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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:

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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The original Buzz Box was designed by a guy called Hemmo, though I first came across it at the Runoffgroove site. It looked easy enough to try on breadboard quickly, so I went for it. Along the way, I tried a couple of little mods, here’s a schematic of what I came up with:

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:

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

Finally managed to get this off the breadboard, onto perf, and into an enclosure. Here’s a shot of the board before wiring to the jacks, switches, and pots was added:

…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:

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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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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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I finally managed to get this built and boxed up…

There were some errors in the last schematic, here’s the fixed version:

As you can see, I decided not to include a bypass switch, because I don’t need it for my uses, and that makes it a little cheaper to build. Including one would not be difficult, if you need help, see this article by R.G. Keen. For my final build, I also decided to leave out the output volume control- again, because it’s not necessary in my setup. When it comes to the modular amp/preamp/distortion device thing I mentioned last time, it would be especially redundant.

Here’s a couple shots of the internals:

A few changes were made since that photo was taken, most notably the wiring for the “softer mode” diodes- I moved the switch wires to a position which more accurately represents the schematic, and changed some jumpers on the bottom which you can’t see in the photo (oops- that was a silly mistake on my part).

Final build notes: I built the perfboard version with the 100pF cap in place on U1a (you may remember I had to remove it on the breadboard build to make the RC4560 work). My thoughts were that it would make trying other op amps easier, and if I had to swap out the 4560 for another TL072 or whatever in the final build, so be it. As it turns out, it works just fine this way, even with the 4560. I have some theories as to why this is, but will let more educated folks tackle that question via google.

The final build is also much less noisy (which makes sense), and perhaps because of that, at least in part, I’ve also noticed much more difference in the sounds between op amps in the distortion section. The differences are just slightly more noticeable with “softer mode” engaged. The TL072 had a, well, softer sound (in either mode) than the RC4560- I suspect due to lower output level, and thus, less clipping. I suppose one could get really crazy here, and make a version that switches between chips- but I think I’ll just keep the 072 in mind for another device entirely. As you can see in the photo, I mounted the board in such a way that I can easily swap out chips, should I change my mind later. Also, since I have no single-row headers to use, I used a 16-pin IC socket so I can also swap out the clipping diodes (there are jumpers on the bottom side connecting the two sides where needed). I’m a little iffy about them holding in place with much jiggling around though, so I may have to think of some way to hold them a little better, without changing the mounting- electrical tape is my first thought. :)

For the audio examples this time around, I plugged by bass in, with the output of this unit going into the Dirty Cow amp, which in turn is plugged into a 12″ Peavey PA speaker. The speaker is then miked with a Sennheiser e835 dynamic- the sound is just a tad dark, I didn’t really get very technical about it.

For the first several recordings, I kept the gain all the way down and the input level lowish on the Dirty Cow, in order to get as little extra distortion from it as possible. I also kept the tone towards the low-boost side.

For the first one, the ReMock+ is set to “dark”, with harder mode on, lo EQ boosted (not quite all the way), high EQ cut:

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In the next one, it’s set to “bright”, harder mode, lo EQ cut, hi flat:

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For the next one, we go back to “dark”, but switch to softer mode, and set the EQ flat:

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The next one is the same as above, but set to “bright”:

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…and for the last one, we go back to harder mode, keep it set to “bright”, boost the lo EQ, then also crank the amp gain and input level:

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In this setup, the difference between “harder” and “softer” modes was barely noticeable- I suspect this was partly due to added distortion from the amp.Also, I really didn’t play with the input level of the ReMock itself, which can make a pretty big difference actually.

All things considered, I’m very happy with this device- big thanks to Runoffgroove for the original inspiration!

Now, here’s a bonus bit of noise, with the Modutronic Messmaker prototype being run through the ReMock+ (starts off in softer mode, with the input level set low- first tweak is the input being turned up, then various controls are swept & switched throughout):

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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.