The rough and dirty VCF discussion (formerly known as the "please build me a VCF" discussion)

After failing again at building a simple VCF for my SK-1 bending project I’d be happy to pay somebody to build me one.
This project has been seeing really little progress in the last years and I would just like to get it done…
What I need is an active filter with voltage control on the Cutoff (like Escobedo’s Q&D VCF – see PDF attached).
So is anybody volunteering to build this for me?

of course I’d be paying for the work

You may want to look into an envelop following filter or if you have a Eurorack system one of these. I think there may also be a version that attempts to track the pitch CV for key-tracking. Or you can use an MS20 mini.

There would be several cheaper options, and they work with more than one synthesizer. :slight_smile:
The real issue I see with the SK1/SK5 is that they do not put out any CV for an envelope. They should put out a digital trigger that could be used to activate one. In the end, you would need to build an ADSR and a filter. Even then it would still be paraphonic.

Also, for 50$, you can get an Ibanez demon wah pedal. It is a great LP filter in a solid metal box you control with your foot. You can adjust the resonance, drive, and freq range too; that is very nice!

yeah I know that I can just attach an external box to the sk-1. What I was trying to do is to build a small filter circuit and solder that to the sk-1
Btw. the sk-1 does have an analog CV internally… which can be used as an envelope for the VCF too. It has one for each voice (4 in total)

Neat, I didn’t know about the CV. With 4 discreet ones, you should have one per voice. If you can live with 6DB filters, you could use 2 LM13700 to build a very simple LP filter with minimal components. There would be no resonance. That would require 4 LM13700; one per voice.

You shouldn’t give up. :slight_smile: Pull out the bread board, you will only need those and your choice of passive components since they already have 2 buffers and OTAs built in. You may need to adjust the voltage to work with the VCC inputs to power it because I am not remembering the voltage available for the SK1. Build one and the other 3 will work.

ok time for a rant: It’s really hard to keep the motivation for such projects intact. I’ve been working on it for over a year. always a bit at a time, with many weeks between one step and the next. Every time I try to get on with it, something breaks, or I’m building something that does not work, usually for the time I have finished a part my available time is over, and then the thing remains there for more weeks.
Unfortunately there is no well documented VCF project that you can just build for something like this. There’s also no PCBs for it. Building the filter on veroboard seemed easy, but the instructions were a bit confused and misleading… and I ended up doing it all wrong. I tried to add a pitch control via LTC1799 but here as well, you find a lot of contraddicting and confused info on the net… and in the end I don’t know where I should start.
Don’t get me wrong, the web is full of great, nice people, and one just has to ask. In fact I was able to solve the LTC1799 problem in the end. but now that I have solved it it will probably take 2-3 weeks until I am able to find the time to actually solder the thing together, and god knows if it will work then.

Well… since nobody wants to build me a VCF, I made a quick PCB for it. Problem is, the circuit in the schematics is different from the one on the veroboard layout, and the veroboard layout is not really that intelligible… so I don’t really get the differences.
This should theoretically work though… if I have not done anything wrong.

since this is kind of circuit bending, maybe you like to have a look what people have done in ultra simple filter designs with their lunetta’s, there’s a forum for this at sometimes just a transistor or diode and a few resistors.

Well I had breadoarded the WSG filter, which sounded quite nice… but didn’t have voltage control. I am thinking about driving the cutoff from the modular… but in case I could also just go with that one indeed…
Yep the lunetta filters are really easy! :slight_smile: thanks for the tip!

Just found a really simple WSG layout for veroboard
I can’t believe I’m doing this again… why can’t I just stop? Somebody help me…

Anyway, have to design a logo first… then do some more panel mockups, so this will have to wait.

but what about eric archer vcf?
its a bit more complicated, but propper lm13700 12db with cuttoff cv, 9v psu with virtual ground (might be good for integrating in device that also uses 9v input) and has quite good documentation.

yeah I have been thinking about that one for some time… I’ve skipped it because of the higher parts count and because from the instructions it looks like a real mess to build that.
Also the SK-1 runs on 7.5V, though I could probably build a small PSU circuit so I get 9V for the things that need that and 7.5V through a regulator for the main SK-1 circuit.

If you want i can etch a PCB - that might be quicker than waiting for some chinese fab….

@fcd72: yeah that’s true… I can’t find the schematics for the WSG filter I had breadboarded anymore… hmm, though it should be the one from the MFOS website… hmm there I can’t find it anymore, but the one on e-m looks just the same. I could make the etchable circuit and send it to you, that would indeed make things a lot easier and maybe a bit cheaper. Fritzing works well for small things like this but a PCB would cost me 20€

Just found out that Shane Williams (one of the celebrities among circuit benders) has boards for sale… I think I’ll grab one of these!

like to post a link? always drooling for info

I’ll do as soon as he gets back to me

So here’s more info about the thing…
He’s selling the VCF board for 5£ and the small PSU board for 3£
The PSU board is to “clean up” noisy voltages and give you a cleaner sound. You can contact Shane via Facebook (just search for Shane Williams) or through the Circuit benders are not crazy we are artistic! Group on Facebook

I’ll keep you updated about how this goes.

I think he used the filter on this project here, if you want to hear how it sounds:

Did you ever get this working? I am trying to find a suitable filter for my Casio SK as well and coming up short.

Well I did build the filter, but as usual I didn’t get it to work. It always happens when I have to source my own components, in the end I did buy an assembled one from Shane, which then worked like a charm!

btw. here’s a demo video I’ve made of my bent SK-1: