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Nasty VCF Schematic & feedthrough question
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Nasty VCF Schematic & feedthrough question
NielsBakx
First of all I'd like to share a circuit with you I've been tinkering with the past couple of days. Schematic is below. I wanted to have a go at designing a VC filter for a while now, both with the eye on expanding my (newly started) modular system as well as using it as an outboard processor when producing music. Most of my synth sounds that are not in the box come from a Korg Monologue, which I love, but the one thing I miss on it is a high pass filter to roll off some low end for lead lines etc.

My feedthrough question: when breadboarding the filter I noticed that there was a small portion of dry signal getting to the output (unfiltered). This got less when I added a few 47u & 100n bypass caps across the rails. Is this just a side product of breadboarding (stray capacitance?) or is more debugging in place before I build it?

The filter
Here's the schematic of the filter I came up with [note the "pots" in the filter network are LDRs!]:


I had a few requirements:
- Multiple filter configurations (LP, HP, BP, AP)
- Simple topology (simple enough for me to understand, anyway! I'm still learning!)
- Resonance

I had done some reading on variations of the Sallen Key topology quite a bit, including the MS20 filter, Doepfer Vactrol filter, Steiner/Parker etc.
Some resources include:
Doepfer Vactrol Filter (and other explanations, very helpful!)
René Schmitz' take on the MS20 filter
Yusynth's take on the Steiner filter

My first challenge was to decide the way to implement the resistive element of the RC filter. I recently got some 2164 chips and at first was really excited to use these, since they respond logarithmically to CV I figured it'd save me the hassle of figuring out the expo converter to get a nice sweep out of the filter. I quickly realised the 2164 was punching above my weight for now, as I kept struggling to get it to work as a simple VCA. I intend to revisit the 2164 at a later date to build something based on Mutable's Ripples module.
I then decided to go the lo-fi route with some home-made vactrols (Yellow LED, LDR and a whole bunch of electrical tape). I knew both LDR's and LED's can have a large error margin when it comes to matching, but I figured this would give me a chance to really see what affects what in the circuit as I was tweaking it. In the schematic you can see I specified adjusting one of the LED resistors so the brightness of the LEDs matches (roughly) at a given input voltage.
I then opted for the MS20/Sallen Key/Steiner style filter, as it seemed fairly straight forward (2 pole filter with a simple op amp booster).
As soon as I started adding the resonance amplifier I quickly learned this circuit is quite a wild beast once it hits the oscillation point (which is different depending on the cutoff frequency). First I went about limiting the oscillation voltage with soft clipping diodes in the resonance amp. This brought the resonance down a bit, still ear-piercing, but a bit more matched with the "dry" input signal in level. I probably spent most of my time on this circuit trying to get a nice balance between various levels of resonance with a small helping of chaos at the extreme (the first version of the circuit had a very large helping of chaos).
Due to the 47k/100r combo on the input the output was still relatively low at this point. So I added a second compensation amplifier to bring the signal (with the filter open and resonance at 0) back to roughly unity gain. In a final strike of inspiration I decided to apply some of my guitar-FX building experience and adjustable clipping to this final amplifier stage. When this control is at 0 the signal is roughly unity gain, but when you crank it up the signal starts to distort, first softly, and then it gets crushed to bits, almost like a gated fuzz or something. With the drive up a legato sequence will be turned into a very distorted and choppy rhythmic sound. Very cool.
As for the VC input I opted for René Schmitz' expo converter from his 4069VCO (found here). I had to spend a few moments figuring out how I would connect this to the LEDs, but I eventually sussed it.

The filter has: LP, HP, BP inputs (and AP when plugging into both LP/HP), resonance & distortion, VC cutoff. In Low Pass mode it sweeps all the way down to mute the signal, in High Pass mode it stops just short from cutting out the signal (if there's an easy way to fix that let me know, but I don't mind otherwise).

Hope this is of use to someone, and any suggestions are very welcome.
As you can see a single circuit uses 5 op-amps, and I like things in even numbers.. So I'll either put two of these in one panel with a "VC link" switch so you can run the filters into each other and globally control cutoff, or just make one for now and live with an unused op-amp seriously, i just don't get it
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