Assorted random project ideas
Before we begin, as general background, if you need pick a project that
requires matched transistor, check this out:
Bill and Will’s Synth
Moog Transistor Matching Tester Construction. If you’re building
a VCO, I can supply you with monolithic matched pairs. If you’re building a
VCA or a VCF, you can probably get away with some rough hand matching.
If you need matched diodes, check out
this
thread on electro-music.
OK, here’s some random ideas. These are circuits that have caught my attention
for one reason or another and that I would enjoy exploring more.
Remember you shouldn’t
consider yourself restricted to picking something on this list; if you
do have your own idea for a project you’d like to pursue, looking at the
things on this list will give you a sense of relative scope. Or, you may
not already have a project idea, and may not be interested in anything on
this list per se, but reading what’s here may inspire a new project idea.
Don’t be scared if you see strange part numbers. A good portion of these
parts are long out of production. I can suggest appropriate substitutions.
- 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 sawtooth output is found at pin 3 of the 4151.
Here
are some notes about one person’s experience building it. (For a
one-person project, I’d suggest just buffering the sawtooth output and not
worrying about the rest of the waveshaping circuitry, and I’d suggest
the one in the upper-left corner that is missing the sync circuitry;
for a two-person
project, I’d suggest doing in the one in the lower-left corner, which
includes the sync circuitry, and also building up the sawtooth-to-pulse
waveshaper. In either case, I’d leave out the 4011-based
ring modulator, and just bring the various outputs to the front panel
instead of using the 4052 switch.)
- 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 are known for sounding particularly smooth and
creamy, perhaps because of a couple of extra parts that Yamaha threw into
the 2nd-order feedback loop.
They look quite compact ince they are using custom Yamaha
IG00156 chips. Here is a
high-level block diagram of
the CS-80 VCF, and here’s
Juergen Haible’s
SPICE
analysis. (The SPICE schematic looks a bit weird
but don’t let that scare you – the triangle {gm1} symbol combined with the
boxes with the little current sources in them appear to be his way of
specifying an OTA, and the boxes with the little voltage sources in them
appear to be regular op amps.)
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. 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; I have a pretty good idea
of how to do this.
(The CS-80 has two SVFs, with the highpass output of
the first going to the lowpass output of the second. A good one-person
project would be to do one of these filters – maybe bring out all three
outputs. A good two-person project would be to do both filters, wired
CS-80 style, with the highpass feeding the lowpass.)
- Like the Yamaha CS-80 described above, the Roland Jupiter-6 has
a filter consisting of two cascaded state variable filters, as you can see
in
this schematic. Each filter has an electronic
switch that lets the musician choose between the highpass and lowpass outputs,
so you can have a 4-pole-highpass filter, a 4-pole-lowpass filter, or a
bandpass filter consisting of a combination of the 2-pole and 4-pole filters.
You could replace the electronic switches with ordinary mechanical switches.
Notice that the voltage controlled resonance is the same for each SVF.
(A good one-person
project would be to do one of these filters – maybe bring out all three
outputs. A good two-person project would be to do both filters, wired
Jupiter 6 style, with the highpass feeding the lowpass.)
- There’s
this
great
thread on electro-music about
building a Korg MS-20 sytle VCO, which includes some schematics of a few
versions of a modularized adaptation.
It has a sawtooth core, but it’s a “thyristor” design that is
different than the sawtooth core we covered in class. (The sawtooth core
would be a good one-person project; a full-featured oscillator that also
included the
various
waveshapers from the MS-20 would be a good two-person project.)
- 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.
(This could be a one-person
or a two-person project, depending on how many circuits are tackled.)
- I ran across information about the
Tau
1005 Utility VCO on
Jim Patchell’s website.
I can’t seem to find any information about this company “Tau Systems” – my
google fu is failing me. (Juergen Haible cloned the “Tau Pipe” phasor
a while back, so most google hits are realted to that).
Anyway, looking at the
schematic,
you’ll see it’s a triangle core like the Buchla 259 core we discussed in class,
except a CA3080 is used instead of the “roll your own” 4-transistor OTA
Buchla uses. The HA2500 op ap is acting as the comparator, and the
FET and BJT at the output of the 3080 are acting as a buffer. I think
the weird configuration of Q5 and Q6 is acting as a voltage clamp. It
appears to have been intended to be some sort of
plug-in
module for a bigger circuit. (This
would be a good one-person project.)
- 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, making it look kind of Sallen-Key-ish.
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. (This is a somewhat big circuit, but probably
still doable by one person.)
- The Roland
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. (This is a big circuit; probably would make a
good two-person project).
- We’ve discussed how some manufacturers used diodes (or diode-connected
transistors) instead of BJTs as way to “get around” the Moog ladder VCF patent.
One such company was Roland. A couple of years ago, one of my EMS students
built the Roland 100 diode ladder VCF, and it sounded totally badass. So
how about one of the others?
You could try the
SH3 filter
(which actually has five stages instead of the usual four), or the
SH1000 filter
(to make sense of “Pack #2” you will need to see how it fits in to the
big picture, as seen on the second page of the
full SH1000 schematics). Or, you could try the
4-pole lowpass
VCF from the SH-5.
- OK, I just
had a crazy idea… We’ve looked at four-pole-LPF-with-feedback
cascades using OTAs in place of resistors. We’ll also briefly talked about
vactrols, which are light dependent resistors combined with a light
emitting diode.
How about putting a single LED in the center of four LDRs, placed
equally spaced around the LED? (Some very old guitar pedals, such as the
Univibe, used this idea). 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. (This would
be a decent one-person project).
- 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. (This would make a good one
person project, especially for someone yearning to do a more original design.)
- I came across a
youtube
video demonstrating the
Farfisa Syntorchestra, which sounds fantastic. There seem to be two
VCF circuits in the
schematics
(you’ll have to rotate them clockwise by 90 degrees
in your PDF viewer for them to make sense),
one in the “Poli” section (upper right corner
of page 6) and one in the “Mono” section (upper right corner of page 8).
They’re quite strange. It looks like they’re using
JFETs as variable resistors; this is usually easy to do in a predicable way
if one side of the “resistor” is grounded, but here they’re “floating,”
which is likely to induce all sorts of odd behavior. The Poli one seems to
be an cascade of two RC stages, but without buffering between them, so
the poles of the filter will be spread out a bit, and I don’t seem to see
any feedback. The Mono version seems to have a feedback loop from the
output back to the input, but that feedback loop itself has some
unbuffered RC stages in it, and there’s a swtich that you can use to switch
this feedback loop in and out. I’d love for someone to rig this up and hear how
it sounds! (This would probably make a good one person project.)
- Sorting through my hard drive, I found this
interesting
transistor ladder that uses PNPs instead of NPNs.
I can’t recall where I found it! (Once you add in a current
source, this would get pretty big, suggesting it would be a good two-person
project.)
- 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). This could be a one-person or a two-person project, depending
on size and complexity of the circuit you want to build.
<!–
this
set of schematics.–>
<!–
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.)
–>
<!–
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.
Edit, 4/17: I ran across
this
thread on electro-music:
Fernando: I have an schematic
from Tom Gamble (EFM) describing a 4th order Moog HPF.
But I can’t recall if it was based on the original circuit or was a guess.
edit: it was a re-interpretation.
yusynth: Yes it was and if the basic idea was fine the
schematic contains some
errors in some resistor values…
I think the reinterpretation by Osamu Hoshuyama is more sensible:
http://www5b.biglobe.ne.jp/~houshu/synth/Vchp0302.GIF
francois: I agree with you that Osamu Hoshuyama’s
interpretation is more sensible, and clearer also.
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.”
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.
–>
<!–
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 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.
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.
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 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).
–>
<!–
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.)–>
<!–
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.
–>