"Eiron" wrote in message
...

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


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

David.

On Tue, 11 Aug 2009 04:16:00 +0100, Ian Iveson wrote:

snip

All this rather a buckshot approach, coz I don't really know what your
question was.

snip


LOL!

Nice one Ian... :-D

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

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?

Now, whether 1nF is enough to make any difference at all I'm not
experienced enough to know. If Jim says:
"if you just put a shunt capacitor across the output that might provoke
oscillations if the amp isn't already stable for other reasons" then
that's good enough for me. :-)

/me has been a good boy and read more of Jim's pages... :-)

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

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

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

"mick" wrote in message
...
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?

Now, whether 1nF is enough to make any difference at all I'm not
experienced enough to know. If Jim says:
"if you just put a shunt capacitor across the output that might provoke
oscillations if the amp isn't already stable for other reasons" then
that's good enough for me. :-)

If you are right in your analysis of Eiron's point (and it sems likely) then
clearly he has little experience of this sort of thing. Yes indeed, a 1nF
capacitor connected directly across the output of an amplifier can easily
cause instability if that amplifier is not unconditionally stable.

David.

In article , David Looser
wrote:
"mick" wrote in message

Now, whether 1nF is enough to make any difference at all I'm not
experienced enough to know. If Jim says: "if you just put a shunt
capacitor across the output that might provoke oscillations if the amp
isn't already stable for other reasons" then that's good enough for
me. :-)

If you are right in your analysis of Eiron's point (and it sems likely)
then clearly he has little experience of this sort of thing. Yes
indeed, a 1nF capacitor connected directly across the output of an
amplifier can easily cause instability if that amplifier is not
unconditionally stable.

Yes. I used to keep a small selection of capacitors with values ranging
from a few tens of pF up to a couple of microF to do quick checks on
amplifier stability, etc, when experimenting. I often found that values of
the order of an nF or so would cause bursts of oscillation with designs
being played with... erm developed. I also had a variety of inductors and
'evil' loads to torture the amp and see if I could annoy it. Usually ending
with a screwdriver to short the output terminals when the amp was playing
to see it if survived. 8-]

Perhaps worth adding that quite often there was a narrow range of
capacitance values that might upset a design. So, say, 2n2F might cause
oscillations, but 1nF or 4n7F didn't. So the problem often isn't that 'any
value above X' causes oscillations. Hence you may well find that an
amplifier that isn't unconditionally stable will work happily into loads
which have a high capacitance, but then burst into song when used with a
smaller load capacitance. So this is rather a matter of chance for users if
the amp isn't unconditionally stable. The possible range of loads being
presented by all the cable-speaker combinations out there is quite wide!

....oh, and just to add to that something I've just recalled. I also at
times used to leave an amplifier design in the fridge overnight and then
test it before it had a chance to warm up. Quite interesting how often a
design would oscillate when cold, but then stop when warmer... or vice
versa.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

"David Loser is a LIAR "

Yes indeed, a 1nF capacitor connected directly across the output of an
amplifier can easily cause instability if that amplifier is not
unconditionally stable.


** However, the TRUTH of the matter is that the likelihood of it
ACTUALLY happening with any commercially made hi-fi amplifier is close to
ZERO !!!!

David Loser is nothing but a lying, ****ing, clueless IDIOT.



..... Phil

On Wed, 12 Aug 2009 19:49:45 +1000, "Phil Allison"
wrote:


"David Loser is a LIAR "

Yes indeed, a 1nF capacitor connected directly across the output of an
amplifier can easily cause instability if that amplifier is not
unconditionally stable.


** However, the TRUTH of the matter is that the likelihood of it
ACTUALLY happening with any commercially made hi-fi amplifier is close to
ZERO !!!!


Absolutely. The idea that anyone might ever disconnect a cable from a
speaker is totally unthinkable.

Here's how it goes. You use spade terminals on the speaker cable. The
cables are heavy duty, with plenty of spring in them. Even with the
screw terminal wound up tight, it is really easy for the cable to
exert enough sideways pull to slacken it. The natural spring of the
cable then flicks it clear of the binding post. I've seen it happen
more than once.

d

On Wed, 12 Aug 2009 09:56:54 +0100, Jim Lesurf
wrote:

In article , David Looser
wrote:
"mick" wrote in message

Now, whether 1nF is enough to make any difference at all I'm not
experienced enough to know. If Jim says: "if you just put a shunt
capacitor across the output that might provoke oscillations if the amp
isn't already stable for other reasons" then that's good enough for
me. :-)

If you are right in your analysis of Eiron's point (and it sems likely)
then clearly he has little experience of this sort of thing. Yes
indeed, a 1nF capacitor connected directly across the output of an
amplifier can easily cause instability if that amplifier is not
unconditionally stable.

Yes. I used to keep a small selection of capacitors with values ranging
from a few tens of pF up to a couple of microF to do quick checks on
amplifier stability, etc, when experimenting. I often found that values of
the order of an nF or so would cause bursts of oscillation with designs
being played with... erm developed. I also had a variety of inductors and
'evil' loads to torture the amp and see if I could annoy it. Usually ending
with a screwdriver to short the output terminals when the amp was playing
to see it if survived. 8-]

Perhaps worth adding that quite often there was a narrow range of
capacitance values that might upset a design. So, say, 2n2F might cause
oscillations, but 1nF or 4n7F didn't. So the problem often isn't that 'any
value above X' causes oscillations. Hence you may well find that an
amplifier that isn't unconditionally stable will work happily into loads
which have a high capacitance, but then burst into song when used with a
smaller load capacitance. So this is rather a matter of chance for users if
the amp isn't unconditionally stable. The possible range of loads being
presented by all the cable-speaker combinations out there is quite wide!

...oh, and just to add to that something I've just recalled. I also at
times used to leave an amplifier design in the fridge overnight and then
test it before it had a chance to warm up. Quite interesting how often a
design would oscillate when cold, but then stop when warmer... or vice
versa.

Slainte,

Jim

I think capacitor resonance is important here. If an amp is
potentially unstable (usually with a capacitive load) at say 30MHz, a
smallish cap may provoke it. A big one, though, probably resonates
below that frequency, so at 30MHz it isn't a cap, but an inductor. The
amp will be quite happy with that.

This sort of brings up another point. It is easy enough to design an
amp that is unconditionally stable - the maths isn't hard. But this
makes some assumptions, one the big ones being that Cdom (usually
about 100pF) has the right phase shift. If the layout isn't great,
though, it is quite easy to end up with perhaps 100nH of parasitic
inductance in series with it. If that happens, Cdom turns into an
inductor at 50MHz, which may well be low enough to start things
fizzing.

So it is not enough to design an amp which is theoretically
unconditionally stable, a parasitic analysis must be made to ensure
that this also applies to the reality of the physical design.

d

" Dung Pearce is a Dead **** "


"David Loser is a LIAR "

Yes indeed, a 1nF capacitor connected directly across the output of an
amplifier can easily cause instability if that amplifier is not
unconditionally stable.


** However, the TRUTH of the matter is that the likelihood of it
ACTUALLY happening with any commercially made hi-fi amplifier is close
to
ZERO !!!!


Absolutely. The idea that anyone might ever disconnect a cable from a
speaker is totally unthinkable.


** What mind ****ing ILLEGAL drugs are you injecting into your face now ???

YOU LYING POMMY ****WIT !!





..... Phil