Some assorted project ideas

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

• 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.
  Interesting…
  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)
  

• 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). Warning: someone tried this in the Fall but didn’t
  get their circuit to work. They seemed to have these strange drifting
  offsets we couldn’t track down that would make the stages slide to and
  smack the rails. I’d try to get a single-pole highpass
  working first.)
  

• Add voltage-controlled resonance so a VCF design that currently has
  only manually controlled resonance (i.e., with a pot). (People did
  this last spring with the Synthacon filter (that’s something we didn’t
  see in class but is pretty cool) with a vactrol, and also with the MS-20
  LPF filter, so if you take this route, try something different.)
  

• 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.”
  

• 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. Someone got a start on it last semester – they did
  get a triangle oscillating – but never got as far as getting it to play in
  tune. You could build off of their work.)
  

• 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.)
  

  
  <!–

• I’m interested in some folks trying to build VCOs. In particular,
  I’m curious if some of them will work with cheaper linear-to-expo converters,
  like converters based on the CA3083 (which Ian Fritz raves) about instead
  of uber-expensive paris like the MATs. –>
  

• 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. (Someone last semester said they got it
  working on the breadboard, but they didn’t get their final in-solder
  version working, and alas they had already ripped up the breadboard0.
  

• 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.)
  (Warning: a people tried this last semester. The state-variable filter
  never got working – the student had a lot of trouble with generating the
  current sink and source; the four-pole got simplified to a one-pole,
  which I decided was pleanty interesting, and I saw it sometimes working
  on the breadboard, but the final in-solder didn’t work. This is a really
  interesting circuit though so it would be cool for someone to look at it.)
  

• 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
  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
  ap
  note) 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 synth
  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 [AARON NEED TO FINISH THIS ONE]

<!–
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
  microcontroller.
  

• 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.

–>