Adaptation of the Balanced Modulator from the Music Easel

Revision 2

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.