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

  • 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

    and the vco schematic is given in the left part of

    (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.)
    are some notes about one person’s experience building it.

  • Many years ago,
    I ran across

    obscure voltage controlled filter patent by Hammond
    years I had no clue what actual product this might correspond to. I recently
    discovered that it was probably the
    . 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
    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
    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,
    highpass and one lowpass. You can find them on
    , 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
    (see schematic
    ). 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
    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

    . 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
    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

  • I have some H11Fx series optocouplers. How about a
    Buchla Lowpass Gate
    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
    (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
    ) 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

  • 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.


  • The Korg MS-50 had a vactrol-based “MVCA” – see the last page of
    set of schematics