Hello,
I am hoping someone with experience on Crown PS-400 power amps could help me solve this problem.
I have a Crown PS-400 power amplifier with a very high frequency ultrasonic oscillation problem on only one channel (channel 2) that occurs with no load connected to the output or anything connected to the input.
I am using a variac to significantly lower the line voltage and keep amp draw below about .5 amp during these tests to protect the output stage. All tests were done with the Stereo/Mono switch in the Stereo position.
If I remove U1 (8-pin DIP dual op amp used for both channels at the input), the oscillation stops. If I short the inverting input (pin 2) to ground, the oscillation stops. If I short pin 2 and pin 3 (non-inverting input) together, the oscillation stops. This happens whether or not I open-circuit the global feedback loop.
I have checked almost everything and cannot find any defective parts. The source of the oscillation is pin 1 of U1 (channel 2 op amp output). If I put a 22pFd cap in parallel with C202 47pFd compensation cap for U1, the oscilation stops. If I remove this extra cap, the oscillation does not return unless I power down the amp and bring it up slowly again with the variac.
Is it possible that a 47pFd compensation cap is not big enough for this application?
If so, why is it that channel 1 does not oscillate when it uses the same size compensation cap?
Is there another explanation?
Thank you, Professor Bizzt
I am hoping someone with experience on Crown PS-400 power amps could help me solve this problem.
I have a Crown PS-400 power amplifier with a very high frequency ultrasonic oscillation problem on only one channel (channel 2) that occurs with no load connected to the output or anything connected to the input.
I am using a variac to significantly lower the line voltage and keep amp draw below about .5 amp during these tests to protect the output stage. All tests were done with the Stereo/Mono switch in the Stereo position.
If I remove U1 (8-pin DIP dual op amp used for both channels at the input), the oscillation stops. If I short the inverting input (pin 2) to ground, the oscillation stops. If I short pin 2 and pin 3 (non-inverting input) together, the oscillation stops. This happens whether or not I open-circuit the global feedback loop.
I have checked almost everything and cannot find any defective parts. The source of the oscillation is pin 1 of U1 (channel 2 op amp output). If I put a 22pFd cap in parallel with C202 47pFd compensation cap for U1, the oscilation stops. If I remove this extra cap, the oscillation does not return unless I power down the amp and bring it up slowly again with the variac.
Is it possible that a 47pFd compensation cap is not big enough for this application?
If so, why is it that channel 1 does not oscillate when it uses the same size compensation cap?
Is there another explanation?
Thank you, Professor Bizzt
Hello everyone,
The "22pFd" test cap that I was paralleling with C202 was not 22pFd, it was 221pFd. Sorry about my mistake here.
Best wishes.....
The "22pFd" test cap that I was paralleling with C202 was not 22pFd, it was 221pFd. Sorry about my mistake here.
Best wishes.....
Hello all,
I soldered a 200pFd cap across C202 and the oscillation stopped. I reassembled the amp, calibrated it, and power tested both channels with a 1kHz sine wave. I then had a look at the small-signal 10kHz square wave performance at the outputs.
Channel 1 produces a nice looking waveform with no overshoot and no ringing and exhibits very slight edge rounding. Channel 2 has a very small amount of overshoot, but is very quickly damped. So this amp still needs more work as far as I'm concerned.
Thanks...
I soldered a 200pFd cap across C202 and the oscillation stopped. I reassembled the amp, calibrated it, and power tested both channels with a 1kHz sine wave. I then had a look at the small-signal 10kHz square wave performance at the outputs.
Channel 1 produces a nice looking waveform with no overshoot and no ringing and exhibits very slight edge rounding. Channel 2 has a very small amount of overshoot, but is very quickly damped. So this amp still needs more work as far as I'm concerned.
Thanks...
Crown amps behave differently when powered up slowly than when switched on to full voltage! I have seen it a couple of times. When under warranty Crown service just changes the entire PC card!
I would check the output loading RC network. Parts may have changed value, if not just try a bigger cap or less resistance.
I would check the output loading RC network. Parts may have changed value, if not just try a bigger cap or less resistance.
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