len@syssoft.com (Len Galasso) (09/25/90)
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" | +---------------------------------------+----------------------------------+
jws@thumper.mlb.semi.harris.com (James W. Swonger) (10/03/90)
You might have fried the output devices, and what you get at the output is the base drive. Maybe you fried one and not the other (asymmetry). Also look for limiting resistors in the emitter legs. These are often weak links.