ACMS Project Suggestions

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.

General project ideas

These could be one-person, two-person, or three-person projects, depending on complexity:

  • 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).

Some three-person project ideas

  • 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.
  • 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; I can send you a PDF of it. 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).
  • Find the Memorymoog schematics, and find the circuitry for the VCF and VCA, which in the Memorymoog are hooked together. 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 reimplementation. 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.
  • 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.
  • 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. You could try the Roland 100 filter, 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.
  • 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. The 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 the upper 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 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 two-person project would be to do one of these filters – maybe bring out all three outputs. A good three-person project would be to do both filters, wired Jupiter 6 style, with the highpass feeding the lowpass.)
  • 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, there aren’t many implementations. Examples include the Moog 904B and this (simpler looking) implementation by “EFM”.


    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.

  • On the SSM2040 datasheet, there’s something called a Cauer filter. I’m not sure what that is, but it could be fun!

Some two-person project ideas

  • 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.
  • 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.
  • 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.
  • 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 two-person project would be to do one of these filters – maybe bring out all threeoutputs. A good three-person project would be to do both filters, wired CS-80 style, with the highpass feeding the lowpass.)

  • 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.

Some one-person project ideas

  • The The Paia 2720-3B Bandpass Filter uses a Twin-T network in a feedback loop, where one of the resistors is replaced by a diode used for its variable dynamic resistance. We’d want to use an op amp to clean up the overly simplistic control inputs.
  • The Paia 2720-3L Lowpass Filter consists of two RC stages, with the second loading the first. Diodes are used for their variable dynamic resistance. We’d want to use an op amp to clean up the overly simplistic control inputs.
  • 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 Korg MS-50 had a vactrol-based “MVCA” – see the last page of this set of schematics.