MFOS Analog Vocoder

Building an Analog Vocoder

  • White/Pink Noise Source
  • Square/Pulse Oscillator
  • Mic/Line Inputs for Control and Audio
  • One lo-pass filter (100 Hz.),
  • Ten successively tuned band-pass filters (154, 208, 285, 395, 542, 720, 1013, 1495, 2000, 2546 Hz.)
  • One hi-pass filter (3330 Hz.)

Ok, you ask “Just what the heck is a vocoder”?

Listen to this first:


I am sure you have heard these before, used since the early 1950s, maybe in science fiction films or TV shows like Dr. Who. A vocoder takes one audio source, like an guitar or keyboard or sound file, and modulates it with another audio source like someone speaking into a microphone (voice). Peter Frampton, and Joe Walsh made a similar device famous, which was the guitar talk or squalk box. Same idea, take a guitar input and modulate it with another source. So vocoder is a “Voice Encoder”.

In short, these things are really cool and just a blast to play with. You can certainly buy one today, almost all of of them however are digital. You can even get inexpensive versions for you PC or laptop. The pure analog versions are not worse or really better than the digital versions (they are certainly harder to control). But they sound very differently, and can create some effects not found in some of the commercial digital versions. A vocoder is definitely not a beginners project, as it requires some knowledge to build one. These are basically giant filter boxes and they are subject to, what I call, “Noise Intrusion”. Anything, microwave ovens, electrical tools, CB radio, vacuum cleaners, cell phones…. man, these things will pick it up if you have not shielded certain elements properly, or have dirty solder joints.

There is a kit available from PAIA and you can check that out. Alternatively, you can order a printed circuit board (PCB) from Ray Wilson. I chose the MFOS (Ray Wilson) design because it had a little more to offer, and I only wanted the PCB, not the whole kit. I like choosing components that, I think work for the sound I want. I also buy them in bulk, so I can reduce costs tremendously. You can purchase the PCB from Synthcube or Modular Addict in North America. Overall, Ray’s design like most of his designs, uses commercially available components. There are many older designs out there, but many of the semi-conductors are no longer made and impossible to obtain anymore.

For this build, I wanted to incorporate it into a large stage rack system I am making. The idea is that it would fit into a standard 19″ rack, along with a couple other synths that I am building. Purchasing the standard rack mounted boxes can get expensive so…. It’s repurpose existing crap nobody wants time! I found an old D-link ethernet switch that was intended as a 1U 19″ rack mounted device. Perfect!!

D-Link internet switch box

We just need to rip out the guts, and repurpose this fine box. Saving the planet one synth at a time! This particular switch box had a great power supply. The voltage would have also worked too, but this supply is a switching type power supply used for digital circuits. These power supplies, although having the exact +12/-12V and 300mA requirements, would have been far too noisy for use in an analog designed filter. So, I saved this for a digital synth I can use later. I have a standard design that I use for synth power supplies. For small single devices I usually make a half-wave rectified version that can work using a 115VAC/12VAC wall wart or a small board mounted transformer. This power supply is capped by the transformer, as it can deliver 280 mA without an issue.

New power supply also added

I also made a new front panel from 0.050″ thick aluminum sheet I keep, and had a vinyl sticker printed up for about $8 at my local print house.

Calibrating the Audio and Instrument inputs.

Once the whole unit was wired up, you can the calibrate the Analog and Instrument waveforms so they have the same shape when viewed on an oscilloscope. Special note here is that most of the wiring from the PCB to the potentiometers and switches on the panel, require a shielded cable with ground at one end. I made up a set, and grounded everything to the panel.

Interesting safety fact: Power supplies with large capacitors can give you a really noticeable shock, even when the unit is off and unplugged. I forgot this as I was adding a fuse, and was delivered a noticeable shock, with a pin point burn on my knuckle. Always discharge capacitors before you work on power supplies.

More detailed images: