New webpage on loudspeaker cables
 

On Mon, 10 Aug 2009 16:49:38 +0100, David Looser wrote:

"mick" wrote

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?

Because it causes distortions *within* the audio band.


Don just beat you with a more detailed explanation. Cheers anyway!


snip

Arggh!... Plonking a 1nF capacitor across the output of a feedback
amplifier is a pretty likely way of making it unstable


:-)
I always knew there was a reason for zero-feedback amps... ;-)



I'm not convinced that knowing the RF cable properties tells you
anything at all about the audio performance.

Nor am I. AFAIAC the whole thing is about stability, and how likely the
cable is to provoke oscillation in amplifiers with marginal stability.

There shouldn't be any RF present

Ideally there wouldn't. But what makes you think that there isn't?. With
active devices having good gain up into the hundreds of megahertz what
makes you think there isn't noise, RF pickup and self-oscillation well
above the audio band present?



A good point. It does make it rather difficult to design unconditionally
stable audio amps if your test gear can't reach those exotic heights!

Eee... when I were a lad 30MHz were *proper* RF!

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

New webpage on loudspeaker cables
 

On Mon, 10 Aug 2009 10:09:01 +0100, Jim Lesurf wrote:

snip
... And if he or others read the *earlier* pages in the series they
can see I have already discussed the use of output networks, causes of
instability, etc.



/me Runs away to read... :-)

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

New webpage on loudspeaker cables
 

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

In article , mick
wrote:
On Fri, 07 Aug 2009 17:31:43 +0100, Eeyore wrote:

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?

The 'output' would come from the amplifier oscillating. If the amplifier is
not unconditionally stable and you connect it to a load that means the amp
and load combination is unstable, then it can oscillate without any input
signal prompting it to do so.


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.

Probably. But 'almost always' isn't 'always'. And the longer the cable
runs, the bigger the lumped values will be for RCLG.


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.

No, sorry. Putting an RC snubber across the amp output won't protect the
amp from the cable-transformed load having dips. Not against other things
like it being a very large shunt capacitance. Indeed, if you just put
a shunt capacitor across the output that might provoke oscillations if
the amp isn't already stable for other reasons.


If you want to rely on a passive output network you generally need the
amp to have an added output series impedance *outside* its feedback loop.
Usually an inductor and resistor in parallel. Have a look at the

http://www.audiomisc.co.uk/HFN/Cables2/OhmAndAway.html

which is the second article in the series on loudspeaker cables and
amplifier-cable-load interactions. This outlines how instability can
depend on the loading, and discussess the general type of output
passive network designers have employed.

FWIW The Isolda cable will normally include its own series inductor
for reasons like this since the cable itself has an exceptionally
high shunt capacitance per metre.


I'm not convinced that knowing the RF cable properties tells you
anything at all about the audio performance.

It allows you to determine the cable RCLG values which you can then apply
at audio and near audio. It also shows you if the transformed loads might
be severe so you'd need to take care with an amp that was not
unconditionally stable.

There shouldn't be any RF present so the information is redundant.

Alas, there will be is the load means the amp and load are an unstable
arrangement and the system then oscillates. if there is a load impedance
dip this oscillation may then draw large RF currents from the amp.

Afraid none of this is simply theory, I've seen it on many occasions. the
good news is that none of it should matter if the amplifier is a *good* one
and is unconditionally stable, etc. So it should be a problem the designer
and maker fixed for you before the unit left the factory. However Eeyore
has said that some amps are "pure ****". I can't state I'm sure he is
right. But I am aware that no-one seems to be checking this in reviews,
etc. So for me the worry is absence of data on this point.


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

Funnily enough, I'm currently casting a skeptical eye over some of the
recent claims by Russ Andrews and Ben Duncan about their fancy mains cables
being 'RFI filters'. I can hint that I don't really agree with everything
that claim. :-)

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
 

In article 4a833da4.554360703@localhost, Don Pearce
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?

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
 

In article , mick
wrote:
On Mon, 10 Aug 2009 16:49:38 +0100, David Looser wrote:


Arggh!... Plonking a 1nF capacitor across the output of a feedback
amplifier is a pretty likely way of making it unstable


:-)
I always knew there was a reason for zero-feedback amps... ;-)

Alas, they may not actually exist except as a wish. :-)

Even individual gain devices can burst into oscillations. Indeed, I've
spent years getting circuits with just one diode to do that. :-)



Ideally there wouldn't. But what makes you think that there isn't?.
With active devices having good gain up into the hundreds of megahertz
what makes you think there isn't noise, RF pickup and self-oscillation
well above the audio band present?



A good point. It does make it rather difficult to design unconditionally
stable audio amps if your test gear can't reach those exotic heights!

Eee... when I were a lad 30MHz were *proper* RF!

Me too. But I've since spent decades working on the basis that anything
much below 30GHz is 'IF'. 8-) I recall once using a system with an IF
bandwidth of over 10GHz and thinking how many of the usual comms bands,
etc, could be fitted into that IF - if upconverted into it. :-)

BTW M$ free here also. :-)

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
 

David Looser wrote:
"mick" wrote

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.

Arggh!... Plonking a 1nF capacitor across the output of a feedback amplifier
is a pretty likely way of making it unstable

1nF. You mean like 12m of Chord Silver Screen or a foot of Townshend Isolda cable?
(Figures taken from Jim's article.)

--
Eiron.

New webpage on loudspeaker cables
 

"Eiron" wrote in message
...
David Looser wrote:
"mick" wrote

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.

Arggh!... Plonking a 1nF capacitor across the output of a feedback
amplifier is a pretty likely way of making it unstable

1nF. You mean like 12m of Chord Silver Screen or a foot of Townshend
Isolda cable?
(Figures taken from Jim's article.)


I really, really don't understand what's got into some of you people. It
seems that whatever Jim says you are determined to see his piece as some
kind of endorsement of exotic speaker cables.

Grow up!

David.

New webpage on loudspeaker cables
 

David Looser wrote:
"Eiron" wrote in message
...
David Looser wrote:
"mick" wrote
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.
Arggh!... Plonking a 1nF capacitor across the output of a feedback
amplifier is a pretty likely way of making it unstable
1nF. You mean like 12m of Chord Silver Screen or a foot of Townshend
Isolda cable?
(Figures taken from Jim's article.)


I really, really don't understand what's got into some of you people. It
seems that whatever Jim says you are determined to see his piece as some
kind of endorsement of exotic speaker cables.

Grow up!

I wasn't criticizing Jim. I was criticizing you.

--
Eiron.

New webpage on loudspeaker cables
 

David Looser wrote:


One problem of being at the end of science is that the
devil
makes work for idle minds. If you really have nothing to
write about, stop writing. Integrity is perishable. Your
magazine will be lucky to survive as it is, without you
taking the **** out of its readers.


What an extraordinary post!

Thanks! I did my best. One of Bob's best songs, but *very*
hard to sing.

You clearly haven't understood a word of any what Jim has
been

....Is there a word or two missing here? What has Jim been?
It's OK though, I wouldn't have known what he's been either,
and he's not been much help in my effort to find out. But it
really, really doesn't matter.

and are trying to cover up your ignorance

eh? In what way does that make any sense, at all? Why would
anyone come here for the sole purpose of covering up his
ignorance? Actually, come to think of it, if I were to find
myself pursued by some murderous anti-ignorant militia, I
suppose I might try to lose myself in a crowd of luminously
ignorant louts...

with a load of sarcasm.

Sarcasm? Oh well...part was an attempt at sati I don't
think they're the same thing but I'm not an expert. It was
supposed to be quite a pointed parody, such that if you had
the wit to follow it, the point would be clear. I don't
claim to be a great satirist, so I may have failed, but I'm
unlikely to take your word for it. Neither will I succumb to
the temptation of suggesting you haven't read my post, and
listened, carefully.

It wasn't all satire. There was some seriously
well-intentioned and carefully-considered advice.

It doesn't wash.

Not with you or your mates, predictably, or with a
professional author who believes he has a reputation to
defend, unfortunately.

I'm not trying to convince you; I don't wish to stoke your
boilers. Just an awkward and slightly embarassed wink to the
wise, should there be any in the vicinity.

Finally, I do have a genuine passion for truth, and a
coherent and cogent point of view. I discovered some time
ago that this is not a worthwhile place to discuss them.
Every now and then I check to see if it's still true. This
has been one of those occasions. I've made an effort. No-one
has returned the favour. This is all you're worth.

Ian

New webpage on loudspeaker cables
 

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