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From: len@syssoft.com (Len Galasso)
Newsgroups: sci.electronics
Subject: Power Amplifier Problem
Message-ID: <1990Sep24.212447.11648@syssoft.com>
Date: 24 Sep 90 21:24:47 GMT
Reply-To: len@syssoft.com (Len Galasso)
Distribution: usa
Organization: Systems & Software, Inc., Irvine, CA
Lines: 29

I need some assistance.  I have a PA designed by Yamaha which consists of
of a six-channel mixer and a stereo power amplifier.  One of the power amps
channels has a problem in that the output is very distorted as well as down
in terms of SPL.  The output stage is that of a quasi-complementary transistor
circuit using two 2N3055s for the final outputs.  I know the theory of this
design fairly well but in trouble shooting this puppy, I don't quite under-
stand what is happening.

The symptom is that when using a sinewave as a test signal, the output doesn't
want to drive a low-impedance load (<100 ohms).  I have removed and tested
the 2N3055s as well as the drivers and predriver.  Everything looks good in
static tests and the signal actually looks good into the pre-drivers.  But, 
when I decrease the load (going in stages--starting at 1K and working my
way down to 8 ohms), the finals can't cut the mustard.  The waveforms get
very distorted--asymmetrical and clipped.

The power supply is good, as is  the output coupling capacitor.  My question
is in essence, how important is output transistor matching in this type of
quasi-comp circuit?  Does it sound like a balancing problem between the
two finals or am I fighting some other problem, like a passive component
radically going out of tolerance.  Please give me some trouble-shooting
clues or tips.  More detail upon request.  My thanks in advance. 

+---------------------------------------+----------------------------------+
|  Len Galasso                          | "Watch the sun go brown          |
|  Systems and Software Inc.            |  smoking cobalt cigarettes..."   |
|  len@syssoft.com                      |    Steely Dan, King of the World |
|  (714) 833-1700                       |    "Countdown to Ecstacy"        |
+---------------------------------------+----------------------------------+