Some ideas for Option 1:

  • 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.)

  • 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. 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).

  • Add voltage-controlled resonance so a VCF design that currently has
    only manually controlled resonance (i.e., with a pot). (Alex will be doing
    this for the MS-20 LPF, so try another filter if you want to take this route
    besides the MS-20. It was Alex’s suggestion of doing that for the MS-20
    that resulted in me writing here, thinking it would be fun to try for
    some other filters…)

  • From David Cornutt’s post to SDIY, 4/1//06: “The EML 101 filter has
    circiutry 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.”

  • The SSM2164 and THAT2180 are interesting “VCA” chips that have
    a current-in, current-out operation (unlike the voltage-in, current-out
    operation of OTAs); they also have exponential control inputs (unlike
    the linear control input of an OTA).
    I’m curious if it’s possible to build a triangle core
    VCO (as in the Buchla 259) using either the SSM2164 or the THAT2180 as
    both the exponential converter and the current switch. (This would be
    pretty ambitious.)

  • The THAT2180 chips are expensive, but high quality. You could build a
    four-pole-with-feedback filter or a state-variable filter with them. (Rumor
    has it that the THAT chips are used in some of the newer Serge designs
    produced by Sound Transform Systems.) (I would suggest the same with
    the SSM2164, but that’s already been done – there are state-variable
    and four-pole-with-feedback SSM2164 based designs out there already.)

  • The EDP Wasp had a bizzare state-variable filter with CA3080 OTAs that
    used CMOS inverters biased in a linear range of operation as if they
    were op amps! Check out
    Rene’s version.
    You could similarly try building a Sallen-Key or four-pole-with-feedback filter
    using CA3080s and CMOS inverters. The CMOS inverters gave it a the Wasp
    filter a weird kind of distortion.

  • I have some H11Fx series optocouplers. How about a Buchla LPG that
    uses the optocouplers (which have light-dependent FETs that act as
    resistors) instead of Vactrols (which have light-dependent resistors)?
    Buchla schematics are messy, and the electronic switches are probably more
    complicated than we really need, so I’d start with, say,

    Peter Grenader’s redrawn schematic (see “Low Pass gate”)
    . Maybe the
    optocouplers would react faster to control signals than the vactrols,
    and give it a different sound.

  • Hmmm… the Buchla LPG is a Sallen-Key. As an alternative to
    the previous suggestion, some four-pole-with-feedback
    or state-variable VCFs using the H11Fx
    optocouplers might be interesting too,
    with the optocoupler replacing resistors in standard op amp integrators
    (for the SVF) or RC filters (for the four-pole-with-feedback) designs to
    control the cutoff. For total coolness, you’d want to be able to control
    the Q with voltages too, but you might just try to get it working with
    a pot controlling the resonance first.

  • The Yamaha GX-1 (or equivalently the MOTM-485, which I put together
    and passed around the first day of class) uses a Sallen-Key filter using
    diode rings as the voltage controlled element. How about a state-variable
    or four-pole-with-feedback filter using these diode rings? (If you’re
    interested, I can get you the salient sections of the MOTM-485 schematic.)

  • I have some ideas, based on things I read in Electronotes, about
    constructing a four-pole filter with “variable slope.” The SSM2164 might
    be good for this, since it gives you four separate built-in expo converters,
    and my idea is based on separately changing the cutoff frequencies of
    each one-pole section (instead of making all the cutoffs be the same).
    Ping me if you want to look into this and I’ll tell you more.

  • The EML-101 we looked at in class had a state variable filter
    using transistor pairs as the voltage (current, really) controlled
    elemements and “differential integrators.” You could perhaps building a
    four-pole-with-feedback filter using these same transistor pair/differential
    integrator cells.

  • Most VCFs I know of use Sallen-Key, state variable, or
    single-pole-cascade-with-feedback topologies. There’s all sorts of other
    filter topologies with names like “Twin-T”, FDNR (Paul Schrieber makes
    a big deal about the importance of the
    FDNR topology in his MOTM-450 Fixed Filter Bank
    , and multiple feedback
    (MFB). For the most part, I don’t think OTAs would work well for making
    these voltage-controlled, since there are typically “floating” resistors
    that would require two back-to-back OTAs for a drop-in replacement.
    But, I have a bunch of vactrols in my office – you could build one of
    these fancy filters using vactrols in place of the resistors.

  • The so-called “biquad filter”
    (see Figure 5 of this
    ) is a variation of the state-variable idea. Notice it consists of
    a single-pole filter and an itegrator; the op amp in the middle is just an
    inverter. I’m not aware of any designs using this topology; is should be pretty
    easy to make this voltage controlled via OTAs. The single-pole part could
    be done easily using the single-pole OTA we’ve seen in lecture, and we’ve
    seen many voltage-controled OTA integrator designs.

  • In class, we looked some “wavefolding” circuits, particularly
    one by Ken Stone that’s closely related to the Serge Wave Multiplier.
    I recall some posts on the SDIY by Ian Fritz, which suggested that you
    could build an interesting [NEED TO FINISH]

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.