Tube DIY Asylum: Re: How to design a passive single-network RIAA (two tubes) by Kurt Strain Do It Yourself (DIY) paradise for tube and SET project builders.

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Posted by Kurt Strain on March 25, 2002 at 12:45:38

In Reply to: Re: How to design a passive single-network RIAA (two tubes) posted by Paul D. Scearce on March 25, 2002 at 11:14:50:

Thanks. I spent a long time on this solution on Sunday. It was beneficial for me because I actually calculated the precise ratios that are needed for an "ideal RIAA" in this topology. The transfer function is a little hairy as it is, with a zero and two poles in the denominator that must be solved from a quadratic equation. I had to program my calculator to iterate the answers. But now I know what they should be instead of just guessing as I was doing. A simulator like Pspice lets you get real lazy and you don't derive the exact relationships that pure math does. So I never knew these ratios until just now. I looked back and saw how I can make my designs a bit closer, and the initial chosen values can now be very precise.

What the simulator does provide that this doesn't is add in all the model components and get a real overall picture of the output compared to an ideal RIAA, like what happens when the V2 grid resistor is chosen and the coupling caps and the Cgp capacitance effects, and the autotransformer step attenuator loading on this output stage, etc. The math balloons exponentially with all those added variables. But the intuition for why something is chosen is not well understood. So the basic math and the rules of thumb are capable of getting you dang close right up front and you understand more about the problems of getting there exactly.

Kurt

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