[sdiy] SSL eq topology

[Lanterman, Aaron]

Dear SDIYers,

I was looking over the SSL equalizer schematics:

http://www.ka-electronics.com/images/SSL/ssl_82E02.pdf

http://www.ka-electronics.com/images/SSL/ssl_82E242.pdf

Particularly the high mid and low mid circuits (at the tops of the pages) but the others are similar. Has anyone seen this topology before? If so what is it called? I can't seem to find anything like it, even after pouring through however-many filter design textbooks and ap notes.

It's built around a fairly textbook and boring single op-amp bandpass filter, with a series cap and resistor at the input and a parallel cap and resistor in the negative feedback loop. But, the output is fed back to the positive input of the op-amp. I don't recall seeing that exact trick before on that particular configuration.

I slogged through the algebra and derived the transfer function. If the caps and the resistors are the same, things simplify, and you get an output at the peak that is between 1/2 and 1/3 of the input, depending on the Q setting, and at most 1/3 of the op amp output can be fed back to the positive input (I haven't looked at the feedback resistors in detail but presume it accomplishes that) before it would self-oscillation (at least "on paper.") The Q starts at 1/2 and increases from there as the positive feedback increases. The resistors aren't exactly the same, but they're close, and the caps are the same, so my approximations are probably on target.

- Aaron