"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.

"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

" 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

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

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

In article 4a8398e0.708789000@localhost, Don Pearce
wrote:
On Wed, 12 Aug 2009 09:56:54 +0100, Jim Lesurf
wrote:


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.

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.


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.

Yes, that is one of the possible Reasons. Another is that it may not always
be the case that a larger capacitance may drag the HF gain too low for the
oscillation condition to be satisfied at HF, but not change the phase at
the point where the loop gain is unity enough to meet the condition.

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.

TBH I always tended to avoid the simple Cdom approach which causes people
to slug the design near the output with a large cap or two. I ended up
preferring using a resistor and cap in series in an earlier stage to drop
the HF gain to a resistor-defined level. Avoids adding to the phase delays
at HF and tends to swamp cap resonance. Means you can also use smaller
value caps that have higher self-resonance frequencies so shift that
problem to where it doesn't matter. I guess in some ways a bit like the
tweaks Bailey did for Radford at one time, but which were missed when
people focused excitement on him using a triode-pentode splitter. :-)

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.

Again, yes. I also found that designs which use current sources, mirrors,
and long-tail pairs had improved stability as they tend to reject parasitic
paths via the power lines. Helps to keep down noise, distortion, and
crosstalk, and makes the operation more independent of the actual rail
voltages as well.

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 Looser wrote:
"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.

But that's not what you wrote, is it?

--
Eiron.

"Eiron" wrote in message
...


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

But that's not what you wrote, is it?

What I actually wrote was:

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

OK I didn't qualify that with the caveat about it not applying to
unconditionally stable amplifiers, otherwise I don't see the difference.
Don't forget this was in the context of the someone suggesting that
connecting a 1nF capacitor across the output would get rid of any RF (from
instability). I was just pointing out that adding such a capacitor could
provoke, rather than, remove, instability.

In any case if your objection to my earlier post was that I hadn't added a
caveat about it not applying to unconditionally stable amplifiers why didn't
you say so?, rather than posting some stuff about the capacitance of certain
cables?

Or would I be correct in assuming that you are simply casting around for
something, anything, to disagree with me about?

David.

--
Eiron.