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;

here is 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
twist.)

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

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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:

The Sallen-Key filters I know of in real synths are all lowpass
designs. The
SSM2040 datasheet shows schematics
for true highpass a bandpass Sallen-Keys, which would be fun to try building.
You could use LM13700s. Note that I think the op amp in the schematics may not
be strictly necessary; I think it’s there to compensate for the fact that you
don’t have access to the positive inputs on the OTAs on the SSM2040.
On the SSM2040 datasheet,
there’s something called a Cauer filter. I’m not sure what that
is, but it could be fun!
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 a few semesters ago – 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.)

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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 LM13700s 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 a few semesters ago 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 breadboard).
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 a few semesters ago. 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.)
Here is the Yamaha patent on the diode ring idea; I think just getting than
one pole filter on the first page working would be interesting.
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 build 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] –>

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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.

–>