Progress on ASM-1

gstopp at fibermux.com gstopp at fibermux.com
Wed Aug 7 23:20:01 CEST 1996


     Here's some comments about the ASM-1...
     
     I just realized that I didn't include any notes in my documentation 
     about actually listening to the thing - Bob's right, the signals would 
     be way above line level and should be attenuated before amplification. 
     It's probably a good idea to include a master volume pot on the final 
     machine. One may be tempted to run the final VCA direct into the 
     mixing board and pad it there, but the board's preamp will probably be 
     overdriven. At this point one may be further tempted to cut down 
     either the input to the VCA or its control CV, then go straight into 
     the board, but your signal-to-noise will not be so good. Better to run 
     the VCA input hot, and the control CV full-strength, then cut it down 
     with a volume pot *after* the VCA output for the best S/N ratio.
     
     The noise source "Level" trimpot will adjust the noise level from zero 
     to max - somewhere in between is a good setting. I would probably 
     adjust it so that the pink noise output sounds about as loud as a VCO 
     waveform. The ear is much better than a scope for noise, since to see 
     the whole picture on the scope you'll probably need to set the 
     vertical to 5 volts/div and the horizontal to a real slow sweep just 
     to get a band of fuzz across the scope screen.
     
     As the LFO rate pot is turned almost all the way down (attenuating the 
     square wave down to almost nothing to the input of the integrating 
     op-amp), the LFO frequency will slow down towards zero hertz. Once the 
     integrator's input is at ground, the LFO will stop and the triangle 
     output will probably slowly drift to either the (+) or (-) supply. The 
     minimum frequency before this happens will be determined by a couple 
     things - first, the resolution of the rate pot near the lowest 
     setting, and second, the leakage currents of the TL-082. If you aren't 
     using a TL-082 here, I suggest that you do because something like an 
     LM358 or 1458 will not go to nearly as low a frequency before 
     "stalling". Also remember to de-flux the circuit board because solder 
     flux residue will cause leakage currents and prevent really low 
     frequencies.
     
     In my drawing I showed a minimum frequency resistor in series with the 
     LFO rate pot on the ground side, but I forgot to mention the LFO in my 
     documentation. You can experiment with this to get the lowest 
     frequency - try around 22 ohms first. Of course you can make it zero 
     ohms and let your LFO "stall" at the lowest setting, providing you 
     with a source of (-) voltage or (+) voltage, depending on where it was 
     when you turned the pot down all the way. Many times in my homebuilt 
     stuff I got confused by something like a pulse wave that was silent or 
     a filter that was shut down, and finally stumbing on the fact that an 
     LFO had stalled taking the pulse width or filter cutoff with it. If 
     you like a certain "danger factor" in your performances, then by all 
     means let your LFO's go down to stall speed. I personally like to be 
     fooled by the machines once in a while.
     
     If you do want to add a range switch to the LFO, probably the best way 
     to do it is to switch in another cap across the LFO's integrator. This 
     may be a little messy since you will need to add wires to the 
     component on the board, or run wires from the capacitor's location on 
     the PCB and mount your capacitors on the panel switch.
     
     Also, bi-color LEDs are always fun to have in DIY synthesizers.
     
     Have fun!
     
     - Gene
     gstopp at fibermux.com
     


______________________________ Reply Separator _________________________________
Subject: Progress on ASM-1
Author:  Bob Zimmer <bzimmer at voicenet.com> at ccrelayout
Date:    8/7/96 12:24 PM


This is some possible suggestions for those of you that are building ASM-1's!
     
     <good stuff snipped>




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