synthgeek’s blog

After a while of being used, all the plugging & unplugging was starting to make the jacks push through the cardboard faceplate. However, with the latest addition (the 555 VCO, see previous post), the frame is pretty close to full anyway, so I decided to take a stab at the aluminum faceplate I’ve been waiting to add.

The main reason I’ve been holding off is that I don’t have access to very good metalworking tools. The best thing I could find for the job was a friend’s SawzAll, which did a pretty bad hack-job… if I had a grinder, it wouldn’t be such a big deal, but since I don’t, it’s pretty rough-looking:

I had also recently come to the realization that this panel wouldn’t have enough room left for the CEM3379 filter I’m planning to put in, which was the main reason I had +/-12V power in there. So, I decided to pull out the AD633 module, make this strictly a 9V machine, and make a second cabinet to hold the +/-12V stuff later. I’m thinking I’ll probably build a 567-based module to replace the AD633, unless I come across something else interesting first. I also put the passive 3-input mixer which used to be external into the frame, and added a global voltage starve knob.

Here is another recording of the new layout- here we have the 555 VCO modulating the 566 VCO, which is then being fed back to modulate the 555 VCO. The output of both oscs is then fed into the passive mixer, and put through the Blue Lantern filter:

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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:

Once again, thanks to Beavis Audio Research- this time it’s a tremolo/quasi-ring modulator circuit built from the LM567 tone decoder chip. As an added bonus, the squarewave oscillator also works on its own, without any input.  Click here to view the pdf from the Beavis audio site (includes schematic and breadboard layout). This chip was traditionally used for touch-tone decoding in telephone & other communication systems (I recently harvested two of them from an old modem), and includes an internal oscillator and bandpass filter.  Here’s what the circuit looked like on the breadboard:

And here’s a shot of the final perfboard build:

I made substitutions for the filter and volume pots: instead of a 50k on the filter and 100k on the volume, I used a 22k for the filter, and a 50k for the volume- the original 50k on the filter seemed to have a lot of extra travel (and so does the 22k, but I’ve got a few to use). The volume pot sub was just because I had no 100k audio-taper pots.

Here’s a recording of the circuit being fed with the APC side of Mimsy Modular:

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My original thought was to make this into another module for the Phoenix project, but now I’m leaning towards just making it a stand-alone fx/noise box. Since there is so much extra room on the board, I’m thinking of adding a simple distortion circuit of some type on the output, which could be switched in & out.

I recently came across some nifty schematic and PCB design software called ExpressPCB- the download includes seperate programs for schematic drawing (ExpressSCH) and PCB design (ExpressPCB). I haven’t tried the PCB part yet, but here is my first schematic (click for full-size):

This is my first schematic, so there may be errors. Both of the basic circuits are well-documented elsewhere, however, with only minor changes in my implementation- so if you build from this and it doesn’t work, google “Atari Punk Console” and “555 oscillator”. I just wanted to try out ExpressSCH, and this seemed a nice, quick way to do so.

Having learned a few things since building this, there are a few small changes I would make if I were to do this now. I may post a “revised” schematic eventually.

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.

First, two Phoenix vs. Mimsy videos:

One of just Phoenix:

…and one of just Mimsy:

This one started from a basic schematic in the 566 datasheet:

For my build, I substituted a 1.2k resistor in place of the 1.5k (because that’s what I had on hand), used a 500k pot for R1, and a 1µF cap for C1. Here’s a shot of the breadboard:

You may notice the second jack in the above photo being routed to pin 5 of the 566, which is the CV input.

The next step was to build it on perfboard. I also wanted to add a saw wave output to the 566’s “built-in” square and triangle waves, and once again, the Experamentalists Anonymous schematic archives came to the rescue, in the form of this schematic, an op-amp and about a dozen components. This is an early shot of the perfboard build, some tweaks were done later:

I’m not getting a very good saw wave with it, which could be due to my having to sub some components. The schemo specified 330nF caps for the square-to-saw converter, and the closest I had were 220nF. Also, there were no specs for the diodes or trim pot, so better results may be possible with different components there- again, I was just using what I had on hand.

After testing, I decided to replace the trim pot with a panel-mounted control, just for added odd sound capability. Here’s what it sounds like through the Blue Lantern filter, with some tweaking of the shape knob and a CV input later on:

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Another 386-based lo-fi bass amp build, again inspired by ideas from Beavis Audio Research and Runoffgroove. This one is a bit more complicated, incorporating power supply filtering (via 220uF capacitor), buffered input (via 2N5951 transistor), and a tone control (from the Beavis Audio article on the Big Muff Pi tone stack, inserted between volume control and 386 input pin).

The input section (including buffer) is like the one from the Beavis Audio Noisy Cricket design, but with the BMP tone stack instead of the one from the Cricket. Also, I used a single 100nF cap in place of the 47nF specified in the Cricket schematic, and omitted the second input cap. The rest of the design is inspired very much by the Grace Overdrive design from Runoffgroove, with a larger output cap.

Perhaps you’re wondering where the name came from… well, like the Candy Land Combo, the Dirty Cow gets its name from its enclosure:

The jack with the red plug in it is the input, output is on the back. There is a power switch and LED, and the knobs (from left) are: gain, tone, and volume. Their placement is a bit misleading, since I went with a “non-master” volume setup (the volume is on the input, not the output). That also means the volume and gain knobs are interactive, allowing for a more adjustable response. Compared to the Candy Land amp, this one is a little louder, but is also capable of more sounds approaching “clean” territory.  Here’s some audio (kinda sloppy playing, sorry)- first up, a cleanish tone (same active Ibanez bass as last time) volume just over 3/4, gain lowish):

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and here’s a more distorted one (volume and gain around 3/4):

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…and some even sloppier guitar playing, gain and volume cranked (passive-pickup cheap Strat copy):

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Another project based on a schematic from the Experimentalists Anonymous archives, with a tone control added to the return path.

The tone control follows the schematic in the Beavis Audio Research article on potentiometers. It’s a very simple passive resistor/capacitor network that works like a guitar’s tone knob.