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In Reply to: OT: feedback resistors in PAs posted by Jacques on August 22, 2006 at 03:41:40:
Jacques, I think that I would rescale the resistors to be a higher value. Power amps are at the end of the amplifiying chain and the difference between a 100 ohm resistor and a 1K resistor would usually be unimportant. At a moving coil input stage, however, even 100 ohms would be too much, but then there is no signal output either.
For my power amps, I generally use a 47K 1/2W Holco (old) feedback resistor with perhaps 1.8K to ground. I might use a 1-2W Resista, if I had to sacale it down to 10K and 330 ohms or so. I would most likely never use a 100 ohm resistor, because of the problem of dynamic overheating, that could be fixed with a heatsinked power resistor, but then you develop other problems such as capacitive coupling to the chassis, etc.
Follow Ups:
Jacques, of course you meant the 100 ohm resistor as the resistor that goes to ground, but if you raised the value of that resistor to 1K or so, the noise in the power amp would still be very low, The main feedback resistor that swings almost all the voltage would then be about 25 times larger or more with modern power amps. This is the resistor that will overheat, if you are not careful.
...was the resistor to ground.
So, the FB resistor itself would be in the <1,000 ohms.
which would suck several watts up to tens of watts... For sure.
sorry for the misunderstanding.
Just to make it perfectly clear: You can use a somewhat higher series resistor in series with the input stage of a power amp, because it operates at a much higher voltage level than a preamp phono stage, or microphone input stage. A 1 Kohm feedback resistor to ground for a PA amp should be OK. Then the main feedback resistor would be 28.2K for a normal home theater power amp, and maybe more or less for some other amplifiers. If you use +/- 100 Volt supplies and presume a full square wave as a worst case output, then the main feedback resistor must have a rating of 100 squared =10000 divided by 28.2K. This is less than 1W, but it is always best to use the largest resistor that is practical in order to keep changes in temperature of the resistor to modulate the output voltage. Still, a 1W resistor can be compact enough to be board mounted and not require an external heatsink. Even a 1/2W resistor (what we use) is good enough with a good resistor and normal operation.
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