Re: New webpage on loudspeaker cables

In article <4a8398e0.708789000@localhost>, Don Pearce <spam@xxxxxxxx>
wrote:
On Wed, 12 Aug 2009 09:56:54 +0100, Jim Lesurf <noise@xxxxxxxxxxxxxxx>
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/Scots_Guide/intro/electron.htm
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.

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