In article , Pooh Bear
wrote:
Jim Lesurf wrote:

In article , Pooh Bear
wrote:

Recently acquired some new dummy loads for amplifier testing.

The measured THD figures seemed rather high. Swapping to an earlier
dummy load reduced the THD.

Concluded that the resistance element was thermally modulating on a
cycle by cycle basis. The cable to the load acted like a potential
divider in series withe lon-linear load.

Measuring THD directly at the amplifer output itself proved the
point.

Was the 'suspect load' THD high at HF or at LF?

Seemed to be pretty much independent of frequency IIRC which seemed odd.
I was more interested in just sorting it to spend too much time though.

OK. It does sound an odd result. If it were thermal then I'd have expected
more distortion at LF, but if due to reactance, more at HF. So a strange
result. Might it have been due to some sort of contact diodes or similar?

[snip]

The ceramic tubular loads appear to be the ones with the problem. More
recent ones seem worse too. Different resistance wire ?

Don't know. It is a curious result...

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

In article , Arny Krueger
wrote:

[snip]

There's an old JAES paper by Greiner of the University of Wisconson that
charted nonlinear distoriton due to fuse thermal effects.

Thanks again for pointing this out. I've now had a chance to read it. Two
things strike me as remarkable:

1) That he mention fuse effects in just a couple of pages at the end of a
paper largely on other things like cables. Yet I'd say that what he reports
there almost as a throw-away aside is *much* more interesting than the main
topic. THD values up to a percent or more in quite plausible example cases!

2) That no-one else seems to have much of an issue of this.

OK, I assume that most designer's reactions would be like mine and just
avoid using fuses where the distortion would show up. But only once they
realise this is a problem. Yet it seems to pass almost without comment in
many textbooks, and in magazines, and I can't recall any systematic and
widely known data on this. Given how some people beat their brains out over
tiny effects, seems weird this isn't better known!

I've now put onto my "must do one day" list to do some more measurements
and try and work out a basic model of the effect. Some of the papers I've
mentioned will help with that if I ever get around to it. I'll also do a
search of Electronics World and see if they've ever covered this.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

"Jim Lesurf" wrote in message

In article , Arny Krueger
wrote:

[snip]

There's an old JAES paper by Greiner of the University of Wisconson
that charted nonlinear distortion due to fuse thermal effects.

Thanks again for pointing this out. I've now had a chance to read it.
Two things strike me as remarkable:

1) That he mention fuse effects in just a couple of pages at the end
of a paper largely on other things like cables. Yet I'd say that what
he reports there almost as a throw-away aside is *much* more
interesting than the main topic. THD values up to a percent or more
in quite plausible example cases!

2) That no-one else seems to have much of an issue of this.

OK, I assume that most designer's reactions would be like mine and
just avoid using fuses where the distortion would show up

Greiner's article suggests that separate fuses for woofers and tweeters
would do a lot of good.

. But only
once they realise this is a problem. Yet it seems to pass almost
without comment in many textbooks, and in magazines, and I can't
recall any systematic and widely known data on this. Given how some
people beat their brains out over tiny effects, seems weird this
isn't better known!

Well, we all *know* that speakers are very dirty, and some of us know that
rooms can be at least as dirty.

However, realizing the audible implications of the fact that audio
reproduction involves a chain that is only as good as the weakest link,
seems to be less common.

Many people seem to think that speaker distortion is a separate breed that
always flies under our auditory radar. In fact nonlinear distortion of a
given magnitude and order is audibly what it is, no matter what the source.

I've now put onto my "must do one day" list to do some more
measurements and try and work out a basic model of the effect. Some
of the papers I've mentioned will help with that if I ever get around
to it. I'll also do a search of Electronics World and see if they've
ever covered this.

The conventions for use of protective devices seem to be separate protection
devices in each driver's line or even more common, just the tweeter line.
This probably makes most of the problems Greiner finds vanish below
perceptibility.

Arny Krueger wrote:

"Pooh Bear" wrote in message

Arny Krueger wrote:

"Pooh Bear" wrote in message


Recently acquired some new dummy loads for amplifier testing.

The measured THD figures seemed rather high. Swapping to an earlier
dummy load reduced the THD.

oops!

Concluded that the resistance element was thermally modulating on a
cycle by cycle basis. The cable to the load acted like a potential
divider in series with the non-linear load.

Been there, done that.

Measuring THD directly at the amplifer output itself proved the
point.

Perhaps.

Not all dummy loads are equal it seems ! The best I've found in this
respect are the alumium clad bolt down types.

Interesting. What were these new dummy loads composed of, exactly?

2 of these in series to make a 600W 4 ohm load. Tubular ceramic
wirewound type.

http://uk.farnell.com/jsp/endecaSear...=1840290&N=401

They're still fine for soak tests.


The biggest problem I've found with my dummy loads is variation of
actual DC resistance with lnger-term heating and cooling.

I have acquired a stash of precision NI wirewound resistors from
Mouser for my next pass at the problem.

These take my fancy. Supposed to be non-inductive.

http://uk.farnell.com/jsp/endecaSear...=3067920&N=401

Hugely expensive though.

Interesting design. Seem to be very compact about 1 x 3".

Thay are in about the same price range as my 300 watters, of which I have 8.
They are huge - about 2 inches in diameter and about 8 inches long.

Here is the catalog page I order precision NI wirewounds from - I have a
mixture of sizes and wattage ratings related to my reactive and non-reactive
loads:

http://www.mouser.com/catalog/620/411.pdf

In my tests these are amazingly stable over a large temperature range. Rated
power puts the surface temp up in the 400F range. The resistance barely
changes. This is quite a contrast with other cheaper NI parts I have tested
(and regrettably used).

Took a look. Those aluminium clad parts look similar to the CGS parts I have
here for some test loads. They are the ones the produce the 'best' results.

The thick film jobs look similar to one of the types I posted above. Very low
inductance by construction. When the budget permits !


Graham