Music Easel Adaptation – Timbre & Crossfade

Music Easel Adaptation – Timbre & Crossfade

Version 0

Original
circuit by
Don Buchla
(used with his kind permission);
adapted by Aaron Lanterman

This is based on the timbre circuit on Board 9 and the single-vactrol
“waveshape” crossfade circuit on Borad 8 of the Music Easel.
You should spend some time studying the
original
schematics.

Warning: This board is designed to be highly flexible; it can be
configured in many different ways. Please read the notes below carefully
and decide what options you want before building.

Demo

Schematic & layouts

Schematic
Complete PCB layout
PCB, silkscreen
PCB, top copper layer
PCB, bottom copper layer

Errors

The schematic and PCBs linked above contain some significant errors, most of
which must be
corrected during construction.

  • The op amp inputs of IC1A (pins 2 and 3),
    IC7A (pins 2 and 3),
    IC7B (pins 5 and 6), and IC6B (pins 5 and 6) need to be SWAPPED to get
    proper negative feedback. Note IC1A is a spare op amp that is meant to be
    wired “off,” so it’s not necessary to fix it; if left unchanged, the output
    will swing to one of the rails, and it will draw a little extra current,
    but it probably won’t hurt anything (I didn’t bother doing the IC1A swap in
    my prototype). However, the other op amp inputs do need
    to be fixed if you want the waveshape crossfader to work. Note that if you
    don’t care about the waveshape crossfader, and only want the main timbre
    circuit, you don’t have to do the swaps, and can in fact omit all the parts
    you see in the lower right corner of the schematic.
  • There is a capacitor missing. If you look on the
    original schematic,
    you will see a 15 microfarad electrolytic capacitor (marked C2 on Buchla’s
    schematic)
    between the non-grounded side of Q6 (equivalently R21) and the negative
    terminal of the op amp (marked IC1B on my schematic). You need to add this
    in somehow. The positive terminal of the electrolytic is connected to the
    negative terminal of the op amp IC1B. If you don’t include this capacitor,
    the VCA won’t have variable gain. (I used 10 microfarad instead of 15
    microfarad in my prototype.)
  • TRIM should be 100K, not 10K.
  • R24 should be 130K, not 33K.
  • R21 is labeled as 6.7K; it should be 6.8K as marked on the schematic.
    I had to up this quite a lot in order to not get too much gain through the VCA
    with the offset knob at the lowest setting. I used 330K, and recommend
    that as a starting point if you’re using a J201 or MPF102 for Q6 (I tried both
    and didn’t notice any difference). Your mileage may vary. Different values of
    R21 may be appropriate for different choices of Q6.
  • TOS and TCV should be 10K linear pots, not 50K.
  • R100 and R101, and similarly R106 and R107, are intended to create
    +13.5 volt supplies. On the original Easel, this is created on another
    board with an op-amp and transistor buffer. Here it’s just a couple of
    resistors, forming “soft” supply, so
    to counteract loading I found that lowering R100 and R106
    from 10K to 1K is a good idea    .
  • The silkscreen labels of R37 and R47 are swapped, as are R41 and R44.
    However, the part values listed inside the part outlines themselves are
    correct, so if you stuff these based on the ohm values you see on the PCB
    you will be fine.

    Notes

    • I am convinced that the 50K sliders marked on the original schematics
      (and this version of the board) should actually be 10K linear.       The 120K
      input and shaping resistors (R102, R103, R104, R105, R108, R109, R110,
      and R110) are off-board in
      the original Easel, but included on-board in this adaptation.
  • The original Easel has a 13.5 V supply, created using an op amp and a
    transistor. If you have such a supply, you may hook it to the two +13.5 pins
    and omit R100, R101, R106, and R107.
    Otherwise, leave the +13.5 pins unconnected and use R103
    and R104, which create a “soft” +13.5 V supply. I found it important to lower
    R100 and R106 to something like 1K to counteract loading.
  • The Q6 JFET is used as a variable resistor.
    It is specified as a 2N4341 in the original, but it appears to be out of
    production. I picked the J201
    since it happened to come with the preinstalled Eagle libraries. I’ve tried
    a MPF102 here too, and didn’t notice a difference. To get a decent control
    range at the lower end, I raised R21 to 330K. Different values of R21 may
    work better with different FETs.
  • I specified Q5 as a 2N3906 since
    I happen to have of them and it also came
    preinstalled in the Eagle libraries. In the original Music Easel schematic,
    it is specified as a 2N4248, which seems to be out of production. You might
    want to try other transistors here.
  • The circuit has been tested with RC4558s, which was deemed to be
    electrically similar to the original RC4136s used in the Easel.
    Other op amps will probably
    work (many will probably work better!), but they have not been tried.
  • D3 is a 1N457.
    I suspect a 1N4148s or a 1N914 will work, but I have not tested them.
  • D1 and D2 are not specified in the original schematic; I used 1N457s
    here, but my suspicions in the previous bullet point apply here too.
  • I have a tradition
    of specifying 2.2 ohm resistors (should probably be 1/2 watt)
    at the power inputs
    to perform power supply filtering along with 10 microfarad
    electrolytics. I picked 2.2 ohms since this choice shows up on some Buchla
    schematics; I did not pick it through any particularly scientific means. Any
    low resistance should work here.
    I actually use “ferrite beads,” as suggested by Ken Stone, and not resistors
    in these spots.

    Connections

    Front panel connections usually have a square and round pad together in a
    white box. The round pad is the signal, and the square pad provides a
    convenient ground.

    TAI – Timbre Audio Input

    TAO – Timbre Audio Output

    TCVI – Timbre CV input; amount of influence is controlled by setting of
    TCV pot

    WCVI – “Waveshape” crossfader CV input; amount of influence
    is controlled by setting of WCV pot

    A1I – Alternate input 1; buffered and appears at A1B

    A1B – Buffered version of A1I input; may be used connected to A2C, or
    not used at all, or connected to a switch

    A2C – This is misleadingly labeled;
    it would be better labeled as A1C.
    It corresponds to pin 10 of Board 8 of the original Easel schematics. It
    corresponds to what you get by turning the waveshape control counterclockwise.
    If you want to set this up like an original Easel, connect B9P4 or TOP directly
    A2C, so turning the waveshape control counterclockwise corresponds to
    the timbre circuit. If you want to always use the “waveshape” crossfader
    as a stand alone crossfader, you can directly hook A1B to A2C. If you’d like
    to switch between both options, hook A2C to the common terminal of a on-none-on
    SPDT switch, and hook TOP or B9P4 to one “on” terminal and A1B to another
    “on” terminal. (The issue of whether to use TOP or B9P4 is complex and
    depends on how you set the resistors OR119, OR120, OR1A, and OR47A; see below.)

    TOP – Timbre Output Pin – connected to A2C, or to a switch, or not used
    (see options listed under the A2C description above). This is the timbre output
    after the gain provided by IC5B (if gain is used).

    B9P4 – Corresponds to Pin 4 of Board 9 of the original Music Easel –
    connected to A2C, or to a switch, or not used
    (see options listed under the A2C description above). This is the timbre
    output before the gain provided by IC5B (assuming gain is used).

    A2I – “Alternate” input 2; buffered and appears at B8P; used if creating a
    stand-alone module. This corresponds to what you get when turning the
    waveshape control clockwise.

    B8P – Input to the Vactrol side of the “waveshape” crossfader. If you are
    using the A2I input, you won’t need to use the B8P pad. If you are trying
    to build an complete Easel, B8P corresponds to pin 12 of IC 4 on the
    original
    Easel Board 8 schematic. This is the pulse, square, or triangle shape
    signal that you’d get by turning the waveshape control clockwise. If you
    hook a signal directly to B8P, you should omit IC7, R112, R113, R114, R115
    (notice this also takes out the A1I, A1B functionality, but that’s probably
    OK since you’ll probably be directly hooked the timbre output to A2C
    anyway). Most users building stand-alone modules will probably not need
    to use B8P.

    MXO – Mixed Output of the “waveshape” crossfader

    CSW1, CSW2 – there’s a capacitor that provides some filtering action on the
    timbre output. You can put a switch between CSW and CSW2 and experiment with
    switching this cap in and out. If you want it to act like an original Easel,
    just short CSW1 and CSW2.

    Resistor options

    • If you are using an op amp with some build in short-circuit protection,
      like the specified RC4558s, then you can use the 220R resistors OR121 and
      OR48A, and use wires instead of 1K resistors for OR115 and OR122. If, on the
      other hand, you are using a different op amp capable of creating much bigger
      currents, I recommend using wires instead of 220R resistors for OR121
      and OR48A, and installing actual 1K protection resistors in the OR115
      and OR122 spots.

     

  • OK, here is where things get really complicated. OR1A and OR47A
    are specified as 15K and 75K; this is as things are in the original
    Easel. This gives the raw timbre signal at B8P4 and whatever it is mixed with
    at B8P a gain of 6. IC5B, OR119, OR120, OR121, and OR122 are not present
    in the original Easel; this is a copy of the circuitry around IC6B to give
    that gain of 6 at the TAO output. If you’d like your external signal input
    at A2I to be subject to the same gain, then you can use 15K and 75K in
    the OR1A and OR47A spots, respectively. However, you may prefer to
    take the timbre output to mixer from the TOP pin, so it already has the
    gain, in which case you can omit OR1A altogether, and use a wire for OR47A,
    which turns IC6B into a unity gain buffer; in this case, IC5B boosts
    the timbre output up to the level of typical signals, and will then be
    on an even footing with most external signals, and IC6B won’t provide
    additional undesired gain. Think carefully about your particular desired
    gain structure.

    Potentiometers

    WOS – “Waveshape” croassfader Offset (Easel schematics and this version of
    the PCB say 50K,
    but I recommend 10K linear)

    WCV – “Waveshape” crossfader CV; controls amount of influence of the WCVI
    input (Easel
    schematics and this version of the PCB say 50K, but I recommend 10K
    linear)

    TOS – Timbre Offset (Easel schematics and this version of
    the PCB say 50K,
    but I recommend 10K linear)

    TCV – Timbre CV; controls amount of influence of the TCVI
    input (Easel
    schematics and this version of the PCB say 50K, but I recommend 10K
    linear)

    Disclaimer

    These should be considered advanced projects, and should only be attempted
    by people with extensive knowledge and experience in electronics,
    particularly
    in terms of practical construction and debugging techniques. The boards
    are
    dense and the documentation is sparse.
    If you are just
    getting started with Synth DIY, we recommend starting with kits
    by Blacet Research or
    PAiA, or boards by
    Music
    from Outer Space    . (There are numerous other kit and
    PCB manufacturers, but those are relatively newbie-friendly.)

    If you try to build one of these projects, you must assume that you will be
    on your own, and be confident enough to tackle the project under those
    circumstances. I am interested in learning about people’s experiences
    in building the boards, and will try to answer questions over e-mail,
    but I don’t have time to do any hand holding.

    Any PCBs made available to the public are provided as-is, with no
    guarantees or warranties whatsoever. Similarly, no guarantees or warranties
    are made about the correctness or usefulness of the information on these
    webpages.

    Any electronic project may present a risk of injury or
    death, particularly when
    dealing with mains voltages. It is important to follow appropriate safety
    practices. The author of these
    pages, Aaron Lanterman,
    disclaims any liability for injury, death, or other damage caused in
    using the PCBs or any of the information contained on these webpages.