A75 Oscillations? (long)

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I am a newbie and this is my first amplifier.

I am in the final stages of building a Pass A75, and I have run into a little snag. I have supplied a signal from my variable headphone on my CD player, from a disc that I made with a 1KHz track. The out put array of transitors is not hooked up. The PCB is from AudioXpress, and is populated per the schematic except for the resistors at the gates which Have been increased from 100 to 220 Ohms. When I apply power to the amp with this 1KHz sine wave, I get a very dirty signal that is full of some oscillations. I tried to trouble shoot this by taking my oscilliscope and probing different areas, here are my results:

1: output=dirty sine wave
2: Regulated+, Regulated- = very dirty DC, which is clean when the amp is not connected.
3: input = also has become very diry.

This shows me that the ocillations are nearly everywhere in the circuit.

What I have tried so far:
1: Increased the gate resistance from 100 to 220 Ohms.
2: included 39pF at C9 and C10

This is moving in the right direction as I have been able to eliminate most of the oscillations. However I'm still stuck with an output that looks like a sawtooth at higher output and a square wave at lower output with oscillations at the peaks. The input seem to have these same artifacts.

Any help suggestions or ideas would be greatly appreaciated because I am really strugling

Greg, I built a variation of the A75 and also had some stability problems at first. I've exchanged email with another builder in the same boat, and we were both able to fix it by adding local power supply bypass capacitors (100 - 220 uF is traditional, and the parts should have a voltage rating greater than 50V and be connected with their + leads toward the most positive voltage) from (+V) to ground and from (-V) to ground, located on (or very close to) the front end board itself. You might try this *without* the two optional 39pF lag caps and see if it works for you.

Even this may not be fully effective if you run the second stage at full gain (no folded cascode). In that case, the optional lag capacitance still may be needed, though I can't be sure as neither my topology nor its physical layout were identical to the original A75. There are other ways of attacking the problem, but I'd start with this approach if I were using the original PCB.

Hang in there, this is an excellent sounding amplifier once it's up and running.

Disconnect the output stage and take the feedback from the gain stage. Track down and eliminate the problems with the frontend before attaching the output stage, this will save you a lot of time tracking the source of those pesky oscillations. Excessive lead length from the board to the outputs may also be a source of problems, requireing larger gate resistors for the output devices.
Good luck.

Joe & Jam Thanks for your reply,

Joe, I will give this a try when I get home tonight, and I will let you know what the results were. Thanks again for your suggestion.

Jam, I do not have the output stage connected. The oscillations seem to come from every where. With the ground clip on the probe connected to ground, not earth, everywhere I place the scope's probe seems to have some kind of oscillation. Thanks for your reply I'll let you know how it going.

Good news,

I placed 100uf at the positive and negative rails (as close as possible), and suprise the ocillations disappeared :)

I removed the lag compensation capacitors and returned the the gate resistors to 100 Ohms without any ocillations.

I guess I should have asked here a while ago, because I have been struggling with this for at least a couple of weeks.

Since I'm building this amp as a learning experience, does anyone have a clue why this work or what is happening to cause this?

Mr. Pass Thanks for your input, you were right I had already hooked up the ground to the output resistor/capacitor, removing that ground removed the RC constant fron the signal. Now I have a nice looking output trace on my scope.

Thanks everyone for your advice,

I am glad you got the amplifier working. Tracking down oscillation problems can be a pain.
Please let us know how the amplifier sounds and let us know the differences between folded cascode and regular operation.

We all owe a debt of gratitude to Mr.Pass for sharing his designs and ideas with us, and I for one can't wait to see what he will unveil next.

Care to comment Mr.Pass?


I agree, it is not too many hobbies that a profesional post designs, let alone gives technical support!

Thanks again Mr. Pass

I have read on a different forum Mr. Pass's comments about possible future work which may include a very low power, possible very pure amp ~1W. From the stance that most people listen to the first watt anyway so why not optimise that area. I guess well have to wait and see.

I will let you know what the end result is, and any difference I can determine betweeen the different operation I try. The only concern I have, is my low-fi ears, as I have never really listened to high quality equipment before and am unsure of the differences. I will extend the invitation to anyone in the Chicago area to listen to this amp at my house.

Greg, you asked why local power supply decoupling cured the oscillation. I believe that the oscillation in this case is caused by reinforcement of the amplfied signal by signal modulation of the power supply rails at certain frequencies. While the rails are regulated, the wires connecting the supply to the front end board introduce series inductance across which signal voltage can develop. Adding capacitance to ground at the front end board suppresses these modulations, so they no longer interact wtih the amplified signal, and the oscillation goes away.
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