Re: New webpage on loudspeaker cables

On 10 Aug 2009 15:30:16 GMT, mick <not.here@xxxxxxxxxxxxxxx> wrote:

On Sat, 08 Aug 2009 09:46:15 +0100, Jim Lesurf wrote:

In article <0079bf3e$0$10261$c3e8da3@xxxxxxxxxxxxxxxxx>, mick
<not.here@xxxxxxxxxxxxxxx> wrote:
On Fri, 07 Aug 2009 17:31:43 +0100, Eeyore wrote:




I'm unsure about drawing any conclusions from graphs that start at 5x
the accepted maximum audible frequency. I hope Jim has included tests
on VHF coax as speaker leads too - it makes as much sense to me...
;-)

The problem is that some amplifier designs can be upset by having a load
at RF which does not suit them. The classical symptom is either
sustained oscillations in the region around a MHz or above, or bursts of
oscillations with particular audio waveforms. This can alter the audio
behaviour. The effects are measurable as well as audible.

Alas, the amplifier designer has no control over what loads the user
connects. And this will change with the choice and length of the
loudspeaker cables.



Fair enough, but surely the only amps that are likely to have much output
(i.e. enough to drive the cable - never mind the speaker) above 1MHz or
so are likely to have severe problems anyway (such as overheating) aren't
they? Ok, maybe not if the oscillations are triggered on audio peaks I
suppose, but how would that be audible?


*If* your amp has much output above 100kHz then it is faulty and needs
looking at - seriously.

I agree. However some commercial amps *have* produced oscillations like
this with some loadings. For all I know, some still do.

And one of the points of the RF measurements is that it allows you to
determine the cable properties which you can then apply at audio
frequencies to assess what changes may occur *in* the audio band even
when the amplifier is stable and happy. So the measurements are useful -
if you understand why they were made and how to use the results. :-)



I'm still trying to get the hang of this - please excuse my stupidity and
put it down to old age! Ok, cable always influences RF performance, I'm
happy with that. I'm incredibly skeptical about whether it affects
audible results though. The L, C and R and/or Z values are just too low
at audible frequencies. This is demonstrated on your graphs, where at
500kHz there is virtually nothing to differentiate one cable from
another, no matter what the cost. AFAIK even the advocates of extended
frequency response can't hear harmonics up there. :-)

Now I have to get around getting enough RF out of an audio amp to make
those values important. At the frequencies you are talking about a simple
zobel, or even a 1nF capacitor across the output should remove enough RF
to swamp the effects. Values are probably very uncritical. Obviously if
the amp has nothing across the output then there will probably be
problems, but how often does that happen when the components are so
cheap?

I'm not convinced that knowing the RF cable properties tells you anything
at all about the audio performance. There shouldn't be any RF present so
the information is redundant. It may tell you whether to expect the
leakage of magic smoke if you connect an unstable amp to a measured cable
though... ;-)

I'd be interested to see measurements for a bit of mains twin and earth,
if the mood to test takes you again... :-D

All very educational. Thanks, Jim. :-)

Provided the amplifier is not made unstable by the particular RF load
of the cable, it will make no difference to the sound.

But when an oscillation happens, the result can be severe. Frequently
the oscillation only occurs over a small part of the waveform, but
when it does, like any oscillator, the amplifier is driven instantly
into saturation. That results in something like clipping of the audio
waveform, which is definitely audible.

When you have only audio test equipment - oscilloscopes etc - the
effect can be very hard to identify. It might be a barely visible
wiggle towards the top of a sine wave. It generally won't be found
when doing a distortion measurement, because when you do that, you run
into a nice resistive load. It only happens into a speaker type load
and the audio-only designer will never find it. Frequently the only
clue is an output device that seems to run a little hotter than it
should.

So here's the thing. For the consumer, knowing the impedance of a
cable is not important, 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.

d
.

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