Don Pearce wrote:


BTW; anyone familiar with the famous Crown DC300A ?

Both channels would emit a long squeal about 10 seconds after switch off
- it sounded much like a party balloon being let down while squeezing
the neck.

There was a note inserted in later service manuals that *no attempt*
should be made to fix the problem, as any possible fix would render the
amp unstable.




So they had an instability they couldn't fix, but sales took it off
the engineers and put it in the market anyway? Why am I not
surprised?


** The DC300A is perfectly stable with any load and all frequencies.

When the AC supply is removed, an internal +10V boost rail above the main positive rail quickly dies removing current from the class A driver stage - which pretty much disables the output stage.

When the DC rails to the input op-amp ( a uA739 ) drop below a working level, the squealing sound arrives. Its not particularly loud and does not depends on load conditions. The frequency is a few kHz, falling in pitch until there is almost no charge left in the main filter caps.

I know about it since I was asked to fix the noise by a customer who owned a DC300A. The solution was to fit a speaker relay with switch on delay and fast off.

I only mentioned this as an example of how trying to fix a small problem can lead to creating a much worse one. I suspect trying to fix inbuilt turn on/off transients in power amps are mostly like this.

IOW - " leave good enough alone ".



.... Phil