frequency shifter

This module is based on a description of a modified frequency shifter (FS) used on The Ed Sullivan Show in the early 70s. One of the sound engineers from the program modified the studio FS so they could generate funny voices for comedy skits, a very early example of circuit bending. It was called the Foster Freqy, after the designer/hacker, Don Foster.
In studios and concert venues, FS are often used to shift a signal by approx. 5Hz to prevent feedback howl. The Foster Freqy used a variable oscillator on the second multiplier stage to enable a shift of approx. 600Hz. This version uses a VCO to give a shift of up to +/-5kHz.
This is a single side-band FS, a lot simpler than the Bode/Moog designs. There are no dome filters. The output does sound similar to a regular FS but has its own character.
The input signal is goes into the multiplier with the 21kHz sine from a fixed oscillator. The output goes into a bandpass filter (approx. 24kHz centre freq), the upper sideband is allowed to pass but the lower sideband is blocked. This goes to the 2nd multiplier along with a sine from the VCO and the output goes to a 10kHz low pass stage to remove all the upper frequencies. Part of the output circuit includes a simple envelope follower and vactrol to help eliminate any high frequency bleed coming through when the input is quiet.
The multipliers are op amp based designs, crude, effective and cheap. I didn’t feel the need to use $20 AD633 chips and LM1496 would have doubled the size of the circuit with all the supporting circuitry they need to work.
Anyway, beyond the vactrol and LM13700, no fancy components required and it sounds great, a bit raw compared to the Bode type designs.
If nothing is patched into IN 2, then the In 2 Pot can be used as a feedback control. Keep it at zero, if you don’t want feedback.

https://www.nonlinearcircuits.com/modules/p/freqy