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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 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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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 project was started as pure experimentation, toying with op amp distortion. Although it takes inspiration from many other designs (nothing entirely new here, just overdriven op amps), it is not a clone or mod of any specific device. Also, though it certainly might work with guitar (and sounds pretty good on my bass I think), it wasn’t designed for it. Its purpose will be as a distortion channel-type preamp for the modular amp project I mentioned in the ReMock+ post.

For the distortion circuit itself, I referenced the article “Cook Your Own Distortion” from the General Guitar Gadgets site, as well as designs by Runoffgroove and Mark Hammer. There are, of course, several things that could be modified- some of them are mentioned in the schematic notes, those and a few others will be discussed here. This is not to say that this post will cover the entire range of things that can be done with an op amp-based distortion unit, just several ideas for tailoring this one to your own purposes. Perhaps the most obvious would be trying different op amps- I tried a few, and preferred the MC1458 for this device- but, as they say, YMMV.

Getting into the rest of the circuitry, it may seem like overkill to have two gain knobs and a drive control- and it probably is- but they are actually all useful.  However, the second gain knob could certainly be left out or changed, while still retaining a good range of different sounds. The configuration shown gives a variable gain factor from about 5.5 (5.45 repeating actually) to 48 in the second gain stage. As mentioned in the above-linked article, the formula for the gain factor is (R1 + R2)/R2, where R1 is the resistance in the feedback path, and R2 is the resistance to ground.  Just as an example, to keep it at the low side of the current configuration, you could change the feedback resistor to 47k, and just remove the 100k pot (leaving the 10k resistor where it is) for a gain factor of 5.7. This would still give a bit of clipping in the second stage.

The capacitors C2, C3, C7 and C8 (along with associated resistors) create simple filters in the feedback loops. Changing these will alter the sound quite a bit. In the current configuration, they act as bandpass filters which act mostly on the lows- the first stage keeps most of the lows intact, while the second stage cuts the lows for an edgier sound. C3 & C8 remove the very high frequencies- around roughly 33kHz.

Getting back to the the drive control, this could probably be left out of you’re building this as a guitar effect, since your guitar’s volume knob would perform the same function. I’ll be going the other way, and leaving out the output volume control, since in my setup it will always be plugged into something with an input volume control- probably a mixer most of the time.

Another thing you could do to simplify things is to remove the LED’s and/or switch from the feedback path of the first stage. You could go the other way and get more complicated here as well, but I personally chose to save that for another project. As mentioned in the schematic notes, one LED I used is some strange multi-color thing that came out of a computer. I only tried it because it was here, it turned out I liked the sound, so I went with it. I also tried two red LED’s, and it was much more subtle.

On to the tone stack: I chose to place it between the gain stages in order to make it like having two distortions in series, with an EQ between them (which is essentially what it is). As a starting point, I used the “Fender” setting in Duncan’s Tone Stack Calculator- first, changing some component values in the simulation to get started, then tweaking values further on the breadboard until I was happy with the sound. My main goal here was to get rid of as much of the mid scoop as possible, while still retaining a decent range of control. The current configuration still gives a slight dip around 400Hz, also known as the “mud zone”, so we can live with that. As is often the case with passive tone controls, there is quite a bit of interaction, though in this case, I consider that a feature. Specifically, what would have been the treble control is now more of a spectrum tilter-turning it one way simultaneously boosts the highs and cuts the lows, and vice versa. The bass control is still a bass control. I replaced the mid control with a fixed resistor- lowering the value here will cut the highs, but if you make the value too low, it will affect the entire frequency range, and become a volume cut instead of a tone control. I would suggest going no lower than 33k, but again, YMMV. Below is the simulated frequency response plot from TSC. The red line corresponds to flat tilt and bass knob settings, the green and pink lines are the two extremes- bass all the way down, tilt all the way to the treble side, and vice versa. The white line is both controls all the way up:

The schematic includes a bypass switch for the tone stack, which could also be left out. Speaking of the schematic, here it is:

With the tone stack between two distorting gain stages like this, it acts more as a way to adjust the character of the second stage’s distortion than an equalizer for the actual sound coming out of the effect. While this is done by design, you could certainly move it to the end of the circuit if you prefer. I had actually considered including a simple lowpass tone control at the end- but there’s already so many knobs, and as part of the larger project, I plan to build at least one EQ-only module.

Here’s a recording of a sine wave from the K2000 being processed, with various controls being swept/switched:

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We’ll follow up with a few bass riffs… first, both stages cranked, with the LED disengaged:

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Now, with the second stage backed off (minimum gain), first one still maxxed out:

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This time, both stages backed off, but not quite at minimum:

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The next two have the LED engaged, first lowish gain on both stages:

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Now with both stages cranked:

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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.

Original circuit here: Q&D VCF

This circuit is a resonant twin-t filter. In this circuit, its purpose (according to the creator), is to give a “decent fake lowpass response”, which it does fairly well. Tim’s schematic shows two variations, one with self-oscillation, and one without. I built the one with oscillation. Since I was breadboarding it with a dual op amp (an NE5532, to be exact), I tried adding an overdrive-able input buffer/preamp, which worked pretty well, so I went with it.

I had a plastic box sitting around with no lid, so I’ve temporarily built it into that for use with Loid. It will eventually become part of the piece I mentioned in the last post. Pots from left to right are: gain, cutoff, resonance.

Here’s an audio demo, with the Q&D filter being fed with an XOR pseudo-ring mod, which in turn is being fed with two squarewaves from Schmitt trigger oscillators- at 1:06, the input is removed, and you can hear just the filter’s self-oscillation (modulation via 4069 LFO):

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EDIT: I have updated the schematic, R17 should come before the output volume pot.

This is a standalone distortion unit, which also marks the beginnings of another project. I haven’t come up with a name for this one yet, but the idea is a modular amplifier and distortion unit. There will be a number of preamp, tone/EQ,  and power amp modules, focusing on circuits suitable for bass guitar and synthesizers- my main instruments. I will probably also make at least one mic preamp for this project eventually. Nothing is set in stone yet, but there will of course be at least one 386-based power amp module- and of course, the circuit I’ll be discussing here (which is a preamp/distortion unit). I expect the going to be slow on this project overall, because I will be continuing with modules for Loid and Phoenix as well.

The distortion circuit itself is a modification of the Mockman V2.0 by Runoffgroove (the name ReMock+ is a nod in the original’s direction, as well as a play on “remix”, with the “+” added to indicate the addition of an EQ). I replaced the 47pF feedback capacitors with 100pF’s to reduce noise, and added a switchable 2-diode “softer drive” mod. I also added a 10k audio taper pot to the input, which allows for something approaching a clean signal, as well as control over the amount of clipping.

The second part of the circuit is a simple 2-band (hi/lo, or treble/bass) Baxandall EQ. If you’ve been paying attention, you may notice that the R/C network which makes up the filters is the same as the one used in the Mossifier, only this time we’re using an op amp instead of inverters for the amplifier part.

Here’s the schematic:

This schematic is suitable for a standalone distortion box, I may or may not break it into seperate distortion and EQ modules for the modular amp I mentioned. In the meantime, I’ll be building one to use while I gather resources for the big project.

Here’s a breadboard shot:

The version in the photo is using a TI RC4560 for the distortion op amp. For this particular op amp, I had to remove the first 100pF feedback capacitor (C4 in the schematic)- no other changes were needed. I also tried a TL072 and an MC1458, and they both worked best with C4 in place. As far as the sound of the distortion, I didn’t notice a huge difference with any of the different op amps, other than slightly lower noise with the RC4560. I also played around with the values of the input cap and the caps in the “bright/dark” section, and ended up going back to the original values. Feel free to change them to suit your own needs though.

I’ll need to wait until I can make another parts order to finish this build, at which time I will be making another post (mostly need more jacks and switches for this particular project). For now, here’s an audio demo of a plain sine wave from the K2000, playing a simple arpeggiated sequence- first dry, then through the ReMock+:

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When the distortion first kicks in, you’re hearing “harder” mode, with the EQ roughly flat and the input gain cranked. At about 0:25, I crank the high EQ, then back it off a little, and so on, messing with the two EQ controls. At around 0:51, “softer” mode is first heard. Thereafter, it’s just a bunch of various knob tweaking.

I also gave it a quick run-through as a bass distortion with the Dirty Cow as the amp, through my Peavy 12″ speaker, and it sounded good. Didn’t feel like putting a mic to it today though, sorry- next time.

So, after a few attempts with different op amps, and different filter configurations, this is the best I came up with for using the 4007 in a filter:

Note that this will not work well with anything other than signals from logic outputs- inverters, gates, etc.- the stuff Lunettas are made from. I was only able to get the BP input to work, so I removed the other two from the schematic. Other changes include limiting diodes in the feedback path, The use of a TLo8x for the filter amplifier, filter capacitor value changes, and input/output capacitors.

Ultimately, though not a complete failure, this is not what I was hoping for either. I’m sure that my n00bness has much to do with it, but I’m also beginning to have my doubts about the 4007 in this capacity.

Here’s another recording of the BPF, first on a squarewave, then on some more interesting stuff coming from a quad AND/OR select gate:

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Perhaps I will come back to this line of research eventually, for now I think I’ll be looking elsewhere. I am still considering adding this circuit to Loid though, as it does pretty well there I think.