DIY stripboard Roland System 100 VCA
Continuing my suddenly obsessive System 100 stripboarding with a boring bit, the voltage controlled amplifier.
I wasn’t expecting this to be amazing sounding, being not much more than a CA3080, but I thought it was worth a go as I’m trying to replicate the original as far as I can.
Here’s the original VCA, with the blue marking on the 3080 presumably to show that it has been selected for low CV feed-through. I’ve seen circuits for matching OTAs, but I’m not clear how best to select for low CV bleed. Maybe it’s just a case of trying a bunch of 3080s to see which is the least clicky.
I wanted to keep the mix input from the 102, so I built the thing around Q337. Also not having a TA7136 SIP opamp to hand, I just subbed in the usual (non-inverting) TL072-based CGS DC mixer on the end, which was a mistake.
As I understand it now, the System 100 VCF inverts the signal from the VCO, and the VCA inverts it again, so it’s all back the right way up. As it stands, my layout inverts it twice, so input to the VCF eventually ends up inverted.
Incidentally I can’t imagine the TA7136 will make much difference, but now I’ve just had some arrive in the post, I’ll build a version with it.
To give an idea of my general confusion, I spent an evening wondering why the output from the VCF was crazily offset by a number of volts, before realising that this would be sorted out by the capacitor C324, which is on the VCA board. Just because it seems all nice and modularised doesn’t mean that it is.
Here’s a demo with the envelope on the System 100 101 keyboard modulating the VCA cutoff.
It gets clicky like my SH-5 with the envelope on a short release and the filter cutoff low, it might be that I need to find a better 3080.
One mod that might be interesting is to boost the input to the VCA from the VCF to see how it overdrives. Maybe messing with R378 on the input to the CA3080 might do the trick.
Here’s a stripboard layout for the inverting version using a TL071, any corrections or improvements joyfully received in the comments below, taaaa.