Power resistors for amplifier testing – the solution

This is the long delayed followup to the earlier post about power resistors as dummy loads and the nonlinearities found.

After some further experimentation an 8 ohm load was built from four Riedon PF2270 series  2 ohm 200W thin film resistors (to give an 8 ohm load) and a surplus heatsink with fans.

With 200W continuous power the heatsink rises about 100F  (55C) over ambient in 30 minutes (and doesn’t seem to level out). With the fan on the temp is stable with about a 25F (13C) over ambient.

Comparing THD+N between the Ohmite wirewounds and the Reidon and Dale (see prior post) parts:

shows that the Riedon parts behave closer to the Dale parts; the thin film parts should have less inductance than the Dale parts but it turns out to not be a huge difference. Note the Dale parts are being overloaded (200W at the highest level applied to 100W equivalent sicne it’s two 50 W rated ones in series) but it is only very briefly that these high power levels are applied. The Dales  do not warm externally under the test conditions.

With respect to inductance, here’s the measured values:

• Dale RH 50W 4 ohm resistors are 190 nH
• Four Reidon 2 ohm 200W in series (8 ohms total) + 4′ of wire 574 nH
• Memcor 450W 4 ohm wirewound 11 uH
• Ohmite 300W 8 ohm wirewound 24 uH

While the Riedon parts perform better the amplifier under test actually has lower distortion than what was measured. Later analysis indicates that the measurement was dominated by noise where as the Ohmite parts the distortion dominated. Determination of the noise in the measurement setup will be in described in a future post (yeah, really).

It was hypothesized that high inductance resistors might affect the amplifier performance for THD+N. An experiment using the Riedon based 8 ohm load in series with a 500 uH coil showed only about 1 dB decrease in THD+N, the values of inductance of a typical low value wirewound resistor will not affect measurement of a robust class D amplifier design. This experiment therefor does leave open the question of why the Ohmite wirewound parts create excess THD.