Adaptation of the Timbre & Crossfade from the Music Easel

Version 1

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

Note this video is of Version 0, so pay no attention to any mention I make of errors on the PCB; those comments are not relevant to Revision 1.

Schematics & layout

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 buffer. 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 R100, R101, R106, and R107, which create "soft" +13.5 V supplies (via voltage dividers made by R100 with R101, and R106 with R107.) To counteract loading I found that lowering R100 and R106 from 10K to 1K is a good idea, so I marked these as 1K on the PCB. You may want to experiment with other values. 
• 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. R21 is marked as 6.8K on the original Buchla schematic. I had to raise this value 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 instead of 6.8K and marked R21 as such on the PCB. I 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 (even instances of the same JFET model type) may be appropriate for different choices of Q6.
• 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.

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 A1C, or not used at all, or connected to a switch
• A1C - 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 A1C, 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 A1C. If you'd like to switch between both options, hook A1C 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 A1C, or to a switch, or not used (see options listed under the A1C 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 A1C, or to a switch, or not used (see options listed under the A1C 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 A1C 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 
• WCV - "Waveshape" crossfader CV; controls amount of influence of the WCVI input 
• TOS - Timbre Offset 
• TCV - Timbre CV; controls amount of influence of the TCVI input 

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, I recommend starting with kits.
• 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 this post, 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.