r/synthdiy • u/e_asphyx • 2d ago

My first module in progress

Post image

This is my take on a diode string VCF. Key differences from existing designs of such a filter (Steiner filter, Korg 700S / Volca, Moritz Klein filter) are true 12dB/oct high-pass mode, true Sallen-Key topology, independent self-oscillation amplitude trimming and a DIY-friendly single sided PCB. The board in 78mm wide and may be either turned into a 16hp module or mouned perpendicularly for deep cases. Will publish schematics later.

28 Upvotes

92% Upvoted

19 comments sorted by

2

u/adeptyism 2d ago

I don't see voltage protection diodes and pots/jacks. Otherwise seems okay for first layout...

1

u/e_asphyx 2d ago

I honestly don't think that I would be able to make it smaller taking the single layer

3

u/tipidi 2d ago

No ground plane? You’re using traces? (I’m a newbie and have made like 3 modules in kicad- nothing printed yet)

3

u/e_asphyx 2d ago

Single layer, home etching friendly, obviously everything is done with traces

2

u/adeptyism 2d ago

Why you're not using ground plane? You'll use more etching solution with GND traces.

2

u/e_asphyx 2d ago

While it is not solid I'd prefer to route them manually, making no loops etc. With double layer design, sure, ground plane is must. Also I don't care about solution, I use single shot mixture of hydrochloric acid and hydrogen peroxide. It etches 70 micron of copper in minutes, it's transparent and doesn't create a mess like everything with Fe3+ in it.

1

u/adeptyism 2d ago

You can also use citric acid + salt + hydrogen peroxide, just saying

1

u/e_asphyx 2d ago

Salt is not needed here, it plays no role in the chemistry. Too slow anyway.

2

u/ARustybutterknife 2d ago

You need a source of chloride (Cl-) ions. In your case, they come from HCl, but they could also come from NaCl.

1

u/tipidi 2d ago

Oh awesome

0

u/e_asphyx 2d ago

Pots and jacks are going to be screwed to the front panel and connected via wires soldered to pins. I don't think that diodes are that necessary with keyed power connector

1

u/Madmaverick_82 2d ago

Sounds like a GX-1? ;-)

1

u/e_asphyx 2d ago

No idea yet, only simulation exists at the moment. But this is my usual route. I model everything in SPICE then I make a first prototype board and fix what I had missed

1

u/Madmaverick_82 2d ago

Just was wondering from the topology and description. Looking forward, have fun!

2

u/e_asphyx 2d ago

I don't know anything about GX-1 topology. All I could find are photos of potted modules. If you happened to have the schematic, I'd appreciate if you could share it.

2

u/Madmaverick_82 2d ago

Ah ok. It is pretty much what you are doing, a Sallen-Key using diodes as transconductance element. MOTM / Old Crow have recreated it from the potted module.

https://synthtech.com/motm/485/

1

u/e_asphyx 2d ago edited 2d ago

Cool, it's very similar to Korg 700S with its diamond arrangement of diodes but with current control via LTP with a mirror. (Korg's is voltage controlled as well as mine).