On Jul 12, 1:10*am, "N_Cook" wrote:
If they are liable to oscillate (rather than howlround) *why is it always
ultrasonic about 50KHz, not 20 to 30KHz or 80-100KHz ?

Plot the poles. An amplifier is a low pass filter -- figure the
rolloff occurs well outside the audio range. Then look where the phase
shift hits 180 degrees.