I agree with the others, it is not a good design. I also consider it useles s.
For one, if you want to detect clipping in an amp there are too many variab les for such a circuit with preset thresholds to do any dman good. It might tell you when a certain power level is exceeded but other than that it tel ls you nothing.
To detect clipping on an amplifier, you must detect a discrepancy between t he input ad output. First of all you need the right point for one of the in puts. That will be the input to the power amp stage after all volume and to ne controls and ANY other signal processing. Usually it is maybe 1 volt P-P for rated output.
The power amp circuit has a specific gain after this point and you have to null it. Here's a qwikndirty of it :
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The box on the left is whatever, the preamp and everything like equalizers, dynamic range expanders and whatnot before the power amp. If you are deali ng with an integrated amp or a reciever without pre out/main in jacks you h ave to go inside and find the inputs to the power ampp itself. This is not difficult usually, except in newer junk where they try to force the space s huttle electronics into three square inches.
R1 and R2 simply take the level of the input side down to the point where i t is easy for the OP AMP to handle. On the output side, R3 and R4 match tha t level so that it nulls, that is to apply the same level to the + and - in puts of the OP AMP. Any difference is rectified by the diodes and fed to Q1 , which lights the LED. That will not happen until and unless it clips.
MOST power amp stages are non inverting, but if you happen to get one that inverts then R1 an R3 are chosen to null right there and the OP AMP only ne eds serve as a buffer.
The REASON you need to do this to detect clipping in an amp is because you never know what the power supply voltages are going to be as they are usual ly not regulated and they vary with line voltage, and how the varying speak er impedance reacts to the input signal.
Another way to do it, more hillbillyish, is to ust use the power supply rai ls in the amp as a reference and when the speaker voltage gets within about a volt of that then trigger something to light the LED. This also works we ll enough for some. The circuit on the back of the proverbial napkin in the picture has an added advantage of detecting if the amp is not working well with an uncooperative load, like certain amps with peizo or electrostatic tweeters, or if there is actually a fault in the amp that is not apparent t o the ears, or if the current limiting is kicking in for some reason.
The ciruit that is (somewhat) depicted does not really detect distortion in the conventional sense, it does but it is simply detecting the differecne between the input and output signal. Yes it is distortion but this is not, well, quite, the IHF method and any measurements you can take with this are not admissible in the official spec sheet.