Wednesday 5 March 2014

CGS77 Serge 1973 Filter Demo

Sounds of the CGS77, Ken Stone's adaptation of the original Serge filter, are scarce. I hope these recordings help anyone considering this DIY build.

I'm really not a filter man, but the Serge VQVCF is something special. This circuit is its 1973 ancestor. The state variable found in today's systems has been refined with gain compensation and a trigger input. But Ken Stone tells me that the CGS77 is essentially the same core and my ears aren't about to argue.

If you want to hear more, please buy the EP for the unedited recordings and pictures of my build.

Building CGS modules for Eurorack presents the DIY-er with the problem of how to accommodate the PCB. Others have opted for 'best-of' panels, combining several PCBs behind one large interface. My solution was initially based on a false assumption - that I could cut below the CGS77's 'dotted line' to remove the unneeded buffers and mount it vertically.

I should have done this before populating the board, so I settled on another tactic. Using stand-offs and making sure the module in the row below is less than 4 cm deep, my '73 filter is a dashing 10HP. Here are the guts and here its installed.

On the mechanics, I used Charlie The Bad Producer's method of attaching the power via a perfboard add-on. I considered normalling the band-pass to the input for self-oscillation, but found in testing that the '73 doesn't make a particularly stable oscillator. The HP & BP are nice, but not as good as the Plan B Model 12, so I might leave them out on a future build. Other casualties might be the DC mode and LED. This filter responds well to modulation, so an additional CV input would be more useful: this can be added in at the points marked S & R on the schematic.

Thank you Serge & Ken for this wonderful filter, negativspace for BOM help and my brother for the boards!


Fer said...

Delightful samples!

Fer said...

Re. the actual filter heard in the samples:
Are you using the buffers for audio input and outputs?
Did you use a smaller (than 68k) resistor for the resonance?
Did you used J201 and 2N3906? Which type of caps for the 100n on the core?
I like how yours sound so I'd like to know to have a reference. Thank you

Fer said...

sorry, I meant which type of 220pF caps (100n are for the low range option)

Navs said...

Glad you like the sounds, Fer.

It was a while ago, so I don't remember specifics about buffers, but I just built it according to Ken's design. I did not use the lower part of the board, if that's what you mean.

Yes, I used J201 and 2N3906. The caps were WIMA MKP4 + FKS2 and Styroflex. At least, that's what my BOM says! Got the parts from Reichelt and Banzai here in Germany.

Fer said...

Thank you. Yes, the buffers are on the lower part of the board.

What about the resonance? Did you use a smaller (than 68k) resistor to get increased resonance?