OTAVCA: Dual LM13700 linear VCA (4HP)
5 in stock.
THIS ITERATION OF THE MODULE IS NOW END OF LINE - an updated version is in the works.
Another module brought to you by the Department Of Keeping It Simple.
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 +/-5V range, although it's relatively unbothered by particularly hot CVs from envelope generators.
Is it perfect? No - far from it; depending on your envelope generator there may be a bit of CV bleedthrough, but then if you want ultra-high fidelity then you're probably better off with the THAT2180, which has a unit cost about 10x that of the LM13700 ... that being said, they work well enough that I actually keep a couple in my own system.
Personally, I find that the LM13700 gives a bit of warmth to a sound, but that could just be me ...
On a technical note, the CV LEDs are driven through the on-board buffers on the LM13700, so require a fairly high voltage to illuminate, so if the CV level is particularly low you may not see any illumination - I've yet to decide whether or not future runs of the board will incorporate the LEDs; the current run will. In addition, both channels are set up so that unity gain occurs at 5V - you can modify this via the GAIN trimmers if you like, but having a couple of attenuators handy may be a good idea depending on what your CV levels are.
Pre-built modules are supplied complete and calibrated - a power cable and mounting hardware are supplied. The panel is dark purple (RAL4007) FR4 fibreglass with gold lettering.
INCLUDING THE POWER CABLE, THIS MODULE HAS A DEPTH OF APPROXIMATELY 50mm - PLEASE ENSURE THAT YOUR CASE IS DEEP ENOUGH TO ACCOMODATE IT.