Assorted random project ideas
- Guitar effects, like wah pedals and distortion circuits,
are often a great source of ideas; sometimes you can
take such an effect and
replace variable resistors with vactrols or JFETs (used as a voltage controlled
resistor).
- The Rhodes Chroma VCO uses a unique “charge pump” design using an
Exar 4151. Here is the
service manual; in particular, the oscillator circuit is described
here and the vco schematic is given in the left part of
this
diagram
(notice each Chroma voice board has two VCO’s;
you’d want to build just one). The pitch input is labeled as “PITCH” A or B,
and the output is found at op amp 26A or 26B (I wouldn’t worry about building
the circuit past that point – it would be just a sawtooth oscillator.)
Here
are some notes about one person’s experience building it.
- Many years ago,
I ran across
this
obscure voltage controlled filter patent by Hammond.
For
years I had no clue what actual product this might correspond to. I recently
discovered that it was probably the
Hammond
102200. I’m really curious how this sort of filters sounds.
It’s a strange circuit; it constructs a lowpass filter by putting
a highpass filter in a feedback loop.
You could build
Figure 1, using the values given in the table in the text of the patent.
You could just use TL08x or TL07x op amps, and J201s or MPF102s for the FETs.
R151 and all the circuitry to the right of R151 just looks like an
amplifier, so I think you could leave all that out; you could replace R151
with
a 1K protection resistor and then put the output jack right there, where
C35 currently is. The only thing that would need added might be
a buffer amp for the control signal input, and we might need to
experiment with some gain on this buffer to give a useful control voltage
range. (This would be a case where you wouldn’t worry about trying to
get a 1 volt/octave control characteristic.)
- The very last slide of my “ladder filter” slide set showed a diagram
from the Moog patent that shows his idea for creating a four-pole
highpass filter; it uses the same overal idea of changing the effective
resistance of BJTs, but is quite different in the details. Although everyone
and their cousin has created variations of the Moog ladder lowpass filter,
the only implementations of the highpass filter I know of are
the Moog 904B
and this
(simpler looking) implementation by “EFM”. I’d suggest trying the latter,
since it uses op amps while the 904B uses discrete transistors throughout.
- The highpass filter part of the Buchla 191, which has four
highpass stages,
uses BJTs configured as diodes
as variable resistors.
Here’s a rough sketch of my suggested version of this idea. The idea
is that changing CV* changes the DC operating point of the diode which changes
the effective resistance, and AC signals will then see a capacitor through
an effective resistance to “AC ground.”
- The
Buchla 227 System Interface
has lots of VCAs made using VTL5C3/2 type
vactrols. The schematics can be found
here.
Look at the Board 4 (Program Buffer) and Board 5 (Monitor Buffer) schematics;
on each of those schematics sheets, you will see four copies of this kind of VCA
circuit. I think it would be interesting to try building just one of these.
On both Board 4 and Board 5, the circuit that generates the control current
for the vactrol LEDs is on the bottom part of the schematic; in fact, it looks
like these are the same. The VCA circuits themselves are also similar;
they use a 6.8K resistor to connect the middle connected point of the vactrol
resistors to ground. (This must give some shaping to the control that Buchla
liked.) Both versions use a 12K resistor in the feedback loop an output op amp,
although some other details around them differ (you could probably leave these
details out at first).
The input op amps (which form inverting amps) on Board 5
have 20K resistors in the feedback loop and 30K resistors
at the input on Board 5. When trying to make a stand-alone circuit, I’d use
a 100K resistor on the input, and then try using 100K on the feedback loop.
I’d tweak these values as needed to get “unity gain” out of the VCA
When applying maximum control voltage to the CV input. I understand Buchla
schematics are hard to read and my explanation may not make much sense; if
you’d like to try this one, I can meet with you to describe what I’m thinking
in more detail.
- The Yamaha CS-80 filter
consists of two cascaded state variable filters,
one
highpass and one lowpass. You can find them on
this
schematic, in the right part of the diagram, labeled as VCF high pass
and VCF low pass. They look quite compact ince they are using custom Yamaha
IG00156 chips. You can also find a similar filter structure in
the CS-50 (see
schematic here) and the
CS-60
(see schematic
here). I haven’t looked at these closely enough to see if these filter
circuits are identical, and if not, what differences there are.
I’ve found descriptions of the IG00156
here
and here (see page 19).
I think it would be interesting to try to replicate this circuit (just one
of the SVFs, not both); it looks
to me like the IG00156 basically had some OTA and buffers and an expo,
and we could guess what is where, and build it using LM13700s and a
voltage-to-current expo converter cribbed from any number of sources. If
this sounds interesting, let me know and I will meet with you and we can
try reverse engineering the circuit together.
- The Korg Delta has a rather interesting VCF; you can find it in
the upper right corner (once you flip it sideways)
of “Section 4: Circuit Diagram” in the
service manual. It’s like a 4 pole cascade, using LM13600s (we would use
LM13700s), but the feedback loops are
weird. I’m not sure what’s going on in this circuit but I bet it sounds
interesting. Anyway, it looks like the audio input is buffered by IC-6 (or IC-8?
I am having trouble reading it), and immediately hits a LP/BP decision switch
Tne the audio output is the output of
the last stage. The 15K resistors R122, R127, R132, and R137 just make sure
current is flowing through the Darlington buffers. The main control voltage
is in theupper right, labeled “ext fc,” along with the “Cut freq” pot – you
could probably drop the “Joystick” input. If this looks fun, I will meet with
you and hammer out the details and go over the smeared and confusing parts
of the schematic. You should also look at
EFM’s version of this filter.
- The Jupiter-4 filter consists of a 1-pole highpass followed by a typical
4-pole-lowpass-with-feedback. You can find it in the
service manual;
it’s on the top of one of the page
that says “module
board,” about 40 pages in. The LPF part is quite similar to the polyfusion
lowpass VCF we looked at. You’d probably want to simply the various bits of
control current generation circuitry. If this one sounds interesting, ping me
and we will brainstorm.
- OK, I just had a crazy idea… We’ve looked at four-pole-LPF-with-feedback
cascades using OTAs in place of resistors. We’ve also briefly talked about
vactrols. How about putting a single LED in the center of four LDRs, placed
equally spaced around the LED? Those LDRs could then replace the resistors,
and you could control all four stages at once just by changing the current
through a single LED. It would be a sort of make-your-own super-vactrol. You’d
probably want to cover up the
LED/LDR setup somehow so external light wouldn’t effect it.
- 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’s the state variable filter of the EML-101;
here is a redrawn schematic of it. It’s a strange circuit that uses
differential BJT pairs to feed “differential integrators” that use op amps
and two capacitors.
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.
- 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.
- 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.)
- I have some H11Fx series optocouplers. How about a
Buchla Lowpass Gate
that
uses the optocouplers (which have light-dependent FETs that act as
resistors) instead of Vactrols (which have light-dependent resistors)?
Maybe the
optocouplers would react faster to control signals than the vactrols,
and give it a different sound. To start off, I’d use regular switches
(connecting wires on the breadboard, really) in place of the CMOS
switches.
(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, so I never
heard what it sounded like).
- The Yamaha GX-1 (or equivalently the MOTM-485)
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.
- 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 integrator; 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. I can help with the
design.
- The Buchla 148 generates harmonics from a fundamental triangle wave
through a series
of strange waveshaping circuits. These circuits are odd; they create deadbands,
not the way we saw in class, but through op amp circuits with diode bridges in
the feedback loop. They are quite strange. Let me know if you’d like to take
a look at some of them and try getting them to work.
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