more MetalPhoto info

gstopp at gstopp at
Wed Oct 23 21:06:11 CEST 1996

     Gosh I knew that comment would stir up some interest....
     Well I was considering using photoresistor couplers (like VacTeks) 
     across or instead of the EG time constant pots, because I know that 
     works great, but first of all I'm not sure how available those are 
     still and if so they probably cost a couple bucks apiece. So first I 
     wanted to see if there was anything I already had lying around so some 
     gut instinct led me to the parts drawers here at work (shhhhhh...) and 
     into the optocouplers. I picked one at random (4N37) and tried it out. 
     It worked!
     The 4N37 is a six-pin DIP package, with two pins for the internal LED 
     and two pins for the collector and emitter for the internal photo-NPN. 
     Well if you connect the emitter to ground and the collector to a 
     buffered cap (like the cap/buffer in the ASM-1 EG's), and charge up 
     the cap, and put a little current into the 4N37, the cap will be 
     discharged at a rate proportional to the LED current. Lots of current, 
     fast discharge, no current, no discharge. Likewise if you attach the 
     emitter to the cap and the collector to V+ and discharge the cap and 
     put current into the 4N37, the cap will charge up at a rate 
     proportional to the current into the LED. Lots of current, fast 
     charge, no current, no charge. Since the ASM EG's are of the analog 
     switch charge/discharge type, these opto's could replace the pots and 
     you just use one control each for A, D, and R to all the respective 
     LED inputs (the S control is already a voltage).
     Looks like the time constants will be from microseconds to minutes - 
     if you put no current at all into the opto's the charge/discharge 
     stops totally. Cool huh.
     I have not tried this on an ASM yet. This has only been done on a 
     protoboard and a scope. One potential problem is the LED currents will 
     add up if you parallel a lot of units, so a simple pot may not have 
     enough ooomph to drive them all. More research must be done.
     The "mess" I mentioned would be the 3 opto's that would have to be 
     added to the circuit board, per EG. Since there's some blank real 
     estate next to the EG chips, I'd probably try to super-glue them 
     upside-down to the PCB and run wires to the solder pads.
     This trick won't work for the VCF resonance or the ADSR depth to the 
     VCF. Since there's an extra VCA on the ASM it can be used for one of 
     those, and the other with an extra piggyback 3080 or something. That 
     should make the ASM useful in polysynth designs, at least as far 
     settings go. Selection of VCO waveforms, VCF output modes, VCO sync, 
     and other stinky problems may have to be dealt with using relays or 
     analog switches, so beware!
     - Gene
     gstopp at

______________________________ Reply Separator _________________________________
Subject: Re[3]: more MetalPhoto info
Author:  Christopher_List at Sonymusic.Com at ccrelayout
Date:    10/23/96 11:33 AM
       > I've discovered a cool way to VC the EG time constants 
       > cheaply but it does require some added mess.
       Please elaborate!!!!
  - Thanks,

More information about the Synth-diy mailing list