First up, two saw VCOs, with an envelope controlling VCA and filter cutoff, LPF output:

cerberus-2oscsaw.mp3

Next, self-oscillation filter blips, controlled by an LFO and 2 different envelopes:

cerberus-blips01.mp3

Finally, here we have a drum loop being processed, starts with the LPF output and low preamp gain, then gain is increased for distortion. Later you hear the BPF output, and around the 1:40 mark the LP and HP outs are used together for a notch filter:

cerberus-dloop2.mp3

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.

chimera-lpf02.mp3

Next, the bandpass output with white noise as the input:

chimera-bpf01.mp3

This first one includes an appearance by the noise generator, some inter-osc FM, both filters, and the distortion/preamp (on the noise, post-filter):

2ndunit-demo02.mp3

The next one is the two Nick W. VCOs into the ring mod/multiplier, processed with the MFOS filter and distortion, also includes feedback via an active mult and a mixer channel:

2ndunit-demo03.mp3

Up next, we have the same two VCOs, this time each goes to a VCA (each modulated with an Ian Fritz AD envelope), and from there each gets one of the filters. More feedback in this one, I think you’ll notice when it kicks in:

2ndunit-demo04.mp3

And here’s the video- same basic patch as the second audio demo above, but different settings:

Recording below. Patch is 1 Nick W. VCO with pitch modulated by the Semi-Random Source, pulse width by triangle LFO, VCO output into audio mixer channel 1, and the MFOS Experimenter VCO (squarewave), pitch modulated by 2 triangle LFOs (through DC mixer) into a second mixer channel. The audio mixer output then goes through the LDR-controlled SVF (lowpass input) at mid gain, with the cutoff modulated by the fourth LFO.

2ndunit-demo01.mp3

w00t!

On the same board, sharing a TL074 with the multiplyer’s input buffers, are a very simple preamp and noise source.

As mentioned in the notes, the noise source is a little quiet, while the preamp can be excessively LOUD. You may want to follow it with an attenuator if not using it somewhere where you can easily patch it into one. With the gain cranked, and a high enough input level, it’s gonna be as close to rail-to-rail as the TL074 will go. Way more gain available than needed, really, but I figure it’s better that than the opposite problem.

Here’s a recording of the Nick W. VCO (pulse wave) and the MFOS VCO (triangle wave) through the multiplyer, which is in turn put through the LDR filter (bandpass mode) with a little overdrive- the filter cutoff and VCO pulse width are modulated with triangle LFOs:

phx-pwrmodtest1.mp3

MFOS VCSVF: I like it, though I think I prefer the sound of the Steiner-inspired one. Not that it isn’t good, I would just say this filter is less agressive-sounding, a bit smoother. There is a good explanation of the inner workings at the MFOS site, on the page for the newer design.

MFOS “Experimenter” VCO: I appropriated this from Ray Wilson’s “SDIY Experimenter Board” project, which again is explained in great detail at the MFOS site. I built it with a waveform switch (as per the schematic), rather than having seperate triangle and square outs as I had originally planned. I did this to keep from needing another op amp. I don’t think I’ll miss the seperate outs much.

Ian Fritz AD/AR: There are several designs out there that provide both AD and AR functions, but this was the only one I could find that didn’t require external logic. Space being a concern, I want to use as few IC’s as possible. I raised the values of the Attack and Decay pots to 500k and 1M, respectively. This increases range, but of course also sacrifices resolution. I will probably build the second one with the original 100k and 500k values.

Here’s a recording of these and the other completed parts playing together: the MFOS oscillator is modulating the pulse width of the Nick Woollaston VCO, which is then put through the MFOS and Steiner filters in series, then a VCA. The pitch of both oscillators and the filters are modulated with LFO’s, and the VCA is controlled with the Ian Fritz envelope.

phx-12092010-1

Here’s a shot of the board, pre-bend, with the clock resistors marked:

There are two resistors there, on a daughterboard attached to the main board at a 90-degree angle- this daughterboard also contains the main IC/ROM, which is of the “black blob” variety. Adding parallel resistance to just the top resistor (thus lowering the resistance there) with a pot allowed only upward pitch changes, while putting the pot between the upper side of the top resistor (top in the photo, that is- it’s actually the bottom side of the device if you were looking at it head-on, like you were playing it) and the bottom side of the resistor underneath it (closer to the main board, not down from the circle in the photo- both resistors are inside the circle) allowed the pitch to be shifted slightly higher and much, much lower, with less resistance here giving a lower pitch.

The yellow wires which lead up & out of the photo are the output connections. I believe the 8-pin IC next to them is the amplifier.

I was unable to find any other bends that were very interesting. However, if one were to find a non-working one (or just not like it enough to keep it around), there are a few things that would make fine salvage. The drum pad triggers are piezo discs, there are a few slide switches, and the smaller board which is screwed in at the corner there, facing the opposite direction of the main board, has the two small slide pots which are used for the pad & rhythm track volumes. Also, the built-in speaker is around 5″, and not too shabby for a toy like this- might be useful for a small lo-fi amp of some sort.

Here’s a video:

So, new plan: I already was planning to add gate-to-trigger converters to the Ian Fritz AD/AR EG’s- now, one will be wired up with a switch on the panel, the other will be patched out. I will replace the WP-20 envelopes with an ADSR by Nick Woollaston. This will allow me to keep the same basic panel layout, since it will require the same amount of jacks and knobs. There are a number of other decent options out there, but I think this will work out fine.

Here’s a recording of one of the Ian Fritz AREG’s gated with a square LFO, modulating the MFOS Experimenter VCO- this is then put through the LDR VCF, which is modulated with a triangle LFO:

mfosexvco-fritzar1

I went with the red LED simply because it was what I had on hand. I also tried adding a power starve pot, and although it did indeed change the sound, I didn’t feel it was a drastic enough change to be worth it. Others may not agree however, it’s worth trying for yourself.

Here is a recording of a triangle wave from an NE566 being processed with this circuit on a breadboard- first, the dry triangle wave is heard, then the effect. The Buzz Box controls are also swept, first the input drive, then the “fuzz” control, then different drive settings are tried while the fuzz control is swept:

buzzboxmod1-tri1

I’ll definitely be building this, I really like the nasty things it does.

…and here’s the inside of the box:

This is the finished device- I decided to go with another aluminum Rat Shack enclosure, mostly because the price is right (bad photo, sorry):

I also have a bonus bit of noise- nothing special, just some racket from the Phoenix modular synth being run through this bad boy:

phx-test1.mp3