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Old 16th March 2004, 05:11 AM   #1
PetarLD is offline PetarLD  Bulgaria
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Default P3a gain problem

Hello, I recently build 2 P3As. After extensive listening I noticed one of my speakers was driven harder than the other. So I got a singnal generator and put 60Hz frenquency through. And I measured the output AC voltage of both amps, one had 1V and the other 2.78V I didn't want to turn the volume up because I couldn't stand it as it was, I did switch the speakers around and it wasn't from them but from the amps. I have measured all of the voltages on both circuits and all the voltages I measured were absolutely the same.

I have used the same parts in both amps, except C4, C6, and C3. C4 and C6 are 100pF in both amps except in one they are rated at 50v and in the other at 100v and they are both ceramic. As for C3 in the amp with the "higher" gain I have used a 100uF rated at 50V and in the one with the "lower" gain I have used a 25V rated film capacitor. Both amps are running from separate trafs, which both have ~25V rails. I have measured them both and the rails are exactly the same. The bias is set to 50mV on both, the pot is set to 1.67KOhms. So as I said I have tripple checked both amps and compared all the parts which are the same with the exception of those 3 caps. I have checked all of the connections on both amps and everything. I am not using Rod's PCBs by the way. I have spend well over 2 days trying to figure out what the problem was and I can't seem to figure it out. I am still a newb since I have been doing this for around a month now, and I am only 17 years old. Any help is greatly appreciated. Thanks!
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Old 16th March 2004, 08:23 AM   #2
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Do you have a volume control pot before the amp? If you have used a logarithmic pot in this position it is easy to wire it the wrong way round. If you have wired left and right differently then you would end up with different voltages at the input to each amp.

Also double check that the resistors in the feedback network are indeed idential and also check the ones connected to the input of the amp.
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Old 20th March 2004, 04:10 AM   #3
PetarLD is offline PetarLD  Bulgaria
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Hello, I figured out what the problem was after reading a lot about amplifiers. Now I am pretty sure I know what every component does. So the problem was the feedback capacitro C3. The print on it said that it was 100uF at 50V, but when I measured it was showing only about 68uF, so I replaced it and now both amps have pretty much the same gain. I am really pleased with the result.
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