[sdiy] VCA crossfade/pan idea

Tom Wiltshire tom at electricdruid.net
Sun Jun 26 19:21:16 CEST 2016

Hi all,

In the SSM2045 datasheet is the following snippet:


This uses a differential pair to provide currents to two VCAs on the chip. Since the total current  depends on the VL input (at the top) this can be used for envelope control of the amplitude. The VOF input (on the right) sets the differential voltage between the two bases, and thereby changes the balance of currents between the two VCAs. This gives a crossfade or panning function.

How difficult would it be to modify this to work with a LM13700?

It seems pretty straightforward to me, but I'm messing with things I don't understand. The overall current needs altering (from <500uA for SSM2045 to <1mA for the 13700) and VOF would need tweaking for whatever PNP transistors were used. But apart from that, should it work as drawn?

I'm asking because I breadboarded it and it didn't. I'll like to know if that's because I've got a mistake, or whether the whole idea is bad.


More information about the Synth-diy mailing list