"Phil Allison" wrote in message
...

"William Sommer******"

"Nutcase Kook "

If they are liable to oscillate (rather than howlround), why is it
always
ultrasonic about 50KHz, not 20 to 30KHz or 80-100KHz?

Always?

Are you familiar with the use of negative feedback, and how an amplifier
needs to be designed to be stable?

** Got absolutely nothing to do with an amplifier's inherent stability
with
NFB.

The Kook is talking about audio power amplifiers oscillating because of
accidental POSITIVE feedback - ie capacitive coupling from the output
to
a non-inverting input.

Each case if different, but something like a 50 to 100pF cap from the
speaker out to the non-inverting input will send almost ANY tube or
transistor power amp into full power oscillation. ( A direct connection
typically turns the same amp into a low frequency ( 1 to 5 Hz) square wave
oscillator. )

The actual frequency of oscillation is determined by many things, but is
typically about 50 kHz ( with stray coupling) because of the combination
of
input and output low pass filters, the amps slew rate and the available
gain. While not a common event these days, I still see occasionally power
amps damaged by HF oscillations. Bad wiring practices, like unshielded or
unbalanced input cables running alongside speaker cables are the usual
culprits.

Also, it is very easy to make a guitar amp oscillate at a high audio
frequency - just put the guitar near the speakers and turn the gain and
treble controls up. Capacitive coupling from the frame of the speakers
injects HF signals straight into the pickups and any unshielded wiring in
the guitar.

Some "combo" amps have the speaker frames connected to the amp's chassis
to
reduce this effect - though with tube amps it rarely damages anything.


... Phil