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.


Graham