Hey Matthew, I saw that exact same problem in my friend's P3A, one channel worked fine and the other channel drew 200mA all the time. It turned out nothing was really wrong and there was a bad solder joint somewhere. However, it was pretty clear the problem was in the CFP output stage. You can verify the Vbe multiplier is working by checking that the base voltages of the output followers are near ground (1V and -1V, approximately). And of course check the input stage current but it better not be that high. 
See, this is where my ignorance kicks in, what's an output follower? Someone else stuffed and soldered these boards for the most part, and has claimed that they work fine. I don't mind checking voltages, but beyond telling me ie between r9 and r10 or something like that, i'm lost. Sorry
-Matthew K. Olson
-Matthew K. Olson
Kilentra, i checked Q5 and Q6 base voltages on both left and right channels. The left channel (the one that works) is about .7V+- the right channel is about -.6v and +.3v does that sound like something's wrong? I'd just like to know where i need to focus, especially if i'm going to be takin the board off the heatsink. Later!
-Matthew K. Olson
-Matthew K. Olson
http://www.instantboard.com/users/rode/viewtopic.php?t=762&highlight=p3a+bias
Almost the exact same problem here, only mine is reverse in polarity, i get -.6v and +.3 instead of the other guy's problem which was .6v and -.4 ????
-Matthew K. Olson
Almost the exact same problem here, only mine is reverse in polarity, i get -.6v and +.3 instead of the other guy's problem which was .6v and -.4 ????
-Matthew K. Olson
Haha, that post is about my friend's amplifier. Anyway, as you can see in the diagrams posted on that page, something in the circuit of Q5/Q7/R11 or Q6/Q8/R12 (those are the CFP followers) is messed up. Though it is really strange that the voltage across the bases of those is negative. Definitely check that Q9's in correctly and its base-collector resistance is 1k and base-emitter is 2k. And also, see if all the other voltages line up similarly to the working channel, or the first diagram in that other post.
Ok, i reheated the solder joints...no help. I checked the 1k and 2k etc....thats fine. Turns out my problem is slightly different from your friend's problem. The voltage at r9 is sorta ok. its about 600-700 mV. The voltage at r16 is the problem. I measured on one side of r16... .35mV and on the opposite side, .08mV BUT...heres the catch, r16 itself measures fine. On the good board, r16 measures 600-700 mV on one side and .22mV on the other.
Also, between r9 and r10 on both boards measure fine...14.7V which is fine because i'm running 29v rails. Now i'm really stumped. If i thought it was r16...i'd replace it. No problem. But thats not it, the pot is fine apparently, etc. ??????????
-Matthew K. Olson
Also, between r9 and r10 on both boards measure fine...14.7V which is fine because i'm running 29v rails. Now i'm really stumped. If i thought it was r16...i'd replace it. No problem. But thats not it, the pot is fine apparently, etc. ??????????
-Matthew K. Olson
Ok, upon further inspection of the schematic, i figured Q4 was a more likely culprit. Instead of replacing everything, i just replaced Q4 to start. Sure enough, i replaced it...everything's fine now. I still don't know if the thing plays music, but it biases just fine. Thanks guys!
-Matthew K. Olson
-Matthew K. Olson
Well, quick update. The bias troubles came back. Turns out the amplifiers was oscillating. After much trouble and many emails it boiled down to the input capacitors which I had mounted off board right at the RCA's. One side of the amp was fine, which is weird, but the other side oscillated. So, after shorting the inputs, oscillations immediately went away. After disconnecting the inputs from the input caps, the oscillations also went away. Strange. Tried cheap intput caps....works fine. No more oscillations. I guess the input cap was attracting some stray EMI or whatever. So I changed out both input caps to keep things even. All is well now, and it plays music just fine Long story short, if there is bias trouble, FIRST short the inputs and see if it goes away, then unplug the inputs all together (w/o shorting them) and see if the bias trouble still exists. When in doubt, try different input caps Much easier than swapping all the transistors out 
anyway, onward and upward. Time to enjoy my p3a
-Matthew K. Olson
Long story short, if there is bias trouble, FIRST short the inputs and see if it goes away, then unplug the inputs all together (w/o shorting them) and see if the bias trouble still exists. When in doubt, try different input caps Much easier than swapping all the transistors out anyway, onward and upward. Time to enjoy my p3a -Matthew K. Olson
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