Original circuit by Don Buchla (used with his kind permission); adapted by Aaron Lanterman. This is based on the balanced modulator circuit on Board 5 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.
Demos
Note this is a demo of Version 0. Revision 2 doesn't require all of those horrible kludges seen in the video.
Schematic & layouts
Notes
I am convinced that the 50K sliders marked on the original schematics should actually be 10K linear. The 120K input and shaping resistors (R117, R118, R119, R120) 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 +13.5 pin and omit R115 and R116. Otherwise, leave the +13.5 pins unconnected and use R115 and R116, which create a "soft" +13.5 V supply. You may want to experiment with changing these settings a bit, for instance lowering R115 and R116, to counteract loading. I found that lowering R115 from 10K to 3.3K is seemed to work well.
If you want to use the FO fuzz output, you should add a 0.1 microfarad DC blocking cap, like the FOA and FOB outputs have. It should be pretty easy to add this somewhere on the way between the board and the FO jack.
The circuit has been tested with RC4558s. Other op amps will probably work (many will probably work better!), but they have not been tried.
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.
MI - Modulation Input
SI - Signal Input
EW, ED - External Wet, External Dry - the balanced modulator has a crossfader that lets the user fade between the "dry" signal input and the "wet" full ring mod signal. These inputs let the user put in alternative "wet" and "dry" signals, so the crossfader may be used as a stand-alone submodule separate from the ring mod.
MIXO - Mixed Output of the crossfader - this is the actual output of the "real Easel" balanced modulator
RMO - Ring Modulator Output - this is the pure ring mod signal.
FO, FOA, and FOB - fuzz outputs - various highly distorted versions of the modulation input signal. These taps were suggested by Grant Richter (who had many helpful suggestions on this particular project.)
CVIN - crossfader CV input; amount of influence is controlled by setting of CV pot
WSC, WSE, WSW - switch to select what goes in the "wet" side of the crossfader, if you want this facility. Hook WSC to the common of an on-none-on single pole switch, and connect WSE ("external wet" select) and WSW (full ring mod select) to the poles. If you don't want this facility, and want it to act like an original Easel, i.e. the crossfader is dedicated to the ring mod, then simply hook WSW to WSC.
DSC, DSE, DSW - switch to select what goes in the "dry" side of the crossfader, if you want this facility. Hook DSC to the common of an on-(none)-on single pole switch, and connect DSE ("external dry" select) and DSW (signal input select) to the poles. If you don't want this facility, and want it to act like an original Easel, i.e. the crossfader is dedicated to the ring mod, then simply hook DSD to DSC.
Note that if you take the "act like an original Easel" approach in the last two bullet points, you may omit IC5 and R104, R105, R106, R107, R108, and R109. Also note that when external inputs are being used, the terms "wet" and "dry" just denote different inputs - those external inputs can be whatever you want.)
B5IC2P8, BIC2P2, BIC5P10 - these correspond to various points on the original Easel schematic, if desired. The ICXPY notation refers to IC X (on the original Easel schematic), pin Y (original Easel IC pins). They might be useful to someone trying to clone a full Easel, but most users will not need to use these.
VI - "variant input" - OK, this may get confusing (uhm, even more confusing, I mean.) On the Easel, the control circuitry for the vactrols in the crossfader for the ringmod drives an additional vactrol, which sort of forms a VCA for the FM input for the principle oscillator. Here, I've set it up as a stand-alone VCA that you can do whatever you want with.
VO - "variant output" - output of the VCA described above
VSC, VCE, VSM - switch to select what to send to the "variant" VCA. Connect VSC to the common of an on-none-on single-pole switch, and connect VCE to VSM to the poles. When switched to VCE, it will send the VI signal to the VCA. When switched to VSM, it will send the modulation input (MI) to the VCA (this is the "real Easel") behavior. Of course, you can omit the switch and just tie VSC directly to VSM or VCE, if you want to do such a thing.
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 OR122, OR123, and OR124, and use wires instead of 1K resistors for OR112, OR113, OR114. 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 OR122, OR123, and OR124, and installing actual 1K protection resistors in the OR112, OR113, OR114, spots.
Potentiometers
offset - crossfader offset (original Easel schematics say 50K, but I believe that is an error; I recommend 10K linear)
CV - crossfader CV; controls amount of influence of the WCVI input (original Easel schematics say 50K, but I believe that is an error; I recommend 10K linear)
SYMTR - I found the easiest way to trim this is to modulate an audio signal with an LFO, and trim it until you get similar amplitude beats.
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.
Original circuit by Don Buchla (used with his kind permission); adapted by Aaron Lanterman. This is based on the envelope generator circuit of Board 3 of the Music Easel. You should spend some time studying the original schematics.
Demo
Schematic & layouts
Notes
The holes and traces on the Revision 1 PCBs are the same as on Version 0. The only changes I made were to the silkscreen.
I am convinced that the 50K sliders marked on the original schematics should actually be 10K linear. The 120K input and shaping resistors (R117 through R128) are off-board in the original Easel, but included on-board in this adaptation.
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.
Buchla went to the effort to specify 1N457 in one instance and 1N457A in another instance, so I'd be nervious about changing what dioes are used. I'd be curious if to know if other diodes, such as 1N4148s or 1N914s, will work.
Bjorn wrote: "I am not sure whether a solution to get the transient function of the envelope generator with a regular gate signal has been proposed yet. Anyways, I have been able to get this functionality by replacing R101 with a 10nf cap."
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.
ATIN, DURIN, DECIN - attack, duration, and decay CV inputs; influence is controlled by ATCV, DURCV, and DECCV settings, respectively
PIN - pulse input
B3P6 - corresponds to original Easel Board 3, Pin 6
TRAN, SUS - connect TRANS and SUS to the extremes of a single-pole on-none-on switch, and connect B3P6 to the common terminal of this switch. This lets the user switch been "transient" and "sustained" modes. (Note I only tested the transient functionality.)
POUT - pulse output
ENVOUT - envelope output
LED - on the Easel schematics, this is actually called "LAMP" and is shown going through a lamp-looking symbol to a +12 V supply. I haven't tried doing anything with this, since it's a low priority for me, but if someone can get something to light up I'd love to hear about it.
+13.5 V - a supply created using a buffer op amp (most users will not need this)
Potentiometers
ATOS, DUROS, DECOS - attack, duration, and decay offsets
ATCV, DURCV, DECCV - attack, duration, and decay CV controls; control influence of ATIN, DURIN, and DECIN inputs, respectively
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.
While sorting through e-mail, I came across this link to a video that some of my Analog Circuits for Music Synthesis students made demoing their final project.
