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Here’s a few more recordings of Cerberus in action.

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

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

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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:

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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.

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

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Here’s a few more audio bits and a video from the new Phoenix modules:

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):

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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:

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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:

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

I have finished all the circuits, and constructed a temporary cardboard faceplate to allow me to play with it while I gather the rest of what I need (jacks, aluminum for panel, etc.).

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.

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w00t!

As some of you may recall, I had built an AD633-based analog multiplyer (ring mod) module early on in the first Phoenix chassis, then removed it in favor of keeping that box single-supply- with the intention of putting it back in rotation when I broke out the dual supply again. Well, that time has arrived. I’ve used the same chip, but on a new board. I used another Ray Wilson design this time- namely the MFOS Analog Multiplyer from his “Ultimate Expander” project. I will be adding attenuators to the inputs, other than that, no changes.

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:

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I managed to get a few more modules finished (3 of which are Music From Outer Space designs, hence the title)- Ray Wilson’s older SVF design, another of his simple LFO’s, and one of the VCO’s from his Experimenter project; also one of the Ian Fritz AD/AR envelopes (without the 555 parts). Some notes follow.

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.

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While breadboarding the MFOS Experimenter VCO and preparing to test the envelopes too, it occurred to me that my plan to use the AREG’s from the WP-20 schematics probably wouldn’t work, because the LM3900 will only swing between +V and ground.

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:

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After some thought, I’ve come up with an overall plan for this second box. The end result will be a decent collection of basic modules which should make a decent “core” for noisemaking. Some decisions were made purely in consideration of space and power consumption (there will be a little PSU headroom, but I’d rather be safe than sorry)- for instance: in a perfect world, everything would have multiple CV inputs, and I would have at least one more of each active mixer. However, this will be a pretty capable and flexible setup, even with some compromises.

With the exception of the MFOS VCSVF, these are all low-cost, low parts count modules, and are not V/oct compatible or anything like that.

Here is a diagram of my plan for the front panel- the big circles are knobs, the small ones are jacks. I got lazy and didn’t completely lay out the middle section, but I left some fanagle room in there to make sure it will all fit. The passive mixer will be 3-in, 2-out.

Some details, and links where applicable:

2x Nick Woollaston saw VCO + pulse shaper- as discussed in part 12.

1x MFOS Experimenter VCO- this is the VCO from the MFOS DIY Experimenter board, it gives triangle and square waves, not 1V/oct compliant. Chosen in part because it uses half of an LM3900, and I will have that left over from the WP-20 AREG.

1x Semi-Random Source- as discussed in part 13.

4x MFOS simple LFO- as discussed in part 12

1x AD633 ring mod- resurrected from my original module, will be adding input buffers.

1x LDR VCSVF

1x MFOS VCSVF- I will be building the older version, without VC resonance.

1x noise source- just a simple transistor-based white noise source, with op amp amplifier.

3x WP-20 VCA- built with RC4580‘s instead of the LM3900, as discussed in part 12.

2x WP-20 AR- these are retriggerable, and can be made to sustain.

2x Ian Fritz AD- one of these can be mixed with a WP-20 envelope in sustain mode to make an ADSR. I will be leaving out the “truncate” switch.

2 buttons + 2 SPDT (manual triggers)- these 2 buttons will be assignable to either an AR or AD envelope for manual triggering.

1x input preamp- for bringing in external signals. Will have enough gain to act as a simple distortion device as well. I might make the input a 1/4″ jack.

2x 1 in, 3 out active mult- one of these will have inverted outputs.

1x 4 in audio mix w/ 2 outs- just like it says, a 4-input mixer. Both outputs will carry the same signal, for sending to two destinations. This will be something like the MFOS Ultra-simple Mono Mixer.

1x 4 in DC mix w/ 2 outs- as above, but DC coupled for mixing modulation sources.

After this, it will be time to move on to a third box and power supply. Current thoughts include a sequencer, a Synthacon filter clone (discrete transistors), and maybe some drum voices. Plenty here to keep me busy for awhile, so I don’t want to get too far ahead of myself.

This is actually just a hard-wired version of something I often patch up using some of Loid’s modules. The module consists of four squarewave oscillators feeding an R/2R ladder, which in turn feeds an attenuator/glide circuit (which I first saw at TSOL, via Richarius). It occurs to me that this is very similar to Ken Stone’s “Psycho LFO” module- though this resemblance is unintentional, it is entirely possible that there was some sort of subliminal influence involved in the original Loid patch. Both modules use multiple Schmitt trigger squarewaves and a glide circuit, the biggest difference between the two being how the squarewaves are mixed. Mr. Stone’s module uses a standard op amp mixing technique, while this one uses an R/2R ladder to give a weighted mix of the oscillators.

You could take a page from Ken’s book and use fixed resistors for some of the oscillators, swapping out the associated timing caps as needed (C1-C4). You could also extend the R/2R and wire up the two remaining Schmitt triggers as oscillators.

Here’s a recording of the pulse oscillator from the “part 12″ post with the pitch being modulated by the SRS, pulse width and filter cutoff frequency (band pass) modulated by seperate triangle LFOs:

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Due to several things I’ve been wanting to try out requiring it,  I’ve decided to break out the +/-12V power supply I scavanged from my dead ESQ-1 again, and resume work on more modules for Phoenix. My initial thought was to re-house the current 9V modules, and replace them with new stuff- however, I realized that I would soon need more space than that, so I’ve decided to start a second chassis instead.

The first circuit I built for the second Phoenix chassis was a 12dB/oct Sallen-Key state-variable filter (SVF)- there are several variations on this basic design (including the MS-20 and others), the one I built is based on a design by Nyle A. Steiner  (original schematic at Experimentalists Anonymous, Philip Baljeu’s mod/redraw with LDR’s in this e-m thread).  I made a few changes, including changing the gain on the output buffer (it’s now set at 100x gain, well into overdrive territory- but the volume pot allows you to attenuate the signal), and changing the res pot’s value. I think it sounds pretty good, but the LDR’s I have don’t react very quickly, so audio-rate modulation is not possible.

Although I do have several modulation sources available, I decided to build another to go with this filter, just in case they’re all in use elsewhere.  I went with a simple LFO found at the MFOS site- it is essentially the “Super-Simple” LFO, with the diodes and associated resistor removed, and a square output taken from the output of IC1-b. I also decided to go with a single speed range- I went with a 470nF cap there, which gives a pretty good range. There was enough room left on the perfboard I used for the filter for me to build one of these on the same board. I’ll probably build at least one more, possibly with a bigger cap for ultra-slow rates.

Another nifty-looking thing I had recently come across was the Simple Saw VCO (and several other cool circuits) from Nicolas Woollaston at the e-m forums. It works great, so I decided to try to combine it with this saw to pulse converter from the EA archives. This also worked quite well, so I’m thinking I’ll build a pair for this chassis. As of this writing, the VCO/waveshaper is still on breadboard here.

Since it had worked so well with Loid, I decided to try building a +/-12V version of the WP-20 VCA I posted about a short time ago. As it turns out, all I had to change was the op amp- I used an RC4560.This is also still on the breadboard.

What follows is a simple recording of the VCO’s pulse output, being processed with the lowpass portion of the VCF, which in turn is then fed into the VCA. Modulation routing is as follows: the MFOS LFO’s square output is controlling the VCA, and the triangle output of the same LFO is controlling the PWM of the VCO. Two triangle LFOs from Loid are then used to modulate VCO pitch and VCF cutoff.Partway through, the MFOS LFO’s triangle out is re-routed to the filter CV, and the triangle LFO from Loid which was on the filter is in turn re-routed to the PWM.

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Here’s a recording of the VCO’s saw processed with the bandpass portion of the VCF- this one also includes some filter overdrive sounds:

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When I get the VCO and VCA off the breadboard, some AREG’s will follow.