In article , mick
wrote:
On Tue, 11 Aug 2009 07:57:10 +0100, David Looser wrote:

"Eiron" wrote in message
...

I wasn't criticizing Jim. I was criticizing you.


Then you are being remarkably obscure. Criticising me for what?



I think his point is that 0.001uF / 1000pF (a figure that I, admittedly,
plucked from the air) across the output is less than some cable loads.
To quote you: "Arggh!... Plonking a 1nF capacitor across the output of
a feedback amplifier is a pretty likely way of making it unstable"

As Eiron points out, that's about 12m of Chord Silver Screen or a foot
of Townshend Isolda cable, which doesn't sound much does it?

Well, to me 12m seems quite long for domestic loudspeaker cables in a UK
context, although maybe not for the USA or elsewhere. And as pointed out in
my article the Isolda cables come with series inductors.

The distinction is that with a cable, although the nominal shunt
capacitance increases with length, so does the series inductance. So it
isn't quite the same thing as shoving a capacitor across the output
terminals. But I'd agree that you may well have to take care if the amp
isn't unconditionally stable and/or has high output impedance. The problem
is the usual one, that 'reviews' often don't tell you what you'd need to
know, or explain how to use the info to help you assess the situation. But
then I suspect that at least some reviewers have no idea about this
themselves, either. :-)

BTW If we accept the values published in 'Hi Fi Plus' in their current
issue, then the "Cardas Clear speaker cable" has a shunt inductance of
278pF/foot. So a run of about 2m of that would get you to well over 1nF.
The good news is that two 2m lengths cost a mere 3,550 UKP, so I assume
anyone who buys that can also afford an amplifier they can ensure *is*
unconditionally stable. ahem If not, use with the cable may let them
know... :-)

That said, as you might expect, the above capacitance is associated with
low series inductance ( 0.0176 micro Henry / foot ) which is nice if your
amp is happy with the capacitance.

Odd that they publish values for the cables in the Cardas 'review' sic
but not - so far as I can see - for the one of 'MAD' cables later in the
issue. I presume this means they are just quoting values given to them by
the maker and don't bother to check any for themselves.

Slainte,

Jim

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