The SG505 is an outstanding very low distortion state variable oscillator SVF developed by Bruce Hofer while he was at Tektronix before being a co-founder of AP. His use of the SVF inspired most of the circuitry in the THD analyzer I designed (Audio article available on my website). As described below in another post, JFET Q1620 amplifies and inverts and amplifies the error signal at the positive input to U1520. This enhances the net error signal applied to the op amp and further reduces the already low distortion of the 5534. The feedforward/feedback arrangement of R1521, R1423, pot R1510 and R1525 comprises what is often called a Baxandall volume control, in which the positive input of the op amp is usually grounded. The distortion-reduction circuit of Q1620 is just for further reducing distortion of the output buffer stage, and is not necessary if the already-low distortion of the 5534 is acceptable (good enough for more than 90% of applications). If not the right JFET is chosen, this circuit may not function correctly and actually increase distortion. You can disable this circuit and still get excellent distortion results. You can disable it by just removing the JFET from the circuit. See if doing this makes the circuit work properly. If you do so, you should probably short R1524 to keep the positive input of the op amp at a low impedance to ground, as normally done in the Baxandall volume control. Note that in general use, the Baxandall volume control provides a good approximation to a logarithmic gain vs. pot rotation using a simple linear pot over a very wide range of gain, making it excellent for use as an audio volume control.
Cheers,
Bob
