I was out at Megadork at the Love Bytes festival in Sheffield on Friday and they had one of the original designers of the Reactable speaking. He explained it in terms of being a modular synth. It does look cool, but it also seems strange to build a tangible computer simulation of an already tangible entity. Programmers will program I guess. Solderers will solder.

In part 1 of this series I wrote about how my quest for a beginner-priced modular synthesizer started me on the road to fight design flab and reduce costs. How could a cheaper modular synth differ from the standard offerings? It seemed to me that the classic design of a rack with many individual modules was always going to be an expensive solution. I wanted something that could still be used in a modular way but where the modules were bigger pieces of the design and the mounting was lower cost.

EMS Synth-E

Researching in to the history of modular synths I came across an obscure teaching product called the EMS Synthi E from 1975. Designed for school and colleges this is a perfect little modular synth built in to a suitcase. It doesn’t do anything too fancy but it has all the core functions needed to get started creating analogue sounds. If it was still available I would have gone out and bought it. As it isn’t available I decided to take the idea of packaging the basic parts in to a self-contained but still expandable system as the basis for my design.

The next challenge was to think about the simplest useful set of functions. The goal was to have something that could start really small and grow. With this in mind the Korg Monotron was the inspiration behind the first module. The Monotron proves that two oscillators and a filter can be a great instrument and a lot of fun to play with and those are the functions I decided to put in the base module. The current prototype design for the base module has the following features:

• Primary oscillator with square, triangle, sine, ramp and sawtooth waves
• Secondary oscillator which either tracks the primary or acts as a Low Frequency Oscillator (LFO)
• Voltage controlled low-pass filter with resonance and cut-off control

How might the other modules look? This is my rough thinking:

• Amplitude based functions: Envelope generator, Voltage controlled amplifier/ring modulator and extra oscillator
• Signal interfacing functions: signal processor and amplifiers, mixer, delay(?)
• Enhanced filters: band pass, band stop, notch and high pass
  

  MS7 Module Plan

Another barrier to cheap systems is the split-rail power supply that most modular synths need. The power supply on its own often costs more than I want to get the first module out for. My solution: two 9V batteries. It’s a hack, but it gives you something that is simple and portable.

What else might change at the system level? Well, I had to make some decisions about signal levels and MIDI support. Tune in to part 3 soon to lean about those aspects.

A modular synthesizer provides many different analogue electronic modules that can be joined together (“patched”) a multitude of different combinations to manipulate signals and generate sounds. When they first came out in the 1970s they achieved a cult following and massive modular synths became familiar in Prog Rock groups like “Emerson, Lake & Palmer”. Over time their popularity diminished partly as a backlash against the excesses of Prog Rock and partly due to integrated analogue synths like the Mini Moog and then the superior functionality of digital synths. More recently there has been a revival in interest in modular synthesizers and several new small companies have started making them.

What drives this interest in a 40 year old technology when computers can synthesize any sound and emulate analogue beasts that few of us can afford? Partly it is a pull towards retro technology that isn’t just about a screen and keyboard. Partly it is the agreeable roughness of real analogue components. The behaviour of analogue modules drifts over time. When the get overloaded they start to fail in interesting ways. As an electronic engineer there is something fascinating about seeing an electrical signal jump around like a string being plucked. For me the physical environment of connected electronics provides the best place for serendipitous discovery of interesting effects and interactions. Most of the things I do with my analogue video synths would never have occurred in a digital environment.

So I want an analogue synth and I want to be able to patch different combinations together. Where to get one that won’t break the bank? There are some nice projects bringing analogue synthesizers down to attractive price points. From the big names Korg have the wonderful Monotron. In the DIY ethos then Music From Outer Space have some cool gear. I like both those products a lot but the Monotron really needs more knobs and the Music From Outer Space kit needs more patch leads and is still more expensive then I really want.

One of the things the British seem to be famous for is taking expensive electronic products and finding ways to create a new market at a much cheaper price point. When I was a kid it was all about the ZX81 and ZX Spectrum. These days everybody is going mad for the Raspberry Pi. In between we had ARM setting the industry standard in good-value CPUs. What all these products have in common is that they challenged assumptions about what was truly essential to their function. By focussing on the essentials and eliminating or simplifying the rest they reduced complexity and squeezed out cost. “Simplicate and add lightness” as Henry Ford is supposed to have said. Where would trying to simplify the modular synthesizer concept take me? Find out more in Part 2 coming soon.

I’ve wanted a modular synth for ages but I could never justify the cost so I decided to make one that was “cheap as chips” (silicon chips of course). This is the perf-board prototype. It makes a great range of sounds.

More on this very soon.

To try and understand the filter better I built a VCF “clone” on breadboard and then moved it over on to a perf-board. It has been an experience that is at times fascinating and at times frustrating. Internally the VCF uses some very low signal levels which are then boosted with a monster-gain amp. Net effect of this is to make the circuit very sensitive to component changes and environmental factors. I had to substitute for the transistors and the FET in the original design and this threw out a lot of other things. I found that swapping the three transistors around could make the difference between the circuit self-resonating or not. It seems unlikely that Korg will be selecting individual components on their Monotron production line so I don’t know how they manage to get good enough consistency in their builds.

The final working model is interesting. Perhaps rather noisier and not as musical as the original, but the sound is certainly striking.

For those that like the details here are the mods compared to the drawn Monotron schematic (component designations are Korg’s):

• Q12-Q14 are 2N3904. As noted above the order of selection of these can make a big difference.
• F3 is J112. As this has a different Vgs from the original I then had to add an AC coupling via a 1uF capacitor to the op-amp with the input DC linked to the voltage reference via a 100k resistor.
• D1 and D2 are red LEDs to increase the clipping voltage on the output.
• R60 is 680k to make sure the circuit will self-resonate
• R73 is 680R to reduce cross-over distortion in the Op-amps output. I think the way that Korg bias the op-amp the output doesn’t go in to the cross-over zone so this wasn’t an issue for them. Because I changed the biasing it becomes an issue for me.

Edit: Another source on the web dates this mobile to 1994 without giving any evidence. From the design and what I know of its history I would say it was a very early GSM mobile and 1994 feels a bit late. There are various date codes visible on the boards and body castings. The latest date code I can see is “21 92″ (ie week 21, 1992) on the main circuit board. Therefore I believe that this is a phone from 1992.

Mike Challis did a very nice build of the Vibrati Punk Console as a Coffee Can Synth.

Sam Freeman made some great phat sounds by looping the Vibrati Punk Console through a Korg Monotron.

Thanks to all the builders and all the campers that put-up with the massed bleeping!

• Interview with me by the charming Josette Garcia about the background to my various projects.
• Look out for Lushprojects.com circuit bending at Brighton Mini Maker Faire and a chance to buy Vibrati Punk Console kits
• Vibrati Punk Console building workshop at Dorklake11 in a couple of weeks
• First batch of Vibrati Punk Console sold out. Working through the second batch now
• Details of a suggested box for the Vibrati Punk Console now available