VCOs for home-built modulars - update

Joachim Verghese jocke at
Fri Sep 1 09:13:04 CEST 1995

> 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.

I've had similar experiences with my home-built Chroma VCOs - no problems
at low frequencies. If I remember correctly, the sawtooth stops at around
5 Hz with the 1000pF integrating cap. I did, however, try to make the
integrator input/charge pump output trace as short as possible.

At high frequencies, the 20us blanking period becomes visible on a scope,
and the ramp less steep since the exp generator 'steals' an increasing
amount of current from the charge pump. This is the cost for having good
control current <-> frequency linearity. 

But as mentioned before, the obscurities with this design are mainly
visible, rather than audible.

>      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.

That's why d*g*t*l synths are what they are.... :)


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