HPF-Pre Series 3 technical info

This is a work in progress. I'm going to upload the schematic for the Series 3 preamp, then add some descriptive text within the next few months.

Schematic Diagram

U1-U4 is a Burr-Brown (now Texas Instruments) OPA1654 quad audio JFET op amp chip. You could use a general purpose JFET op amp here, but the OPA1654 has very low noise. For longer battery life, a TLC2264 quad op amp would be a good choice.

U1 is wired as a unity gain buffer if you omit C8 and R15, and let R14 be zero Ohms. Otherwise you can choose components in this section for more gain if you need it.

U2 is a unity gain phase inverter.

U3 and U4 are classic Sallen-Key high pass filters. There are a lot of recipes for choosing the R's and C's in the Sallen-Key topology. I wanted equal resistors so I could use a dual pot. I also wanted unity gain in the passband, to avoid headroom issues. But choosing those things leads to a Bessel response curve, i.e., a fairly soft "knee" or transition from the passband to the stopband. It turns out that providing a bit of global feedback with R6 let me tailor the Q of the circuit, and I chose a value that gives Butterworth response (optimally flat in the passband) with the frequency knob at its lowest setting.

I believe that the R6 trick is novel -- an invention!