[sdiy] ASM-1 Questions

[sbernardi at attbi.com]

It's not that unusual for a JFET to work with source and drain reversed when 
used as a switch. I think you might see some difference if it were used as an 
amplifier or source follower.
As for saw to triangle converters, I have had good luck with the circuit shown 
in http://home.attbi.com/~sbernardi/elec/lockbox/lockbox_waveshaper.gif. You 
just need the part surrounding U4:D and U4:A, which form an absolute value 
circuit. This takes a sawtooth symmetric about zero (like the ASM-1 +/-5v 
sawtooth coming off U2:B (before the 1K R24). It doesn't matter whether the 
sawtooth is an up-ramp or down-ramp, as long as it is symmetric about zero. 
U2:B is like U4:C in my circuit. The output at U4:1 will be a triangle going 
from 0v to +5v. U4:C amplifies it and shifts the center so you have a +/-5v 
triangle at U4:7. 
The -VREF is a -5v source, but you can also use -15v and make R41 a 301K.
The TRI SYM trimpot is used to trim the triangle to a nice point, if you have a 
scope. If you don't, just tweak it until it sounds the most "mellow".
You can see a CA3080 based sine shaper in there too (U6 and U11:B).

Scott Bernardi
> Hi all,
> 
> I am a beginner attempting to build an ASM-1.  I have constructed the VCO
> and the VCF on a breadboard and both seem to work quite well.  I haven't had
> this much fun for several years!  I just have a few questions...
> 
> Two different schematics for the VCO design specify the JFET orientation
> differently (switched S & D) so I have flipped back and forth to test (I am
> using NTE466.)  There does not seem to be any difference in sound with S & D
> reversed.  Is that a bit odd?
> 
> For several days now I have been trying to construct a saw to tri converter
> with no luck (using many different designs I've found on the net.)  I can't
> seem to get rid of the reset at all.  I noticed that my sawtooth wave from
> the VCO is a 'falling' waveform - is this correct?  I assume this would
> affect any waveshapers.
> 
> Unfortunately I do not yet have a scope, but I am looking at waveforms using
> my soundcard (There is a 465b on the way though!!)
> 
> Thanks!
> aaron
>