In article 4a7eb1fc.387537093@localhost, Don Pearce
wrote:
On Sat, 08 Aug 2009 10:17:24 +0100, Jim Lesurf
wrote:

It is all too easy to make an amplifier that looks OK on a test bench
connected directly to a test load - then find it bursts into
oscillation, or its other properties alter - when given some other
load. I've also seen this happen when someone was using an oscilloscope
that didn't reach the oscillation frequency. So the audio waveform
became distorted, but with no visible sign of the RF bursts until they
tried a faster scope.

There is a general rule in design that everything will oscillate. The
only consistent exception to this rule occurs when designing an
oscillator.

Yes. That is one of the maxims I explain to undergrads when teaching about
feedback. The distinction between having built an amplifier and an
oscillator is that you want one of them *not* to oscillate. :-)

The problem for designers of commercial audio power amps is that they have
no idea what actual loads will be connected.

Slainte,

Jim

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