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My Paia keyboard is a great sound generating source. Sure would be nice to be able to process those sounds through some voltage controlled filters and amplifiers, with MIDI and some other stuff thrown in for good measure, right?  And it would be nice if the processing could be contained in a reasonably small box that sits on top of the keyboard.  The Chameleon was born!  After some research, I decided to use Ray Wilson’s excellent modular synth board designs from Music From Outer Space (MFOS) as the basis for the Chameleon. The unit contains the following MFOS boards:
  • Noise and Random Gate generator
  • Low Frequency Oscillator (LFO) with extensive routing possibilities to the other modules
  • Sample and Hold (S&H)
  • Stereo Auto-Panner with custom modifications for status & rate LEDs
  • Two State Variable Voltage Controlled Filters (SV VCFs)
  • Two Attack, Decay, Sustain, Release generators (ADSRs) with custom modifications for ADSR LEDs and an auto-repeat circuit
  • Two Voltage Controlled Amplifiers (VCAs)
The boards are integrated in the unit so that no patch cord changes are necessary to reconfigure the modules - this is called "normalization" in analog synth-speak. Everything is routed and connected through selector switches and potentiometers on the front panel. This is accomplished through a custom control voltage (CV) and audio mixing and distribution system. There is also a custom three channel MIDI interface that includes two Attack-Release-LFO (AR-LFO) generators and a MIDI trigger and gate implementation with yet another LFO.  DC power (+/-12V and +5V) is provided from an external power supply, with power switching and filtering built into the Chameleon. Everything is packed into a 20½ inch wide by 6½ inch deep by 3 inch tall box with 77 knobs and 17 switches.

Front Panel View
Front Panel View

Here's a photo of the front panel.  Click on the picture for a larger view.
Back Panel View
Back Panel View

Here's a photo of the unit from the back.
Inside Top Layer
Inside Top Layer

The top layer of circuit boards is mounted to aluminum rails that can be dismounted from the main unit and tilted upward.

Inside Bottom Layer
Inside Bottom Layer

The bottom layer of circuit boards is mounted directly to the bottom of the unit.  You can see the top layer tipped up in this photo.

This is a block diagram showing how the modules are interconnected.  The main modules are shown in gray.  The switches that allow selecting the routings between modules are blue.  Mixer elements to combine audio signals are tan.  Mixers for control voltages are in pink.


MIDI Interface 2
MIDI Interface Schematic

Here is the schematic of my custom three channel MIDI interface that includes two Attack-Release-LFO (AR-LFO) generators and a MIDI trigger and gate implementation with an LFO.

While developing the circuit, I ran out of PIC inputs, so the input expander shown in the second drawing was added to allow multiplexing of six switch input states into a single PIC input pin.

The color code legend at the top right of all the schematics identifies which function (MIDI, Noise, LFO, etc) to the connection goes.  The connection point circles on the various schematics use this color coding.  Having all this functionality packed into one normalized unit makes a good interconnection roadmap essential!

Here is the schematic for the noise and random gate generator.  It's basically a "Noise Cornucopia" from MFOS.   I added a level shifter to the random gates output to make it swing a bipolar +/-12V instead of just from 0 to +12V

Here's the MFOS Low Frequency Oscillator (LFO) schematic.  The extensive routing possibilities to the other modules is realized in the way it's hooked up to the mixer & control voltage routing circuit.  There's only one actual LFO oscillator in this circuit, but it generates triangle, saw, ramp, sine, and square waves that are selectable independently as LFO1 and LFO2.
Sample & Hold
Sample and Hold

Again, a pretty much stock MFOS Sample and Hold (S&H) circuit.  You can see the start of the routing possibilities with the rotary switch in the center of the schematic that selects the sample voltage from several sources, including LFO 1 and 2 and external inputs 1 and 2.

There are two independent MFOS State Variable Voltage Controlled Filters (SV VCFs) in the Chameleon.  "State Variable" means that the VCF can act as a highpass, lowpass, or bandpass filter.  The input mixer provides a separate mix of the 4 audio inputs to the VCF.  Noise can be mixed in as well.  Each VCF can be frequency modulated by its own Attack-Release-LFO (AR-LFO) generator which is part of the MIDI interface, LFO1 or 2, external input 1 or 2, or the sample & hold.  Resonance modulation is selected from the same possible sources. 

There are two independent MFOS Attack, Decay, Sustain, Release generators (ADSRs) in the Chameleon. I added modifications to provide LEDs that sequence through the A-D-S-R states, and also provide an auto-repeat circuit which is effectively yet another LFO.

The ADSRs provide one of the sources of modulation for two Voltage Controlled Amplifiers (VCAs).  The other modulation sources include the LFO, S&H, and an external input.  The audio input for the VCAs is the VCF outputs.  In simple terms, VCF1 feeds the audio path for VCA1, and VCF2 feeds the audio path for VCA2.
Panner 1
Panner 2

The MFOS stereo auto-panner takes the VCF1-VCA1 and VCF2-VCA2 outputs and makes them into "stereo" by panning them left and right at a selectable pan rate.  Another LFO!  There's a nifty feature that lets the pan rate increase and decrease to simulate a Leslie rotary speaker effect.  Of course, I couldn't leave well enough alone and applied a modification to add a rate LED.
Mix 1
Mix 2
Mix 3
Mix 4

If the MIDI interface is the brains of the synthesizer, the mixer is, well, the stomach and intestines.  You were expecting me to say "heart"? 

The mixer provides CV distribution of the noise, LFO, VCF, and external modulation sources. 

On the input side, it has the ability to mix the four input sources to either one of the VCFs as well as directly to the master output bus. 

On the output side, the master mixer takes the outputs from the VCF-VCA normalized stacks and routes it to either the master outputs or through the panner or an external effects loop first. 

There are also a couple of external inputs that can be mixed through to the master.  This is where I connect my Fatman and Modulus inputs and mix them down to one stereo output.  I found that these inputs needed to be a little hotter - that's what the gain stages on schematic page 4 are all about.

The master outputs are line level, suitable to go an amplifier.  There's a headphone amplifier onboard too.

Power Input

The Chameleon runs on +/-12V and +5V provided from an external supply.  I needed a 4 pole power switch, but they don't make those in a mini-toggle form factor.  A relay does the trick!
Panel 1
Panel 2
Panel Connections

Like I mentioned earlier, having all this functionality packed into one normalized unit makes a good interconnection road map essential!  This front panel interconnection diagram makes sense of all the interconnections between the individual schematic elements.
Panel Artwork
Panel Artwork

Here's the artwork used for the front panel.  It's printed on paper, laminated with clear contact paper, holes are cut out in both the aluminum panel and the laminated paper, and spray glue is used to adhere the paper to the aluminum panel.