Amplifier Dummy Load for THD measurements / non-inductive?

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I have experimented with various film type power resistors. My experience is that you need to derate them. They do not handle transients well. It was an expensive mistake.
I used a precision ESI videobridge to quantify the cheap wirewound resistors.

If you want to buffer the Pico output the question is linear or a fast switch. Two powerfets make a great very fast power switch for fast transients. Otherwise I would look at a tpa6120a2 since it has power and bandwidth beyond what you need. You can get the demo board to make implementation quick.
 
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The buf634 is very different from the tpa6120a2. The buf is meant to be inside the feedback loop of another opamp. The tpa is actually a current feedback amp for driving dsl lines. The tpa can deliver more current at lower distortion to very high frequencies. It can be hard to use. That's why the demo board is nice. Remember you aren't listening to it. The buf would need an opamp to do similar and you would need to deal with stability issues getting the upper bandwidth from it.
 
Iven if the tpa6120a2 would be the mor suitable device, the package would require a PCB and the demo board is too expensive.

The BUF634 I can just put on perfboard and the TO220 has the best thermal performance.

BUF634 can be used open loop according to the application node. The open loop gain is 1 (almost). So following all the design rules there, that should do it, don't you think? Lowest THD is not a requirement, at least I don't see a use case for that.
 
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If you are using the buf open loop its pretty simple. You may need a ground plane and good bypassing to get the performance. I would airplane over some copperclad.

I don't know what the performance of the Picoscopes arb is. Its probably better than 1% but in many applications that's not critical.

Breadboarding the tpa is not an option. However I found an assembled board for 20 euros on eBay. You are right he demo board is too expensive to buy.
 
UXP600-8RK - 8-Ohm 600W Ultra High Power Resistor (EBG) at The Electrostore.com - Electronic Surplus Parts & Equipment

I used this type for my dummy load. Mine are 4 ohm instead of 8 ohm and require a heat sink in which the resistor can sit flat on with 100% contact.
I have used a bank of these in a series /parallel config to test large power amps.

I was able to purchase some surplus UXP/600/9 4RJ the 4 ohm version.

My large load consists of 8 of these resistors mounted to an old JBL heat sink which is cooled by 3) 5 1/4" fans.
 
Several dusty pictures of my dummy load.
 

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Once you add cables all bets are off. If you have the right instrumentation you can characterize to whatever you need but its not cheap. Above a few KHz you really are best served with a vector network analyzer. Connecting with a cable then adds to the problem since the cable will be 50 Ohms or higher.
I'm away from my shop for another 2 weeks but I can plot the resistors to 3 GHz when I return if its of interest.
Slew rate into a resistive load is interesting. Do you sweep to 3% HD to find where the output departs from feedback control? How does capacitive loading affect the slew rate? Lots to explore.
 
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