Modular Synthesizer

2009-12-24-modularAlthough I’ve owned and used synthesizers for ages, I grew up in the 80s, and unfortunately back then modular synthesizers were not cool. I lusted after the DX-7, and due to price eventually got (and still have) a Juno-2. DCOs, membrane switches, CPU-controlled devices, no knobs… these are the synths of my childhood. It wasn’t until recently when I began designing music instrument electronics full time, that I started to realize that there really is value in the synthesizers which came before. Sometimes it’s ok that it’s hard to store patches. Sometimes it’s ok that not everything has a MIDI port or sounds perfectly in tune. The trade-offs in terms of sound, flexibility and direct hands-on control more than make up for it.

The analog modular synthesizer concept is one of the oldest but still I would argue one of the best formats for a synthesizer. It’s flexible, expandable, and can be patched in virtually limitless ways to create a huge variety of sound and control possibilities. It’s hackable because the control and sound signals are simply voltages which can be manipulated with simple circuits. It’s completely pure, because most audio and control paths are full-bandwidth dedicated channels without digital sampling limitations. And arguably one of the most important aspects of analog synthesis is that it’s not perfect. Little weirdnesses that cause pitch or amplitude variations, noise and distortion are sometimes just the things that add warmth to the sound that purely digital systems often lack. It’s only recently that digital synths can even begin to emulate the sounds of simple, elegant analog circuits.

So in October of 2009 in order to understand this whole area of electronic music more clearly I decided to embark on what will probably be a long-term project of designing and building my own modular synthesizer from scratch. As I find it possible to do so I will update this page and hopefully post newer (and better) sound samples as I get more involved in the instrument. One of my goals was to create every module from my own design, using my own hands to build it. Another goal was, for the most part, to use easily obtainable materials. I’m not against using new and innovative parts when it seems like the right fit, but I really like the idea of making something out of parts which have been, and probably will be available for years and years. Below you will find a description of each module.

Modular Synthesizer Video

Sounds

001 Power Supply

October 17, 2009

A simple linear power supply that produces +15V, +5V and -15V. This should be enough power for one cabinet depending on the modules installed.

There are eight internal output headers and a 5-pin XLR on the front for connecting to the bench for development. I’ve chosen to use a 5-pin Molex header for the power connections because they are cheap and easy to use.

001-power_supply 001-power_supply-inside

002 Mixer

October 17, 2009

A 4-input mixer with volume and pan on each channel, plus sensitive signal LEDs. The mixer is designed to handle high signal levels (more than 10Vp-p) usual in modulars and should have enough headroom to handle large signals from all inputs without clipping.

002-mixer
002-mixer-inside

003 MIDI to CV Converter

November 2, 2009

Providing a much needed bridge between the MIDI world and the analog world of CV and gate, my MIDI to CV converter provides two CV/gate signal pairs as well as a clock output. Some of the notable features are:

  • Dual 12-bit DAC provides two high-resolution outputs. (34 steps per semitone) The output level can be trimmed with multi-turn pots for accurate output calibration. A local voltage regulator for the DACs provides a stable output voltage reference. The max output is +10V.
  • Multiple MIDI modes including: unison mode – both CVs output the same note, the second can be shifted up or down one octave, 2-channel mode – for independent control of two CV/gates, and split keyboard mode – with selectable split point.
  • Last-note Priority – remembers keys in order pressed and produces the correct output voltages as keys are released – works independently in 2 channel and split mode
  • Internal and MIDI clock modes – Generates an internal clock or responds to MIDI clock sync
  • Internal tuning mode – uses the tempo knob to set output voltage for tuning – the octave shift switch moves up and down by one octave for quick oscillator tuning – disables the clock during tuning
003-midi_to_cv 003-midi_to_cv-inside

004 Mono VCO

November 5, 2009

The first really challenging module was a VCO. I started on this a few weeks ago on my breadboard but then realized that I needed a stable voltage source that I could easily tune in 1V increments. So I went back and designed/built the MIDI to CV converter. The most difficult part of the oscillator was the exponential converter for the 1V/octave CV tuning. I found a lot of helpful websites explaining the challenges and offering possible solutions. I finally ended up using an LM3046 transistor array thermally coupled with a KTY81 thermistor. There are two trim pots accessible from the front panel which affect both the scale span and the high frequency (emitter resistance) compensation.

This VCO uses the XR-2206 monolithic function generator IC. The fundamental waveform is that the XR-2206 makes is triangle (I assume using an internal constant-current source) and then can make a sine using an internal sine-shaper. The distortion of the sine is really quite low and sounds nice. Then I added a ramp shaper by using the SYNCO pin to alternately flip and shift the output. This avoids any problems with the lost reset time causing frequency errors, however the frequency doubles when the ramp mode is on. You can use the sine output into the ramp for a different sound.

Also included is a PWM wave (not shown in the picture) which has a dedicated PWM width input jack and level control. Also included is an FM or sync input (switchable) which can either sync the oscillator or provide a linear FM input.

The unit has the following features:

  • Coarse and fine tuning controls
  • PWM input with level control
  • Waveform switches include:
    • PWM on/off
    • Ramp mode on/off
    • Sine/triangle waveform selector
  • FM or sync input with switch to select
  • PWM input
  • Log CV input (1V/octave)
  • Master output (10Vp-p)
004-vco 004-vco-inside
004-vco-waves

005 Quad VCA

November 10, 2009

Once I got my oscillators working it was pretty clear that the next thing to make was a VCA module. I deliberated for a while and looked at other VCA modules, all of which have different sorts of modes and features. I was not sure how to fit in all the types of control possible until my friend told me: “It’s a voltage controlled amplifier!” and thatsettled it. It would just be voltage controlled. There are lots of other modules that can be made to process control signals.

I figured I could fit four channel into a 2 space panel. I implemented some LEDs to showwhich amps are passing signals. Also included is a handy send feature for sending control signals to other VCAs for saving on cables and splitters.

The unit has the following features:

  • 4 channel (quad) VCA
  • 0-5V control (-100dB to 0db)
  • ability for gain (with input voltages greater than 5V)
  • ability to chain channels together for shared CV control
005-vca 005-vca-inside

006 ADSR

November 15, 2009

Every synthesizer needs envelope generators, so after I had something to control with an envelope (my VCAs) I thought I’d build some. At first my goal was to make a completely analog ADSR circuit. I started with some op amps, then realized that I needed at least a flip flop, then found some examples that used 555s, and then after not getting quite what I wanted in SPICE, found myself resorting to good old PIC microcontrollers. The PIC16F690 was one I had never used before, but was in my bin of micros, so I had a go. For $2, 8MHz internal oscillator, lots of analog inputs, PWM, etc. etc. and only an external bypass cap required to make it run, it seemed like a good choice. My module has 2 PICs each handling one channel.

The code didn’t take too long to get the basic stuff going. I tacked a passive low pass filter on the PWM output and made nice DC ramps. Everything seemed fine until I tried it with my VCA. The dynamic range of the VCA is designed for about 100dB. So the 0-5V linear ramp of the envelope meant that most of the ramp results in very low output. For musical note attacks and so on, the useful range of amplitude variation is much smaller. So I added an exponential converter tuned to make approx. -10dB at the mid point of the ramp. This sounds a lot more natural. The low end cutoff (no sound) can be calibrated internally.

Each ADSR can be used separately, or the gate signal from the first channel can be paired to the second with a switch. Also, a retrigger mode starts every new gated note at 0 amplitude instead of the latest running level. (from the previous release still tailing out)

006-adsr 006-adsr-inside

007 Headphone Amp

November 22, 2009

In an effort to make a completely self-contained synth (minus a keyboard) that could be used without other audio gear, I wanted a headphone amplifier for private playing. This amp buffers the line output signal from the synth (with a passive loop-through) and doesn’t load down on the signal lines much. It then contains two separate amplifiers with level controls for two pairs of headphones. It can drive loud into a 32 ohm load. The circuit actually uses opamps but with the +/-15V rails it’s quite easy to make lots of voltage swing. Some small resistors in series with the headphones limit the current and the result is a strong, low distortion amplifier using common parts. The outputs are coupled to the headphones through large caps. Not the most elegant design perhaps, but it sounds good.

007-headphone 007-headphone-inside

008 Digital FX

November 17, 2009

Part of my efforts to be able to jam on my synth without needing external gear, I thought it would be nice to have on-board digital effects. Not too many modulars have that, even in 2009! So I adapted one of the boards out of my commercial guitar effects pedals. I removed the guitar preamp and adapted the board to handle stereo inputs and larger signal levels used in the synth. The effects unit is permanently wired in an analog effects loop so that the dry signal never gets digitized. (to please myself and other analog synth purists) The effects are implemented as a combination of reverb and delay effects with switches to select between two reverb and three delay sounds. A small programming cable dangles out of the back so that it’s easy to update the program.

008-digital_fx 008-digital_fx-inside

010 Voks VCF

December 4, 2009

A friend of mine introduced me to a most curious filter that was used in the Polyvoks synth from the USSR. The strange aspect about it is that there are no capacitors. Most filters (and especially active filters) use capacitors as the main resonant parts to create the filtering action. But this filter creates low and band pass filter outputs from a quite interesting use (abuse?) of a programmable gain amplifier. By running it out of gain, the slew rate is limited. By putting negative feedback around the circuit, the overall gain is 1, but the frequency response is adjustable by the current into the programming pin. I made my own version of this circuit and it sounds unlike any filter I’ve heard before. It’s as though there are creepy soviet devils inside somewhere. Truly a unique sound. Certainly not great for every type of patch, but I was able to create a lot of reedy sounds not unlike a clarinet which sounded very mellow.

010-voks_vcf 010-voks_vcf-inside

Proto Breakout

October 17, 2009

A prototyping breakout box with 3x 1/4″ to banana adapters and power breakout wires.
proto_breakout