OTAVCA revision 2: Dual LM13700 linear VCA (6HP - updated version)

An updated version of the OTAVCA module - functionally the same as its predecessors but now quite a bit cheaper.

As one of my 'high turnover' modules (ie. I sell a lot of 'em) the bulk of the assembly is now handled by the board fabricators with the final assembly and testing taking place 'in house'. There are several advantages to this: first, I can take advantage of the fact that parts from the fabricator are very cheap (economies of scale etc.) and second, I spend far less time faffing around with surface-mount components, leaving me to do final fitting of any through-hole parts plus any testing.

The LM13700 is one of those chips that has been knocking around for years, minding its own business, despite the fact that it is a mind-bogglingly useful and versatile piece of silicon. Granted, it can be a bit of a pain to work with, but it's still useful nonetheless.

This is a dual VCA, each channel of which uses one of the transconductance amplifiers on the 13700. Other than the addition of a couple of linear current sources for CV control this isn't massively different from the circuit you'll find in the datasheet. It works best with signals in the 0-5V range, although it's relatively unbothered by particularly hot CVs from envelope generators.

Is it as good as, say, something built round a THAT2180 or similar? No ... but it's a hell of a sight cheaper.

On a technical note, the CV LEDs are driven through the on-board buffers on the LM13700, so require a fairly high voltage (~4V) to illuminate, so if the CV level is particularly low you may not see any illumination, even with low forward voltage LEDs - if this bothers you then I recommend setting the CV level put fully clockwise and adjusting the output using the gain pot.

The module is 6HP wide and approximately 15mm deep - a partial build with SMD parts pre-soldered is available for the DIYers