Motivation
This is the Eurorack version of my Sims VCA. This VCA is AC coupled. I made my own implementation of the Sims-VCA introduced by Mike Sims in the EDN Magazine January 1995. With this architecture it is possible to achieve THD of 0,02%. Unfortunately I can not confirm the statement from Mike Sims that trimming the circuit for minimum THD achieves minimum control voltage feedthrough. Trimming for minimum THD causes an constant DC bias at the output. I have had to add a output capacitor to avoid the bias at the output. And you need test equipment to measure the THD for correct trimming. If you can not measure THD better build my DC-VCA. You can achieve 0,2 % THD there. Still good. Or, for Eurorack, my Quad VCA.
Specs and features
- AC coupled, 0,02% THD
- Two inputs
- Lin and log response
- CV 0..+5V
- CV attenuator
- Adjustable bias
- Power consumption below 25mA each rail
Implementation
Schematic
Sims style VCA schematic: Control board
Sims style VCA schematic: Main board
Description:
Please refer to my first implementation or the original article from Mike Sims
Calibration
- Insert 5Vpp sinus signal to one off the inputs. Set pot to max. Choose linear response. Set Gain to max.
- Adjust TR1 for lowest THD
- Choose log response. Set gain to zero. Adjust TR2 for zero signal bleedthrough.
Building hints
- The LM13700 differ in quality from part to part. You might want to select the IC for lowest THD
Special parts
- None
Download
Sims style VCA main board documentation downloadSims style VCA main board Gerber files download
Sims style VCA control board documentation download
Sims style VCA control board Gerber files download
Sims style VCA *.fpd file
Sims style VCA: Front view
Sims style VCA: Populated control PCB
Sims style VCA: Populated main PCB
Sims style VCA: Side view
