A “Cordell” low distortion oscillator

Recently, I accidentally broke my low-distortion oscillator, the “Williams Memorial”. The breadboard was such a mess, that I decided it would be almost as easy to just build the Bob Cordell design.

I altered the design a bit: I replaced the switched attenuator for a plain volume pot, made the output balanced, and swapped the three LM318 op-amps for a single TL074.

I put a rectifier, smoothing capacitors and regulators on board. I’m hoping that my THD analyser mainframe will have a couple of spare transformer windings to power it, and the whole thing can fit in the empty left-hand bay where the SG505 would have gone, if I ever had managed to find one.

For the frequency control, I plan to try a 50k Alps “Blue Velvet” pot. I’m hoping the superior tracking and wiper resistance, combined with Cordell’s non-linear amplitude control scheme, and the integrating nature of the state-variable oscillator, will make it usable. I’m also hoping it can be taken apart and reassembled with the shaft coming out of the other end, to make it reverse log.

If that doesn’t work, I’ll use a binary set of switches, or something. That worked surprisingly well on the Williams Memorial. Eventually I’d like to try with 4066-type analog switches in current mode.

Anyway, here’s the schematic and a preview of the board.

Low distortion oscillator schematic

Image of low distortion oscillator PCB

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