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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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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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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.

Here is my latest little standalone noisemaker:

This is a small 3-oscillator logic synth, based somewhat on the same concept as the TrigPulser module in Loid. Like the original TrigPulser, it is based on the CD4093 quad 2-input NAND Schmitt trigger. In this synth, one oscillator is turning a second oscillator on and off, and the second oscillator feeds one side of  a NAND, while a third independent oscillator feeds the other side.

Here’s the schematic:

The range of sounds it can make is pretty amazing- though it can do the beepy & droney stuff you might expect, it can also do much more (especially when controls are swept)- harsh static-like noise, vocal-like sounds, oscillator-sync type stuff, etc. Here’s a couple of demos- audio, then video:

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As I once again need more money for parts, this one is going on eBay- I do plan to build one for myself as well though.

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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These circuits currently exist on Loid’s breadboard extension. As I’m out of space in the containers, and am also reaching the limit of the power supply I’m using, Loid will be getting another piece added soon. In addition to the modules discussed here, I also plan to add a 4017-based sequencer, a noise source, a couple of VCO’s, and most likely some other stuff. If all goes according to the current plan, the result will be a sort of utility/”brain-box”, which will also function as a take-away sequencer and beat box.

The simple VCA is in this series of pages, towards the back of the “book”. This is from a project Ray Wilson was involved in before MFOS came into being, called the WP-20 mini-synth.

Logic-only VCA and AREG at this thread. I will only be using the EG. I used an LM358 for the op amp. There is also a similar AREG in the WP-20 book which is based on the LM3900 and uses a 555 as a gate generator.

The following simple recording shows the AREG being used as a highly controllable LFO, using a slow squarewave as the trigger/gate source, modulating the frequency of a CD4046 VCO. At first, a 40106-based square LFO is being used directly as the gate, later I switch it over to a divider for even slower modulation.

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No recording of the VCA, but it just controls the volume, nothing super exciting. I can, however, say it works quite well with this simple EG. My plan is to build a quad version using all 4 op amps in the MC3401 I’m using, which is a relative/equivalent to the LM3900.