synthgeek's blog

Fixes some errors, and adds some minor enhancements.

I’ve decided to start selling these on ebay to raise some money for parts. Here’s a link to the current auction… and here’s a photo of the current model:

Here’s a revised schematic showing additions/changes, as well as a parts list:

Switchgate is a simple dual gate based on a 556 in monostable mode (with trigger input), with the monostables triggering two switches. The monostable outputs are also accessible via their own outputs.

Here’s what half of it (a single monostable/switch circuit) looked like on the breadboard:

I put a 4019 quad AND/OR select gate (wires straight to pins) on the same perfboard with the dual gate VCA, and put them in an old external modem case:

I’ve also added a 4051 multiplexer wired straight to the pins to the Melody Generator/ring mod can.

Here’s a recording of the Switchgate/4019  in action with the other modules- the Melody Gen and 4051 were left out:

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Some recent research led me to another conglomeration of circuit snippets, which I’m currently calling the PO-555N (Phoenix Oscillator, 555 based, N for “nasty”).

The square-to-saw (which you may recognize from the 566 osc) and the saw-to-triangle bits are tweaked versions of circuits found at the Experimentalists Anonymous archives. This one has a much wider range- from LFO rates to high audio rates- and also adds more waveshaping options.

Like the previous 566 osc, this one is not compatible with any modular synth standards (such as 1V/oct CV response), but the pitch/rate can be modulated via CV input. This one also adds a fine-tuning knob.

Here’s a recording of the “triangle” and “ramp” waves, with the starve knob set low to show the effect:

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Towards the end (after 2:00-ish), you can also hear the “tri glitch” feature, which is simply positive feedback through a capacitor on the saw-to-triangle side of the op amp, a little something I came across while messing around.

This will be replacing the APC oscillator in Phoenix, which was just there as a placeholder because I wanted more sound sources right away. I’m also going to be reconfiguring the front panel so that all the controls are on the left, and all the jacks are on the right.

So, the plan for the 567 changed a little bit. Meet the Modutronic Messmaker- I added a 555 oscillator to make it a synth (with audio input for the AM part). Here’s the schematic:

I actually used 2.2k pots where 5k’s are specified in the schemo (more scavanged parts). Here’s what the final circuit looked like before going into a case:

I recently came across a plastic VHS tape case, so that became the housing.

There are a total of four jacks- from left in the above picture: 555 CV in, 555 output, 567 AM (pseudo-ring mod) in, 567 out. The inputs use switched jacks, so when nothing is plugged into them, they receive input internally. With no inputs, the 567 is fed from the 555 output, and the 555 CV input is fed from the 567′s output. There are controls for the frequency of each osc, voltage starve, 555 CV input level, 567 output level, and a control for the frequency of the 567′s simple filter.

Here’s a video of the Messmaker in action:

I’m writing this as much to help myself keep track of all this as anything else…

Anyway, in a recent discussion at the electro-music.com forums, a member by the name of ion_nine posted a link to an interesting 555-based oscillator. The interesting thing about this circuit is that it provides variable slopes for the triangle wave output. Combined with the square to saw wave converter from the Experimentalists Anonymous archives (the same one I previously used on the Phoenix 566 VCO), this could make for a pretty cool low-cost, lo-fi VCO with variable wave shapes. I plan to build one eventually, of course the results will be posted here when I get around to doing so.

I decided to try something different, this time with a 555 timer chip. I removed the second side’s synth (the non-APC one), and replaced it with a super-simple 555 osc, very similar to this one from the Get Lofi site.

The controls are now arranged thusly: on/off switch at the top, second switch for choosing triangle or square wave, red knob for voltage starve, upper silver knob controls frequency, lower one controls CV input level.

This osc has a very wide frequency range, from sub-audio LFO freqs to very high. Here’s some audio which shows this range using this osc to modulate the APC side. The very first bit demonstrates the LFO sweeping from low frequencies into the audio range.

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Here’s a couple more recordings from Mimsy Modular, showing some sounds made using the CV input on the APC module. These two recordings also show some of the interesting randomness & instability which is made possible by the voltage starve effect.

Up first, the APC being modulated with the output from osc 2 of the other module:

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And now, the APC being modulated with the output from the Coffee Can of Doom:

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Well, it’s done, and the last bit of experimentation yielded some unexpected results. During my initial tests with using the other side of the second 556, I accidentally found a way to use it as a pseudo-noise generator of sorts. I noticed similar behavior on the APC side at certain settings, but thought it had something to do with the interaction between the oscillator and monostable- apparently it’s possible without the monostable as well.

There are various strange interactions between these new controls and the existing ones- for instance, the new switch not only acts as a pitch drop for the new osc, but also enables the touch points on the first one.  There also seems to be a sort of cross-modulation effect going on (see the audio example further down). All in all, things seem rather unstable with the second module now (certain things seem to make the second half stop working until the power’s cycled, various other strange things happen), which leads me to believe I’ve done something wrong- but it also makes things interesting, so I’m going with it.

Another “happy accident” that happened was that I came accross a nice touch point to make use of the spike- in fact, I found two, so I added a second spike.  I also added some other decorations. Yep, ugly as hell, just like the sounds it makes. :)

Back panel- there are now two outputs for the second module, as well as the output and CV in for the APC:

And now the audio (random touching & twiddling)- first “osc 1″ of the second 556 module (note the sort of AM-type ringing):

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and then “osc 2″:

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This update includes some cosmetic changes, as well as a voltage starve mod (the knob on the top) and a CV input to the oscillator which drives the APC (the silver knob on the “faceplate” controls its level). I’ve decided to name this synth the Mimsy Modular, in honor of the inventor of the Stepped Tone Generator the APC is based on (Forrest M. Mims III), and in reference to my favorite poem (Lewis Carroll’s “Jabberwocky”).

The black stripe is electrical tape, the spike is just something I had in a junk drawer- though it currently serves only as a decoration, it may be used as a touch point in the future. The batteries are now held in place with metal clips which are hot-glued to the box. The metal strip with the voltage starve knob is also held in place with hot glue, and also holds the on/off switch for the APC in place.

Here’s a shot taken during the latest update showing the output of the right-hand module being fed into the APC’s CV input:

And here’s a recording of some knob twiddling with the second module plugged into the CV input, no effects:

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My next plan is to wire up the unused side of the second 556 in a similar fashion as the first, but pitched lower. I will also be trying to find a way to make interesting use of the CV inputs.