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Old 19th November 2005, 02:01 AM   #1
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Default Weird amp testing hypothetical

I came across an interesting circuit for a constant current test load recently, which would be great for testing DC supplies like computer PSUs and so on.

For some reason a thought about using it to test an amplifier popped into my head. I'd be interested in peoples' thoughts about the sanity of doing such a thing, as well as what would happen and whether the amp would be damaged by doing so.

For DC stuff, the load basically behaves like a variable resistor - except that you set the current through it, not the resistance. However, if you loaded up an AC supply it would let that amount of current through at any voltage, meaning that your current waveform should look like a square wave right? This means that your power dissipation would be a fair bit higher than a simple resistive load.

What would be the effect of loading an amplifier in this way? It's certainly not a nice load for the amp to be driving, I'm wondering whether it could cause any instability in the amp's operation. Obviously you'd need to do a few sums to work out how hard you could drive the amp without going out of the SOA of the transistors, you'd reach it more easily than a resistive load.

Sounds like a dumb idea to me but thought I'd share anyway
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Old 19th November 2005, 02:10 AM   #2
anatech is offline anatech  Canada
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Hi Geoff-AU,
I've used electronic loads in the lab (HP). They are normally unipolar for one, you can't feed them AC. I don't know how they would react frequency wise, I haven't paid attention to that spec - ever.

If you got one that would accept AC, it would be very hard on an amplifier. Until you reached the drop out voltage, the resistance would appear to become extremely low at low voltages. A short. Not a reasonable test really.

To be of some use, amplifier testing should reflect the real world in some way. Good outside of the box thinking. Try to design a dynamic load. In essence, another power amplifier to be the load in a controlled manner. That way you could model almost any loudspeaker as the load. This also means your test amp (load) would have to be silly powerful so as not to be destroyed.

-Chris
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