Amplifier Dummy Load for THD measurements / non-inductive?

[Zymorg]

Hi,

for my first THD measurements with my QA401 I need a dummy load. I am thinking of 16x 50W 8,2Ohm Widap WD resistors to get a Power rating of:
8Ohm 200W (4x)
4Ohm 400W (8x)
2Ohm 800W (16x)

Unfortunately, these resistors are inductive and its value is not mentioned in the data sheet. With up to 16 resistors, I can imagine it sums up to 100-200uH. I cannot measure it because I haven‘t bought these parts yet.

But my actual question: Should I care for such inductivity during THD measurements at all? What is the inductivity limit which can be regarded as negligible?

The alternative, if pure resistive load is advisable, is to make an own resistor with resistance wire in a bifilar fashion or to buy expensive non-inductive resistors.

How are you dealing with that?

[woody]

If after you have the parts couldn't you just nutralize any inductance with a zobel ? Or you could just use parts like these . 5pcs 100W 50ohm Dummy Load RF Resistor 100Watt 50R HF Power terminator 1109 | eBay

[jazbo8]

You might be interested in this:
Building my own noninductive 8R 150W load using wire wound resistors

[Mark Johnson]

Zymorg, I built dummy loads using the exact same approach you have proposed.

Building my own noninductive 8R 150W load using wire wound resistors

The key result was: wirewound resistors don't all have the same inductance, it varies quite a lot from unit to unit. Therefore, plan to test and tune the inductance-neutralizer on every final resistor-assembly. I did the testing and tuning using a signal generator, oscilloscope, and a PCB layout that let me add more and more neutralizer capacitance, very gradually. Don't lay out one PCB footprint for a capacitor; lay out three or four capacitors in parallel.

[Zymorg]

Thank you very much, I did not think about compensation... I will do it like explained in the mentioned thread.

[Enzo]

Or use non-inductive resistors to start with:

NH0508R000FC02 Vishay / Dale | Mouser

[Zymorg]

Originally Posted by Enzo

Or use non-inductive resistors to start with:

NH0508R000FC02 Vishay / Dale | Mouser

Way too expensive, the non-non-inductive ones cost 2,50 each. If the Zobel has no drawback which I don‘t see then its perfectly fine.

[Mark Johnson]

It's a reasonably straightforward problem to simulate in LTSPICE.

First make a conservative overestimate of the inductance to be neutralized, e.g., 3 microhenries.

Next, calculate the correct value of compensation capacitor for that inductance and resistance. Build this into a simulation .circuit

Apply a ±65 volt, 50 kHz square wave in simulation, and monitor the transient current through the compensation capacitor. Also monitor the power dissipation in the compensation resistor. Use these simulation results to choose a compensation capacitor with plenty of safety margin on the ripple current, and to choose a compensation resistor with plenty of safety margin on the power dissipation.

(also double check that your compensation capacitor value was calculated correctly. Plot (V/I) for the assembly and verify that it's 8 ohms before, during, and after an edge of the square wave.)

[DF96]

Originally Posted by Zymorg

With up to 16 resistors, I can imagine it sums up to 100-200uH.

Inductance combines like resistance, so only adds up for resistors in series. In parallel the inductance goes down, just like the resistance. 100's of uH are unlikely.

A bit of inductance probably does little harm. Real speakers are not pure resistances. Anyway, you can compensate using capacitors - although watch the bandwidth.

[Zymorg]

Originally Posted by Mark Johnson

It's a reasonably straightforward problem to simulate in LTSPICE.

First make a conservative overestimate of the inductance to be neutralized, e.g., 3 microhenries.

Next, calculate the correct value of compensation capacitor for that inductance and resistance. Build this into a simulation .circuit

Apply a ±65 volt, 50 kHz square wave in simulation, and monitor the transient current through the compensation capacitor. Also monitor the power dissipation in the compensation resistor. Use these simulation results to choose a compensation capacitor with plenty of safety margin on the ripple current, and to choose a compensation resistor with plenty of safety margin on the power dissipation.

(also double check that your compensation capacitor value was calculated correctly. Plot (V/I) for the assembly and verify that it's 8 ohms before, during, and after an edge of the square wave.)

I was rather thinking of trial and error using my capacitor box. But the lowest value is probably already too high. However, I have placed the order for the parts and I will hopefully find the right capacitors in my shelves. Why I should need anything more than trial and error with a scope and signal generator?