On Thu, 08 Oct 2009 13:38:08 +0100, Jim Lesurf wrote:

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



Jim Lesurf wrote:

Hi,

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.

What IDIOT thinks the reactance at 30 MEGAHERTZ has any influence on
the listening experience.

I wouldn't want to buy an audio amp that could potentially do anything
at 30mHz. It's madness.

Then you may have a problem to worry about. :-) Many of the gain devices
in audio power (and pre) amps have gain at frequencies reaching up into
that region. As a result *unless* the designer/maker has ensured
unconditional stability and no other problems affected by RF loading, then
changing the cables can affect the amplifier behaviour.

Yeah but bandwith can be tailored. Medical instrumentation amplifiers are
an example where attention is closely paid to when designing
equipment. You can fix the bandwith in any audio amplifier, you can even
have software filters with monstrously steep drop offs/skirts for digital
equipment. If an audio amplifier is comprised of a number of carefully
designed stages no single stage having a very large gain and designed
with a roll off beginning at about 25kHz then I don't see why any
appreciable rf currents should come into it.


If you wish to avoid the 'madness' then you have two choices...

1) never buy or use any audio amplifiers.

No, it is possible to use an audio amplifier which will not appreciably
boost rf signals.

or

2) only buy ones where the maker/sellers/reviewers have told you the
unit is unconditionally stable and that behaviour is unaffected by RF
loading.

I noticed in the 70's how the fashion for extended frequency response in
audio amplifiers was creeping up and up to 200kHz and more and am not
surprised that the uneducated now think higher is better and who now may
be wowed by amps with a high gain response up to 30 mHz or more. It's a
foolish and pointless pursuit.


Otherwise you have just 'plug and prey' to go by. :-)

Ideally you want the gain to tail off at just above normal human hearing
~ 20kHz. It's not well known but ultrasonic sound at high intensity is
capable of causing tinnitus (I'm talking power levels found in
ultrasonic burglar alarm systems which I have repaired and found out
from personal experience).

The problem, alas, is that the amplifier is still connected to the load,
via the cables, at higher frequencies. The gain devices will have gain at
these frequencies. And the length and type of cable will affect the load
presented to the amp at RF. Change the length or type of cable, and the
load seen by the amp will change. So if the system isn't unconditionally
stable then it may misbehave without you explicitly trying to put into it
frequencies above 20kHz.

Yes but I suspect you are talking about exceedingly poorly designed
circuits.


Chances are, anyone who has spent long designing audio amplifiers will
have seen them oscillate or otherwise misbehave at such frequencies with
some loadings. And that can then affect the audio behaviour. I've
certainly witnessed this. I have also seen someone puzzled by an amp
having high levels of distortion *at audio frequencies* that appeared with
some speakers but not others. Only to find later that using an
oscilloscope with wider bandwidth showed that the amp was producing bursts
of oscillations in the region well above 1MHz with one load, but not
another. He could not see the oscillations with the first scope he used,
so was baffled by the behaviour. The better scope showed the reasons and
helped him fix the problem.

I have also seen this with a real-world commercial amplifier that had a
high reputation and sold at a fancy price. So if the designer/maker don't
understand this and take care, it can get out into the home.

I have used and tested premium op-amps when designing
instrumentation for medical research and didn't have any difficulty with
rf springing up anywhere along the line. A single high gain block of
course is asking for trouble. You get what you pay for.


Slainte,

Jim