Welcome to Robot Dialogs. This is a place where I will record my progress, mistakes, errors, failures, lessons learned and hopefully occasional success in my designs and implementations of robots and other technical endeavors. I will try to make it as educational, informative and entertaining as I can...

Friday, September 9, 2022

Eurorack VCA Module - AS3360

Every synth needs a VCA. Since I've been making a few modules around Alfa chips that are largely clones of Curtis chips I picked up some AS3360's to use for this module. The AS3360 has two VCAs in it. It's fairly simple to use compared to the VCF and VCO in the same series of chips. 

Much of the design is straight from the data sheet with some bits borrowed from the Look Mum No Computer VCA but I've changed some elements to deal with CVs in my system going 0-10V and for various component differences.

As usual I put this thing together on the breadboard first. In this case it's pretty much a complete mockup of what I have in the PCB. 

The PCB design goes quite smoothly when I can work out the details on the breadboard. The best part of looking at the LMNC design before going into the PCB design was realizing that that design inverts the input and inverts the output for a net non-inverted output. That's a nice simplification I wish I had thought of myself, it's probably common on lots of modules but the first time I've seen it and understood it.

Here's a simulation of the input CV circuit. And another simulation of the output which is a bit different from other modules I've made since the chip provides a current output.

    

  
Most designs I make now that can be panelized are panelized before I order them it's a simple step to do and I get more boards for the same price for doing it. In this case I made a design decision early that allows me to use the panelized PCB for a quad version of the VCA.
  
For this to work there's a bit of extra space between the two PCBs on the panel and some extra through-hole solder points where jumpers can pass power to the second PCB. That means the two boards can share the power supply components. I also have a variant of the front panel for the quad VCA.

The quad version is desirable in my skiff for dealing with the output of the polyphonic envelope:
Issues on v1:
  • The voltage divider for the CV's shows 400k and 50k resistors (R24/R19 and R29/R20). After some testing, 390k and 33k worked better. The v1 values didn't account for the gain on the non-inverting buffer correctly.
  • On the silkscreen U1 and U3 are both op-amps but the chip name TL074/LM358 are swapped.
That's not much to complain about. I'm happy with the design. I haven't put too many hours on it yet so there may be issues down the road.

12V Current: 33mA
-12V Current: 13mA
5V Current: None

Schematic: vca_as3360_v1.pdf