In article 4a7cf91f.274676328@localhost, Don Pearce
wrote:
On Fri, 07 Aug 2009 10:40:08 +0100, Jim Lesurf
wrote:
1. If you open circuit the cable at the loudspeaker end, it is better
if the cable is somewhat lossy, as this will prevent the quarter wave
Mod Z dropping to too low (potentially damaging) a value.
In general, cable losses will reduce the amplitude in the presented
impdance changes with frequency in the RF region. The snag being that
if too large, they could also upset performance in the audio region.
I can think of certain brand amplifiers which would sing a loud and
shrill song to such a load.
Would you care to Naim one? :-)
IIUC one aspect of DNM's argument is that his cables do include RF
loss. (My interpretation of what he says is this is due to internal
impedance as the cable uses solid core wire. Thus introducing a loss at
RF.)
I can't see that impedance per se introduces loss. Maybe he meant
resistance. Do you suppose he was talking solely about skin effect, or
does he use a particularly lossy dielectric?
It is difficult to be sure as I've had to 'interpret' his descriptions for
myself. But I think his argument is essentially a mix of things which
include.
A) The cable series inductance and the cable having a high characteristic
impedance. This tends to shove up all the peaks and dips.
b) The internal impedance ('skin effect') adding in higher series
resistance at RF than at audio. So damping the peaks and dips at RF without
affecting audio so much.
FWIW what I did find interesting here is that the mulltistranded cables
also showed signs of internal impedance pushing up the series resistance at
RF. So faith that multistrands suppress internal imedance effects may be
unfounded for - I assume - the simple reason that, usually, there is no
insulation of the individual strands. So the bundles act like a single core
with a rough boundary.
[snip]
Ok, I can see that. The inductor is taking the place of the one that
should really already be inside the amplifier. And of course being low
loss and low Z, this cable will invert the high Z speaker resonances
very nicely into an extremely low Z dip. If only speakers were
resistive, we could avoid all this - sigh.
Yes. Hence my reference back to the 'SCAMP' article. Loudspeaker designers
have a lot to answer for, but rhwy usually chuck the problems onto the lawn
of the amp designers and pretend it is nothing to do with them! 8-]
FWIW I'd have liked to also do results with C and L loadings as they can
give even nastier peaks and dips, dragging them down to lower frequencies.
But I could not find any in time that were sifficiently 'pure single value'
over the range to make the results easy to assess. May return to this in
the far future, though...
Curiously, this was all a nice preparation for some later work on the
claims about mains cables behaving as 'interference filters'. :-)
Slainte,
Jim
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