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Help! My PPA isn't working properly

post #1 of 7
10/22/04 at 8:13pm
Thread Starter 
From time to time these days I had a feeling my PPA had something strange. I wasn't able to realize what, so I though it was suggestion (returnig to 8610's after months with 627's).
Today I decided to check my PPA with RMAA to have a performance benchmark for the other headamp I want to mod.
The results were not very good and looking closer at graphs I realized that the left channel was performing really bad.
I mesured the left channel's offset and is huge: 150mv. I was puzzled because I mesured all the channels' offset some week ago (when I swapped 627's for 8610's) and it was everything OK.
I connected my V6 (that are more revealing than the hd580 I wear usually) and a clear hiss is audible on the left channel.
I swapped opamps and buffer all around the channels but I ever get the same problem from the left one.
Strange things: while G and R op-amps are running at +/- 13 volts, the L rails are at +/- 12.1. When I biased the op-am in class A I chose the resitors to get exactly 1.1 mA in each channel but now through R9L I read 0.95.
What have I to do?
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post #2 of 7
10/23/04 at 1:09am
Remove the cascode input resistor. If that helps, the cascode JFETs are probably dead.

Temporarily jumper across the Q3s. If that helps, those JFETs are probably dead.

And, don't plug headphones into an amp with known DC offset problems! You can damage them.
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post #3 of 7
10/23/04 at 10:48am
Thread Starter 
Thank you Tangent.
It's the cascode. No class A 'til I get new jfets. Through RMAA I see a small channel unbalance between L and R (less than 1 db): I have to better resitor's matching.
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post #4 of 7
10/24/04 at 2:35pm
Quote:
Through RMAA I see a small channel unbalance between L and R (less than 1 db
Almost certainly it's due to your pot. Anyway, even trained ears can't discern sounds less than 1dB apart.
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post #5 of 7
10/24/04 at 8:53pm
Thread Starter 
You are right. It's just paranoia.
If I can trust RMAA the umbalance is around 0,3-0,4 db.
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post #6 of 7
10/24/04 at 9:45pm
If you can hear a 0.3 decibel difference, then I would just hire live musicians to stay in your living room instead of bothering with records and amps and headphones
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post #7 of 7
10/25/04 at 8:44am
Thread Starter 
Actually I can't hear any umbalance so I can start listening to records again and enjoing but: do I have to tell Bruce Springsteen that he can stop playing guitar and singing in my bathroom while I take shower in the morning?
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