Jim Lesurf wrote:

mick 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.

These are simply mis-engineered designs. The methods to make them stable have
been known for more decades than I can remember.

Amy amp that does it simply needs throwing in the junk or conceivably
repairing.

Graham

due to the hugely increased level of spam please make the obvious adjustment to
my email address

Jim Lesurf wrote:

Rob wrote:
Jim Lesurf wrote:

FWIW it means very little to me. You seem to assume a correlation
between frequency, resistance and sound.

Not sure what you mean, I'm afraid.

That much is apparent. Doubt you know much about the concept at all, nor the
bases of stability.

Graham


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Jim Lesurf wrote:

Eeyore wrote:
John Phillips wrote:

Alternatively, perhaps, that a well-designed amplifier will have about
2 uH of good quality inductance in series with its output to avoid
such a case becoming damaging?

Funny, that's very similar to the value I use. And it'll have a series
R-C to ground to stabilise the load the amp 'sees'. This technique has
been known for many decades.

Yes. I have always done the same. However some designers don't, and it is
possible for people to choose incorrect values, or use an inductor whose
self-resonance is too low in frequency.

Then those are simply badly or incompetently designed. You'd be amazed at some
of the pure **** that has been sold as 'audiophile'.

Graham


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John Phillips wrote:

Eeyore wrote:
John Phillips wrote:
Don Pearce wrote:
Jim Lesurf wrote:

I've just put up a new webpage that provides some measurements on the
properties of a variety of loudspeaker cables. The page is at

http://www.audiomisc.co.uk/HFN/Cables3/TakeTheLead.html

It is an expanded version of the article published in 'Hi Fi News' a few
months ago.

No conclusions section there, but maybe as follows?

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.

Alternatively, perhaps, that a well-designed amplifier will have about
2 uH of good quality inductance in series with its output to avoid such
a case becoming damaging?

Funny, that's very similar to the value I use. And it'll have a series R-C to
ground to stabilise the load the amp 'sees'. This technique has been known for
many decades. It is even used in long line-level drivers.

Yes - I agree. It seems we are in harmony about the need for an amplifer
to see a well-defined load at frequencies well above the audio band. Hence
my use of "good quality inductance" which needs to avoid self-resonance
at too low a frequency to maintain isolation for whatever the user throws
at the amplifier in terms of cable and loudspeaker.

I learned a lot from designing and building my first power amplifier.
I saw undesirable behaviour into the many MHz region whenever I failed
to pay enough attention.

So, I'm surprised at your reaction elsewhere. Even in the audio band,
loudspeakers can present impedances from near zero to high enough to
be considered infinite.

WRONG. Show me one.


Out of the audio band this gets no better,
from what I have seen.

By which time the RLC network in the output stage will be doing its job, so the
point is academic.


So it seems to me that investigating loudspeaker cables with loads
from zero to infinity, and at frequencies well above the audio band,
is perfectly reasonable.

I do not remotely agree. For most speakers 4 - 60 ohms +/- 4 - 60 j ohms would
suffice.

I trust you know the meaning of the j

Graham


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"Eeyore" wrote in message
...


Not sure what you mean, I'm afraid.

That much is apparent. Doubt you know much about the concept at all, nor
the
bases of stability.


I wonder why it is that some people on this NG post simply to be offensive
to others? Of *course* Jim knows the basis of stability, at least as well
as you do, probably a lot better.

David.

Eiron wrote:

Phil Allison wrote:
"John Phillips"
So, I'm surprised at your reaction elsewhere. Even in the audio band,
loudspeakers can present impedances from near zero to high enough to
be considered infinite.

** Fraid that is absolute crap.

Only a FAULTY speaker exhibit shorts or opens in the audio band.

How about this, the only impedance curve on Trevor's site?
http://www.rageaudio.com.au/index.php?p=1_12

I can barely believe how anyone could have produced such a diabolical
impedance curve.

The alleged 0.8 ohms at 30 something Hz is especially odd. I suspect
negligent design of the crossover too. Par for the course for audiophools.

Note that the impedance doesn't peak over 9 ohms.

Graham

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On Sat, 08 Aug 2009 19:58:06 +0100, Eeyore
wrote:



Jim Lesurf wrote:

Rob wrote:
Jim Lesurf wrote:

FWIW it means very little to me. You seem to assume a correlation
between frequency, resistance and sound.

Not sure what you mean, I'm afraid.

That much is apparent. Doubt you know much about the concept at all, nor the
bases of stability.


Can you explain then? It has me puzzled too. And given Jim's
background I am going to tell you he knows precisely what stability
means, and how to measure and predict it.

Stability circles are the second thing you learn about on the Smith
Chart.

d

"Eiron"
Phil Allison wrote:
"John Phillips"
So, I'm surprised at your reaction elsewhere. Even in the audio band,
loudspeakers can present impedances from near zero to high enough to
be considered infinite.


** Fraid that is absolute crap.

Only a FAULTY speaker exhibit shorts or opens in the audio band.

How about this, the only impedance curve on Trevor's site?
http://www.rageaudio.com.au/index.php?p=1_12


** TW drags that absurd, false example up as a marketing ploy for a brand of
amp he supplies.

In fact and as he admitted once, the impedance dip at the low end of the
range ( circa 35Hz) was due to a wiring MISTAKE made by the factory in a
few samples sold only.

Cause havoc with owners amplifiers blowing fuses and output devices until
they had them fixed.



...... Phil

Phil Allison wrote:
"Eiron"
Phil Allison wrote:
"John Phillips"
So, I'm surprised at your reaction elsewhere. Even in the audio band,
loudspeakers can present impedances from near zero to high enough to
be considered infinite.

** Fraid that is absolute crap.

Only a FAULTY speaker exhibit shorts or opens in the audio band.
How about this, the only impedance curve on Trevor's site?
http://www.rageaudio.com.au/index.php?p=1_12


** TW drags that absurd, false example up as a marketing ploy for a brand of
amp he supplies.

In fact and as he admitted once, the impedance dip at the low end of the
range ( circa 35Hz) was due to a wiring MISTAKE made by the factory in a
few samples sold only.

Cause havoc with owners amplifiers blowing fuses and output devices until
they had them fixed.

Thanks for the correction.
So the only reason for buying an ME 850 is to drive a broken Infinity Kappa. :-)

--
Eiron.

In article 4a7eb1fc.387537093@localhost, Don Pearce
wrote:
On Sat, 08 Aug 2009 10:17:24 +0100, Jim Lesurf
wrote:

It is all too easy to make an amplifier that looks OK on a test bench
connected directly to a test load - then find it bursts into
oscillation, or its other properties alter - when given some other
load. I've also seen this happen when someone was using an oscilloscope
that didn't reach the oscillation frequency. So the audio waveform
became distorted, but with no visible sign of the RF bursts until they
tried a faster scope.

There is a general rule in design that everything will oscillate. The
only consistent exception to this rule occurs when designing an
oscillator.

Yes. That is one of the maxims I explain to undergrads when teaching about
feedback. The distinction between having built an amplifier and an
oscillator is that you want one of them *not* to oscillate. :-)

The problem for designers of commercial audio power amps is that they have
no idea what actual loads will be connected.

Slainte,

Jim

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