Don Pearce wrote:

Phil Allison


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 ".



I certainly agree that there is very little point for the end user to
try and fix this. But it is a design flaw and should have been fixed
by the original engineers.


** It was not a problem that needed fixing.

The DC300A was marketed as a laboratory & professional use amplifier - not a home audio one.


Speaker relay? Yes, that will stop you hearing it, but it doesn't fix
the problem.


** Speaker muting relays have been standard features in countless home and professional audio amplifiers for decades. Their main purpose is to stop transients at switch on/off from being heard.

The last Onkyo 7.2 channel receiver I serviced had 9 chunky relays, one for each power amp, one in the AC supply and one in the main AC secondary for standby.

They are very much the solution to the problem.



..... Phil