Stripboard CGS30 Bandpass Filter

Cgs30stripboard

I’ve been meaning for ages to get a PCB for Ken Stone’s Cat Girl Synth Bandpass Filter, and never got round to it. It’s based on the Roland SH-5 bandpass filter, which I love. Here’s mine.

Rolandsh5bandpassfilter

Phwoar, etc. The SH-5 (and the System 100) look so good.

Because it’s a pretty simple circuit, I thought I’d try and do a stripboard version. It’s the first decent sized schematic I’ve tried to put together, it took about a week of scratching my head and checking.

CGS30 Stripboard layout

It’s meant to be run off +/-15v, but it works happily off my +/-12v stripboard MFOS wallwart power supply. I didn’t have any 47k resistors for the input buffer op amp so I used 43k resistors instead. The resistor marked 800-ish is originally 4.7k in the SH-5 schematic, but Ken suggested taking that down to 1k-ish to increase the resonance, which is pretty subtle otherwise. I could only get a 47k linear pot for the resonance control, and I was sent a dual 100KA (log) pot instead of a linear, so the frequency jumps very quickly to the high end.

Look at the state of this.

Cgs30stripboard-from-the-top

I know, it’s ugly. But it worked first time, so (raises glass) here’s to ugly.

How does it sound? Here’s a square wave from the SH-5, sequenced by the CSQ-100 and sweeping the SH-5 BPF at full resonance.

And this is a pulse wave from an Analogue Systems RS-95, through the stripboard CGS30, sequenced from my dodgy Arduino sequencer.

The hiss in the background is probably a bit of spring reverb, whups.

I prefer the SH-5, it’s got a bit more of a resonant edge than my CGS30, but it does sound pretty similar. There’s not much point in chasing the exact sound given the other variables involved, but I’ll do it anyway. I’m wondering if running it on 15v might help, and I’m using TL072s, I’m wondering if using something like the original 1458s might get me a bit closer.

There’s some more useful commentary on the CGS30 in this synth-diy post – especially like the idea about tapping the output of OA3 (presumably IC2, pin 1?) for a low-pass output.

Anyway. Here’s some SH-5 picture filth.

Rolandsh5vco

Update: Sep 2018

Muffwiggler user kragg has had a go a making a more compact layout, their attempt is in this thread here.

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5 comments

  1. 5th October 2013space says:

    thank you so much for this fun project .this circuit is easy and fun to build. sounds great too. any love on a circuit to add control voltage inputs for frequency?

  2. 8th October 2013ua726 says:

    I guess you’ve seen my attempt at controlling the frequency with some homemade vactrols.

    It worked out ok, kind of, just that vactrols aren’t the most linear. Good enough for a bit of variation, anyway.

    An alternative could be to use FETs, although the linear region is small so you’d still have to carefully scale and offset the input voltage. Ray Wilson’s WSG mods page has a bit on this.

  3. 22nd June 2018chemosis says:

    the 1458 produces a little bigger better sweep in my bassballs filter and dod fx 25 clones so im thinking that the 1458 could make a noticeable difference. i swear though that some 1458s produce bigger fatter sweep than others. for example my ha 1458 does not give out what my lm1458 does in my bassballs filter. its mentioned here. thanks alot for making this page

    http://vulcanofx.blogspot.com/2013/10/dod-fx25.html

  4. 4th December 2018timo says:

    this site is great thankyou. has anyone ever turned this into a lowpass. can this be turned into a lowpass instead of a bandpass?

  5. 5th December 2018ua726 says:

    Maybe IC2, pin 1? I can’t remember if I tried it – I think it might be underwhelming in comparison to the bandpass. There should be a highpass output lurking as well.

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