"Jim Lesurf"
Eeyore

Aside from incompetently designed amplifiers that oscillated with low
inductance cables (was it Naim ?)

More precisely, the problem was that the amp(s) tended to oscillate with a
capacitative load of a given order.
It is easy enough to avoid the problem by a mix of sensible amp design and
the simple use of a series inductor. But the Naim amp(s) in question did
oscillate into some capacitative loads. So simply using some cables caused
problems.

The amp(s) would be OK if the total load did not look like a suitable
capacitance at HF. The simple solution would be to fit output inductors if
you want the rest of the amp design to stay the same. But that might look
like you'd not made the amp stable in the first place perhaps didn't know
what you were doing... :-) So the 'solution' was to tell people that
'high inductance' cables of at least a given minimum length were needed
and
that this 'sounded better'. ;-


** A film cap of approximately 0.01 to 0.1 uF connected *directly* across
the speaker terminals will induce parasitic or even continuous HF
oscillation ( typically in the MHz range) in many amps ( valve and SS )
which are not designed to be "unconditionally stable". In some cases, the HF
oscillation is severe enough to cause overheating of the output devices and
eventual failure of the amp if allowed to continue.

However, connecting such a cap direct to the speaker terminals is a crazy
thing to do to an audio amp and falls well outside normal operating
conditions - so many amp makers did ( and still do) not consider it
important to make their products immune from this kind of abuse.

The arrival of high capacitance / low inductance woven speaker cables ( like
Tocord ) caused a few amp makers to issue warnings against their use -
notably Naim and Phase Linear.

A great many other less pretensions makes and models of amp would also burst
into HF oscillation when such cables were used - a rather serious problem
that resulted in many hi-fi dealers removing the cable from sale as they
could never be sure if the customer's amp could tolerate the high effective
capacitance.

The fix is ridiculously simple for anyone technically minded, just add a
zobel network (say 0.022uF and 10 ohms in series ) across the actual
loudspeaker's terminals - ie at the far end of the cable.

This loads the cable ( really a 8 ohm transmission line) with close to its
characteristic impedance at HF and hence neutralises the parallel
capacitance.



........ Phil