On Mon, 10 Aug 2009 16:49:38 +0100, David Looser wrote:

"mick" wrote

Fair enough, but surely the only amps that are likely to have much
output (i.e. enough to drive the cable - never mind the speaker) above
1MHz or so are likely to have severe problems anyway (such as
overheating) aren't they? Ok, maybe not if the oscillations are
triggered on audio peaks I suppose, but how would that be audible?

Because it causes distortions *within* the audio band.


Don just beat you with a more detailed explanation. Cheers anyway!


snip

Arggh!... Plonking a 1nF capacitor across the output of a feedback
amplifier is a pretty likely way of making it unstable


:-)
I always knew there was a reason for zero-feedback amps... ;-)



I'm not convinced that knowing the RF cable properties tells you
anything at all about the audio performance.

Nor am I. AFAIAC the whole thing is about stability, and how likely the
cable is to provoke oscillation in amplifiers with marginal stability.

There shouldn't be any RF present

Ideally there wouldn't. But what makes you think that there isn't?. With
active devices having good gain up into the hundreds of megahertz what
makes you think there isn't noise, RF pickup and self-oscillation well
above the audio band present?



A good point. It does make it rather difficult to design unconditionally
stable audio amps if your test gear can't reach those exotic heights!

Eee... when I were a lad 30MHz were *proper* RF!

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
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