[sdiy] Ultrarough draft of my hot-rodding of the Music Easel ring mod

[Aaron Lanterman]

Yes, I know my Eagle schematic looks like crap. Yes, I plan to clean  
it up. :)

http://users.ece.gatech.edu/~lanterma/easelrm_draft.png

(Note the 5532s are really supposed to be 4558s, which are the closest  
thing I found to the out-of-production and weirdly-pinouted RC4136  
used in the original easel; I just didn't get around to making a 4558  
symbol in Eagle)

I've added a few features:

* The wet/dry mixer now gives the user the option of using switches to  
override the original wet and dry signals and jack in external inputs  
instead. There's also a separate straight ring mod output. So if you  
want you could use the ring mod and use the vactrol mixer (stuff in  
the upper right of the schematic) as entirely separate units.

* The modulation input is buffered.

* The original circuit has a second vactrol (VT5) which, if you look  
at the way it is used in the rest of the Easel, acts as a VCA on the  
modulation input, to drive the FM input of the principle oscillator.  
Here, I've added some circuitry to the board to let it act as as VCA,  
and a switch that lets you choose the modulation input or a totally  
separate (buffered) input.

* If someone does want to try duplicating an Easel as exactly as  
possible (something I have no personal interest of doing, but I know  
people get all giggly over the Easel), you can leave out some of the  
components I added and use various Easel-compatability points I put  
in. For instance, B5P1 means "Easel Board 5 pin 1," and "B5IC2P2"  
means "Easel Board 5 IC2 Pin 2," all referring to the original Easel  
schematic markings. Whoever embarked on such a mission would be on  
their own to do the various bits of switching circuitry.

Questions:

* Any other cool mods I should add?

* For my inverting buffers, I use 68K on the input and feedback, since  
that's what Buchla used to buffer the signal input. Based on the  
various bits of theory discussed on this list, to compensate for input  
currents, I should put 68K || 68K from the + terminal to ground. I've  
done so. But, I notice Buchla's choice of + to ground is much lower -  
namely 4.7K (although that is shared between several op amps). What's  
going on there?

* If I want a noninverting buffer, I'm guessing it's bad form to just  
plug the input from the jack straight into the - terminal of an op amp  
in case someone plugs something bad in. So I put in a 1K please-don't- 
blow-up-the-op amp resistor. I also put 100K from the - to the +  
terminal, and + terminal to ground, because it seemed like modules  
should have some noninfinite input impedance because, well, they just  
all seem to do. Is any of this sensible? Should I choose other  
resistor values, or do something different? Should I put anything in  
the feedback loop of a non-inverting buffer, or do the textbook thing?

And yes, I'll happy produce extra PCBs for whoever wants them for a  
nominal fee to cover cost of production+shipping+hassle, if and when I  
ever get around to laying this out and getting it made.

- Aaron

P.S. I'm working on some other Easel-related boards along these  
lines... I have a whole series planned... ;)