Music and Synth DIY

Eurorack, Synth DIY, Through hole

CEM 3350 dual resonant VCF (finished)

AS3350 (CEM3350) Eurorack filter DIY – from schematic to PCB and panel production in KiCad.

The Alfa AS3350 is a clone of Curtis Electromusic’s CEM 3350 dual state variable filter chip from the ’80s. It’s recently been released and is available from Electric Druid.

It was this VCF chip that made the Synton Syrinx and Crumar Spirit filter sections so interesting, compared to some of the other monosynths of the day.

Using a mix of the schematic in the datasheet and (mainly) the schematics from the old Digisound 80-16 module, plus a standard Eurorack power filtering section, I knocked up the circuit boards and panel in KiCad and ordered them from JLCPCB.

Uploading the panel was a headache – as it’s not the typical electrical circuit JLC’s online ordering system expects, it wouldn’t recognise the board dimensions until I added a via as a workaround. Something to do with expecting two layers.

Now waiting for the boards to arrive from China…first time I’ve tried to make a skiff-friendly split main/pots board PCB joined by male/female headers, so whether v0.1 works is anyone’s guess.

As a side note, if you’re reading this and you have a copy of the Syrinx schematics with the component values filled in, I’d love to see it!

Update #1

Boards arrived.

OK, I wasn’t expecting to get this right first time, and I didn’t…but it wasn’t too far off.

The panel fits:

…and the upper and lower PCBs line up with the headers.

Again, human error crept in: I got an electrolytic the wrong way round in the power section (bang), set the resonance pots up at a divider between +12v and GND when they should be +12/-12v, mislabelled a 200r resistor as 200k, and missed a ground connection on one of the IC pins.

But – with a couple of flying wires – it works.

I’ve (hopefully) corrected the mistakes and am now waiting for the version 0.2 boards to arrive.

The Syrinx and the Spirit both had two of these 3350s, with a switchable mixing circuit to give combinations of low pass, band pass etc in serial or parallel. But let’s get this simple version working first…

Update #2

Version 2 boards arrived. Forgot to take the bloody JLC order number off the panel. Never mind. Built them with an interactive BOM I exported from KiCad’s PCBNEW, so I could test out how someone else might build it.

The second part of the filter worked, but the first didn’t. In an object lesson about the importance of version control, I pulled the IBOM from the board with the mislabeled 200k resistor mentioned above, which took me an hour to find with an audio probe and an oscilloscope…

Swapping that out – it all works. I will now allow myself a brief period of rejoicing.

Update #3

Here are the corrected build documents – the KiCad files, gerbers and IBOM for the main board and the jacks/panel board.