Jim Lesurf wrote:
So here's the thing. For the consumer, knowing the
impedance of a cable
is not important,
I'd say "*should* not be important", but with a concern
that a cable should
not have an unusually high series impedance or shunt
capacitance. Most will
be fine, though.
although it is clear from Jim's work that you are less
likely to run
into trouble with some than others. But the article
should be a wake-up
call to anyone in the design business that a blinkered,
audio-limited
view can be a recipe for a very poor amplifier, whatever
the
measurements may say.
That is one of the concerns I have. I'd be happier if
reviews checked
stability, or at least got makers to state if their
product was
unconditionally stable. I think it likely most (and
probably almost all)
are. But what about the off-the-wall designs with high
output impedance,
etc? Are they all fine? Who knows? I certainly don't.
The other is that in some cases people may be paying for
cables that, say,
have a high enough series impedance to audibly alter the
sound, and then
presuming this was because it was a very expensive cable
made of
unobtainium - but where a cheap resistor and/or inductor
would have had the
same result. Ditto for a cable with high shunt capacitance
used with a
valve amp that has an output impedance above an Ohm. Yes,
some do have
such high output impedances. Is it a reflection of high
impedance inside
their feedback loop (inc transformer)? I don't know. Who
does?
Menno, probably.
But who does know what? Is what a reflection, the ditto or
the high impedance?
Whatever, I strongly recommend Menno Van der Veen's "Modern
High-End Valve Amplifiers" for a state-of-the-art analysis
of valve amp output stages, and transformers in particular,
including feedback considerations.
My view (but I'm about as popular on r.a.tubes as I am here)
is that the crucial story of modern valve amps has been
centred on the problem of driving modern speakers with
sufficient full-power bandwidth to accomodate modern
bass-heavy music.
Proper loading of the output valves (being a compromise
between efficiency and distortion) largely determines the
winding ratio, and hence the open-loop mid-band output
impedance. Output impedance of the closed-loop circuit then
depends on how much feedback is used, which in turn is
limited ultimately by the quality of the transformer.
Transformer quality can be expressed as a ratio of upper and
lower bandwidth limits. The former is enforced by the
winding capacitance in shunt, and the leakage current in
series with the output, transformed appropriately depending
on whether you're looking into the secondary or the primary.
The latter hinges on the primary inductance. Both influences
are scaled by the impedance ratio, which is the square of
the winding ratio. Considering that the inductance and
capacitance both tend to rise with the number of turns, you
can see perhaps how bandwidth (rather than top or bottom
limit which in isolation are both easy) ends up depending on
arcane details of winding patterns and core shapes, mostly
led by efforts to reduce the ratio of primary to leakage
inductance. Ultimately, all other things optimised and
equal, quality and thus bandwidth depends on winding ratio.
Hence the fascination for archetectures that reduce that
parameter: various ways of arranging output valves
effectively in parallel. Old-style valve amps expect the
kind of speakers and cables prevalent in their day, and
quite right too, IMO. Those who insist on matching old and
new epochs, especially on the cheap, deserve their narrow
margins and gross distortions. Fancy cables would be like
fitting Ohlins suspension units to a Lada, when what you
really need is a Ford.
It may be worth stressing in the context of this thread that
the OPT places capacitance across the output, and leakage
inductance in series (in addition to the comparitively very
large inductance in shunt). Also that the global feedback in
a modern valve amp is taken directly from the output, that
is *after* the series inductance, which may be a crucial
difference compared to SS amps?
All this rather a buckshot approach, coz I don't really know
what your question was.
Anyway, everyone should read Menno. His derivation of the
complete transfer function of a typical valve amp output
stage, including transformer, might help to answer your
question, although it is likely that, at frequencies above a
few MHz, some parasitics he safely ignores may become much
more significant.
Ian
I'm happy enough just to get people thinking about this
rather than leaving
it in the dark corner its been left in out of sight of the
public for
years. If that leads to people finding that all the
current/recent
commercial amps are fine, that is an excellent outcome
from my POV. If it
gives some designers a 'heads up' that also suits me fine.
Back, say, in the late 70s or early 80s it was routine to
check stability.
Since then it seems to have slipped though being taken for
granted into
neglect. Has it been forgotten by some? From the reactions
I've had here I
have the uneasy feeling that, yes it has by at least some
people.
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
New webpage on loudspeaker cables
Linear Mode
