Ouch! Full-power ultrasonic oscillation! I guess that pretty much rules out
all of my suggestions.

Of course, the fact that it oscillates on both channels strongly suggests
something common to both channels. I wouldn't completely rule out the
possibility that something is wrong with the B+ switching circuit.

Maybe you should convert the amp to power an ultrasonic cleaner!


"Tim Schwartz" wrote in message
...
On 1/4/2011 9:35 AM, William Sommerwerck wrote:

Both channels or just one channel?
All speakers, or just one particular model?
Does it occur with a purely resistive load? Does it occur when the amp is
unloaded?
Are you using oddball speaker cables? (I've seen pathological
interaction.)

Here's a thought... Could it be the amplifier rapidly switching between
the
high and low rails? Have you worked through the circuitry that "decides"
when to make the switch? If the oscillation were at 2x or 4x the input
frequency, that would strong evidence.


Hello,

I've only run the receiver on test equipment. Sine wave in, 8 ohm
non-inductive load (Dale NH250 resistors) and I'm looking at it on a
scope. Both channels, both outputs. The test set up is the one I
always use, and other amps do not have this issue. I have tried one
channel at a time with the same result. Speaker cables are 16 gauge
'zip cord'.

I've never left it up for more than a moment, because the current draw
is 8+ amps on the 120V line. It's a very high frequency oscillation,
but I can't really see the waveform to describe it, as I don't want to
blow up the amp. All semiconductors are original.

Thanks for your input,
Tim Schwartz
Bristol Electronics