VCOs for home-built modulars - update

gstopp at gstopp at
Thu Aug 31 23:49:11 CEST 1995

     Hi 808,
     Oh cool you have a Chroma cuz I'm curious - what would you estimate 
     the lowest possible VCO frequency to be? I certainly don't doubt that 
     the Chroma can go lower than my test circuit but the only substitution 
     I made was to use a TL082 instead of a 4558 in the reference current 
     amp in the expo converter. As for layout I think it's okay but I'll be 
     the first to admit that sometimes it can be a "black art". Don't see 
     any obvious problems, though. Probably sumthin' dumb.
     Yah yer right even if the saw gets a flat spot around 15 khz the first 
     possible harmonic is 30 khz so really it shouldn't matter spectrally. 
     I concede that I probably sound like a waveform snob and should tell 
     everybody that if I give the impression that I think that any machine 
     is less expressive than any other that's not my intention nor belief.
     Anyway for the curious this is what the scope shows:
     Low frequency saw (~60 hz):
               *  *  *  *  *            *  *  *  *  
           *                        *
         *                       *
       *                       *
      *                      *
     *                      *
     *                      *
     High frequency saw (~15 khz):
                 /|            /|
               /  |          /  |
             /    |        /    |
           /      |      /      |
     _____|       |_____|       |_____
     I can't back this up but I think that human perception goes beyond 
     20Khz. Maybe it's just a tingle at the back of the neck or something 
     that bypasses the ears but I swear there's something up there. If I 
     frequency modulate one VCO with another and start hypersonic and come 
     down, as I pass through the limit of hearing there are some wheeps and 
     squeals that I can only get on modulars with good-lookin waveforms. 
     I'm not sure how such things can make their way out of the 
     reproduction system and into the air waves in the first place. Maybe 
     it's some interaction that happens way up and I'm hearing the 
     artifacts that splash down into the audible region. Anybody have any 
     thoughts in this area?
______________________________ Reply Separator _________________________________
Subject: RE: VCOs for home-built modulars - update
Author:  SuperBAD MoFo <mr808 at> at ccrelayout 
Date:    8/31/95 11:48 AM
gstopp wrote:
> >     THE CHROMA VCOs:
>      I got the Exar 4151's in the mail (thanks Ric!) and made a dual
>      sawtooth VCO per the Chroma schematic. This dude has a sweep range 
>      from around 60 hertz (try to go lower and oscillation stops) to
>      just above 30 khz. Waveshape ranges from a very jittery
>      rounded-off rising saw at the low end, to a fairly normal-looking 
>      saw in mid-frequencies, to a half-sawtooth half-square-wave mutant 
>      thing around 15 khz, to a train of narrow pulses around 25-30 khz 
>      before they dwindle away into thin spikes.
Sounds like you have a problem w/ either your construction techniques or 
your substitutions - my Chroma has no problem oscillating at low 
frequencies.  I think the sawtooth does flatten out on the bottom a little 
bit at high frequencies, but this doesn't really have an audible impact on 
the harmonic content.  Not sure what happens in the ultrasonic, but what 
are you doing that requires an ultrasonic oscillator anyway?

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