[sdiy] 'reset' for multivibrator LFO?

[Happy Harry]

Hi Nick:


>From: Nick Zuccaro <nmz77 at yahoo.com>
>To: synth-diy at node12b53.a2000.nl
>Subject: [sdiy] 'reset' for multivibrator LFO?
>Date: Tue, 5 Jun 2001 04:20:10 -0700 (PDT)
>
>
>hey there, i'm working on a few additions to the standard
>2-opamp multivibrator LFO circuit, one feature i wanted to add
>was a reset (or sync-to-tempo), basically when you hit a
>momentary switch the LFO would go to either the high or low
>voltage and start to ramp from that point. how i was thinking
>about doing it was to connect a point in the loop to ground or
>either rail, forcing that node to a certain potential and
>letting it run from there. the thing that i'm not sure about is
>the integrator, if sudden change in voltage would screw up the
>triangle waveform. basically i need to see if the concept will
>work and which point is the best to use, somewhere in the
>comparator stage or in the integrator stage.

Hmm... sounds like "Nick Danger" to me. You have to initialize
the integrator cap and force the schmitt trigger to the polarity
you want.
>
>another mod i was thinking about is 'quantizing' the frequency
>into discrete tempo multiples, there would be a pot to set the
>tempo and from there switiches to divide/multiply that, e.g. at
>a certain tempo the LFO would cycle in 4 bars, 2 bars, 1 bar,
>half note, quarter note etc. etc. that shouldn't be too hard to
>do, probably by replacing the rate pot with a switched resistor
>network to correspond to the frequency multiple and maybe a few
>trimmers.

should i be worried about temperature stability with
>the passives?

Yes... worry about the stability of everything.

Maybe a ramp style generator and triangle converter will work
much easier for what you want... you can take a standard VCO and
make the timing cap bigger to get a low range.


another way i was thinking of doing this was to
>change the cap value or connect some in parallel, which should
>change the rate, but it might introduce some noise/weirdness by
>swiching it in and out.

Yes it will... the added caps will not be charged so they will stagger the 
triangle output. You might be really clever and precharge them to minimize 
this effect.  This would be easier with a ramp generator with
charging on the "high side" of the cap... like a PNP current source feeding 
a 555 timer oscillator. One end of the cap is always ground, so you can 
parallel them. Drive the other caps with the buffered output of the ramp you 
are using, so when you switch, they are at the same voltage already.

I would not start doing it this way, I'd go with a low range on a VCO... one 
cap.

Good luck !!

H^) harry

anyway, just a few ideas to make the LFO
>easier to work with when making music that has a defined tempo,
>not that there's anything wrong with music without a tempo.. :]
>l8r...Nick
>
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