"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?

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.

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

"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).

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