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.


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

Agreed. I think I know the type of loads you mean, and if so, they are the
sort I used to use many years ago. However the reason I ask the above
question is that I recall a review in HFN of the Armstrong 600 range where
the reviewer got much higher THD values than the company had measured.
Investigation lead to us deciding that the reviewer's load had a high
series inductance which was changing the distortion. (However it may have
been a thermal effect, despite our conclusion at the time.)

I used to think that too. I even measured the inductive component of some of
our loads. It wasn't that high.

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

Graham