I originally wanted to call it “Chimera”, but then I remembered there’s already a synth company using that name, so I went with Cerberus to avoid confusion, and because “Tricephalic Filter” just doesn’t have the same ring to it. I didn’t bother renaming the mp3′s below. But I digress…
Like its namesake, this filter has three “heads”, or in this case, filter modes. The main influences here were René Schmitz’ Late MS-20 Filter, Osamu Hoshuyama’s VCF 1984B and fake SSM2040 VCF, and Ray Wilson’s VCSVF (this was another reason I wanted to use Chimera, to mark it as a composite beast- oh well). Like all of these filters (as well as many others), the Cerberus is based on OTA’s (Operational Transconductance Amplifiers)- in this case, the LM13700.
The LM13700 datasheet contains a schematic for a single-ended voltage controlled resistor, which is how it’s being used here- in fact, the circuit used in the filter(s) is not much different from the application circuit shown in the datasheet. Each filter stage (of which there are four here) consists of one such VC resistor as part of an R/C filter network, with each stage buffered before going on to the next. This is a handy thing about the LM13700- it includes a buffer for each OTA on the chip. These four stages arranged in a serial configuration give us the “core” of a 4-pole lowpass filter. That’s cool, but what if we want other filter types?
This is where Ray Wilson’s filter comes in. Nice guy that he is, he explains how his VCSVF works quite well at his site. In a nutshell, the highpass filter is achieved by subtracting the lowpass signal from the input, and the bandpass is the highpass signal through the first filter stage. This actually creates a BPF which is steeper on the highpass side than the lowpass side. Since the Cerberus filter has four stages instead of two, we’ll use the second filter stage instead of the first for this.
On to the resonance. This is, in my opinion, what makes a filter. I noticed that the René Schmitz filter, as well as other MS-20 clones and similar filters, uses the trick of using diodes (green LEDs here, I also tried red) in a soft-clipping configuration, like many distortion stompboxes. I decided to give it a try here as well (with the amplifier part configured a bit differently)- works great. I added a potentiometer to the feedback path so you can increase the gain for more distortion. There is another interesting thing about the MS-20-style filter resonance, which is that it is returned through the first stage capacitor, which means the feedback is, in effect, bandpass filtered. Ray Wilson’s filter goes about this in a slightly different way, but the resonance is still bandpassed. I went with a configuration like the MS-20-style filters, taking the feedback signal from the output of the final filter stage. I also tried it with the feedback being taken from the bandpass output, but I preferred the former. This means that the bandpass filter’s resonance is filtered differently from the filter itself, but it sounds good to me, so I’m going with it.
Here’s an early shot of the breadboard (tweaked since then, stuff added also):
… and here’s the schematic:
This may get tweaked a little more from here, but probably only as far as changing the input stage (U1a)- I’m thinking of making the gain adjustable, and adding a pair of diodes which can be switched out (I liked 1N914′s here instead of LEDs), to add another possible “color”. Also, you may notice that the resonance amp and input stage share a dual op amp- this is done so that you can swap out different chips, and in doing so, perhaps alter the sound of the distortion in both places.
You may wish to replace R41 with a 1k resistor, and add an inline 1k trim pot to make the CV response adjustable. I don’t care about 1V/oct compatibility or anything, and I liked the response when I tried the 2k, so there it is.
Here’s some audio… note that I haven’t added a cutoff knob yet.
First up, the lowpass output with a saw wave input, modulation from a triangle LFO and the Semi-Random Source. Later, the lowpass is turned down and the bandpass comes in (using the Phoenix audio mixer), then input is turned down to nothing, and you hear just the filter’s self-oscillation (first BP mode, later LP). The resonance gain is varied throughout. Sorry for the pops, still on breadboard, so there’s occasionally some handling noise when using the pots.
Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.
Next, the bandpass output with white noise as the input:
Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.
