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Re: Trimming is to kill tolerance of realworld Cs & Rs.

OK so now you have to build an inverse RIAA EQ better than the design for the amplifier you are testing because the test equipment should have an order of magnitude better accuracy than that being measured otherwise you are introducing errors at the signal end.

So if you're aiming for +/-0.1dB, then you gotta have equipment that can measure this EQ deviation accurately. That's equipment accurate from 20Hz to 20kHz to 0.1% or better (0.1dB = 1%). I very much doubt many hobbyists have "calibrated" equipment to that standard. Now you have to ensure that no other strays enter the measurement setup otherwise you're going to be wrong here as well. Just work out how small stray capacitance has to be to put all this effort to waste. It is minutely small.

OTOH, if all you're aiming for is +/- 1dB then the above is a bit much but I think there is an easier and way less expensive way. Just simulate your circuit. The better your model, the better your result. I aim in the model to get the curve inside a 0.02dB window. However once it's as good as I can get it, I then rely on the circuit to look after itself. Buy 1% or better accurate components and bung them in. Chances are the worst it will be is +/- 0.1dB ish.

As most people build high Z EQ networks that are quite circuit dependant, all of this in many cases can seem academic. Fer instance, valve drift can change the EQ dramatically over time never mind strays etc.

For those hobbyists with a battery of test gear to the quality mentioned above (resistance and capacitance meter, freq gen, AC voltmeter t0 0.1% accuracy) then maybe the build and tweak method can work but I certainly don't have such good quality test gear that would better the model and build approach.

OTOH, maybe I got the worng idea here. If so, another viewpoint would be welcome.

I would also want to include for the cutter head rolloff as mentioned by Allen W. Jim Hagerman has a good paper on including this in the inverse EQ.

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