LM13700 dual VCA

• Nov 14, 2023

This is a module I built quite a while ago in January 2022. I forgot to clean up and post the schematic and layout back then but coming across the verified stripboard layouts thread on Look Mum No Computer’s forum inspired me to go back and do it now.

It’s based on the circuit from the Design a Eurorack “Vintage VCA” with the LM13700 article on Electric Druid with a few changes.

Schematic

Download: LM13700-dual-VCA-schematic.pdf (0.1 MiB)

My changes

• Left out the 470n DC blocking capacitor on the input because I wanted the VCA to be useful for CV as well as audio.
• Removed the non-attenuated CV input.
• Increased R2 and R3 to 680 ohms since that’s a more common resistor value to have around. This increases the voltage going into the OTA from 62mVpp to 68mVpp, increasing the gain of the circuit by 0.80dB.
• Increased R4 to 22k because I didn’t have any 18k resistors on hand. This increases the gain of the circuit by 1.74dB.
• Increased R6 and R7 to 470k to make the potentiometers’ response curves more linear. I also compensated for this change by replacing R10 with a 47k so the overall behaviour of the current source stays pretty much the same.
• Replaced R12 with 100k to keep the BOM simpler and to get a bit more available gain on the level adjustment.
• Reconfigured the bypass capacitors. I didn’t really give any thought to this, just did what I’ve always done and put one 100nF across each IC’s power rails as well as two 10uF caps right next to the power input.
• Doubled the whole thing to make it a dual VCA since there are two OTAs on a LM13700.

Stripboard layout

BOM

I’ve included the original values for the parts I changed in the third column.

Panel

Here’s the template I used to make the panel: LM13700-dual-VCA-panel.pdf (0.0 MiB)

Finished module