opamp rescaling/variable saw-tri-ramp lfo

Rob cyborg_0 at iquest.net
Sat Nov 13 19:36:41 CET 1999

----- Original Message -----
From: Tony Clark <clark at andrews.edu>
To: Synth-DIY <synth-diy at mailhost.bpa.nl>
Sent: Saturday, November 13, 1999 12:35 PM
Subject: Re: opamp rescaling..

> > Ok, heres the deal.. Im trying to rescale a 741 based opamp lfo.
> > I want it to run off of +5v and 0v. Now, according to most things I have
> > read, it seems that all i really need to do is to reference the unused =
> > input of the opamp to about middle voltage ie in this case, 2.5 volts. =
> > This should work. Or should it?
>    Yes, this will work the way you would expect it to.  But just to point
> it out, most non rail-to-rail opamps will swing only to 1 volt of the
> rails (ie 1 to 4 volts) sometimes it is worse (up to 1.5 volts).

Yes, you are absolutely correct.. My findings reflect this. The opamps are
definitely not up to snuff as far as the total Vout p-p. Its still useable

Im actually considering using the old 7400 nand gate type oscillator as a
pwm square wave output source then feeding that into an integrator to get
what was discussed earlier as far as a variable saw-tri-ramp output so I can
save components and panel space.

The variable slope integrator idea was a godsend. Good that you all
discussed it! ;)

For a LFO, too, this is cool because you can sort of tweak the time in which
the peak mod point occurs in the music.

FWIW, I think this method would be highly problematic at regular vco
frequencies, so you can try it, but be forewarned.. I could see it becoming
unstable at some freq.


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