Some assorted project ideas

This is just to get your creative juices flowing. I’ll add to this as ideas
pop into my head.

Here’s some “modularization” ideas:

  • The timbre generator of the Buchla Music Easel. Your input
    will go where you see a little sinewave squiggle and “4v p.p.” written on
    the schematic. You will need to include some input circuitry to buffer the
    input and bring the input from the MOTM 10 V p-p standard down to 4 V p-p,
    and some output circuitry to scale the 0.8 V p-p output to 10 V p-p. It might
    be fun to put in a switch to let you switch out C3, so you can get it with
    and without the low pass filtering effect at the output.
    Q6 creates some kind of voltage controlled amplifier.
    I’m not sure what the Timbre In voltage range is; you’d need to experiment
    to see what voltage activated all of the bandpass stages. I’d add a mixer
    stage at the Timber In part so that you could set some base timbre with a
    knob, and then modulate it using a control voltage. I have some RC4136s you
    can use. (Careful, the RC4136s have a non-standard pinout!)

  • The timbre
    generator in the Buchla 259
    . Your signal input would come in where you see
    the sine wave squigglies; there’s two of them. I have some CA3130s (which are
    out of production) that you can use if you want to try this, and can order you
    a 2N3565. Look at the lower left where it has the symmetry control; I think
    your symmetry CV woulg go in that lower left input, and you’d have the pot
    between 0 and +15 setting the main symmetry setting. You would ignore the CMOS
    switch above it. The trick to this one is I’m really not sure what’s going
    on with the timbre CV. If you’re interested in pursuing this, let me know
    and I will poke at it more.

  • Ray Wilson designed a nice
    4-pole OTA-based lowpass VCF.
    It looks
    vaguely based on the Polyfusion lowpass VCF. Polyfusion also made
    a highpass VCF, which I’ll send out a PDF of.
    You could build a highpass version of Ray’s circuit,
    using inspiration from the original Polyfusion highpass VCF circuit.
    (I think this would be interesting since there are very very few 4-pole
    highpass VCFs).

  • Here are the schematics for the
    Memorymoog. Look at the upper right hand corner of Sheet1. You will find the
    circuitry for the VCF and VCA, which in the Memorymoog are hooked together.
    (They are huge scans,
    so you will need to open them up in an image viewing program and zoom in.)
    You could clone this combined circuit by putting your input right to the left
    of the 0.22 microfarad cap (C26, I think it is). Note the “40 mv p-p” notation
    on the diagram; you’d need to put some circuitry to buffer the input and
    divide your 10 V p-p input to 40 mV p-p. Use a LM13700 instead of the two
    3080s. For the control current generator for the ladder, you could simplify
    the control input circuitry. You’d also need to add a buffer for the
    audio output. The emphasis, filter cutoff, and VCA control
    in the Memorymoog are generated by
    a microprocessor. It makes sense to just buffer your VCA control input, but
    for the emphasis and filter cutoff, I’d add some circuitry to allow you to
    mix a control voltage with a voltage set by a knob.

  • Here are the schematics
    for the Moog Rogue.
    Check out
    the lower right corner of page 7; you’ll find the VCA/VCF, which is ripe for
    cloning. Most of the advice I give for the Memorymoog applies here. I’d leave
    out the master volume pot, and maybe change R145 (or is it R143?) to 20K to
    get a +/- 5 V output instead of a +/- 1 V output. If you use an LM13700 instead
    of a 3080A, you’ll have a second OTA handy, so you could replace the pot in
    the feedback path with a VCA to give voltage controlled resonance.

  • In class, we briefly looked at the state variable filter of the EML-101;
    you can find a redrawn schematic of it
    . To clone
    this, you’d need to completely replace the circuitry creating the currents
    for the transistor pairs of the integrators, since the uA 726 is long out
    of production (and I think the circuit they use is probably unnecessarily
    complex.) You could replace it with any number of the more modern exponential
    current sinks
    we’ve looked at in class or that you find on the web.

  • Buchla 291 Dual Bandpass VCF. To modernize it, I’d use
    TL08x or TL07x op amps all over. Also, see the dual FET pairs with the 680 ohm or 150 ohm resistors; those are just buffers. No modernize it I’d make
    those amp amps set as noninverting buffers. (I’m not 100% sure what the
    topology is here; it looks sort of like a state variable filter with a

    Here’s some modification ideas:

  • Add “hard sync” capability to an existing VCO design that doesn’t have it.
    (Do a google search on “hard sync” to see what I’m talking about.
    the first link actually talks about doing a digital emulation of the hard
    sync effect, without aliasing! This would be an analog project, but that
    paper would probably provide some insight even though it’s DSP focused.)
    (I’d like to encourage some people to build VCOs, since the designs last
    semester were mostly filters)

  • Add voltage-controlled resonance so a VCF design that currently has
    only manually controlled resonance (i.e., with a pot).


  • From David Cornutt’s post to SDIY, 4/1//06: “The EML 101 filter has
    circuitry to prevent it from self-oscillating
    at high resonance settings. Figure out how to add a control to
    disable or change the self-oscillating threshold, and how to
    compensate for the atttentuation that occurs when the anti-self-
    oscillating circuit is working.” –>

More original project ideas:

Here's some project ideas suggested by the folks on the SDIY list
that would involve microcontrollers. I'd only
recommend these if you are already intimately familiar with microcontroller
programming (say, for instance, from John Peatman's class). They are
fairly ambitious since they involve a combination of hardware and
software; I'd recommend these be two-person projects.

  • Most synth modules have no way of saving and restoring settings. Modify
    an existing modular synth circuit so that its parameters may be controlled
    via a computer (maybe over USB) in addition to the front panel knobs. You
    could have a microcontroller read the front panel knobs via the A/D
    converters built into the microcontroller. In place of the original pots
    in the original circuit, you could try using
    “digital pots” (do a google search for more info) controlled by the

  • Use a microcontroller to modify an existing VCO design
    to make it self-tuning. The microcontroller
    could control some digital pots that could replace the manual trims in
    the original VCO circuit. You’d need a way to bypass the front-panel voltage
    control and make a reference voltage corresponding to some desired frequency;
    the microcontroller could sample the VCO output via its built-in A/D converter
    and determine the frequency. There’d be a feedback loop that adjusts the
    digital pot to get the frequency right by just pressing a button.