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.
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
- 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
- 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
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
- 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
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 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.