[sdiy] Concertmate MG-1 lfo rate mod: Part 2

[Scott Bernardi]

Yes, doubling the value of the integrating cap (by placing an equal value in
parallel) will half the frequencies.


The frequency of a triangle square oscillator (integrator schmitt trigger osc)
depends on three things:
1. The charging current  (basically the output square wave level of the schmitt
trigger divided by the feedback resistor (the frequency adjust pot)
2. The value of the integrating capacitor
3. The trip points of the schmitt trigger (which will be the amplitude levels of
the triangle wave).

The governing equation is:
dv/dt  = I / C

Say you have the output squarewave levels of +/-10v,  triangle amplitude of
+/-5v,  an integrating cap of 10uF, and a 1M pot in series with a 2.2k resistor
for the frequency adjust (the series resistor is necessary because the opamps
have a limited current that they can supply - figure around 5mA max as a good
rule of thumb).
The lowest frequency will occur with the smallest charging current (highest
adjustment of the frequency pot), or 10v/(1M + 2.2K) = 9.978uA or about 10uA.
With a 10uF cap, this will produce a ramp (slope of the triangle) of 10uA / 10uF
= 1v/sec.
A full cycle of the triangle will be from its lower trip point (-5v) to it's
upper (+5v) and then back down to its lower again - that's a total of 20v.   20v
/ 1v/sec = 1 cycle every 20 sec.

For the highest frequency calculation, you use the lowest frequency adjust
resistor, which will be 0 on the pot and just the series 2.2K resistor:
10v/2.2K = 4.5mA.  4.5mA / 10uF = 454v/sec.   20v / 454v/sec = .044sec for the
period, or a frequency of 22Hz.

For the a wider range between high and low frequencies, use a bigger pot - say
10M. You could go to a 1uF integrating cap then and get a range of 1/20 Hz to
220Hz.

For the ultimate in range, go to an oscillator driven by an exponential
generator. You can literally get these to go from subaudio to supersonic  (say
like .1Hz to 30KHz).  Here's one:
http://home.attbi.com/~sbernardi/elec/og2/og2_lfo.gif
The basic triangle / square osc is just U1 plus Q1 and Q2 for the exponential
generator, and U5:B, U2:B, U3 for the oscillator.

DTK wrote:

> Hello again...  Alright, I piggybacked a 10uf cap onto the existing 6uf
> discharge cap, and it works like a charm :) Now I can do really looong nasty
> filter sweeps... yay!
>
> Someone had mentioned making this mod switchable, which I think is a good
> idea... I'd actually like to set up something where I can have 3 selectable
> ranges: normal, slow, and slooooooow :)
>
> My questions:
> - Is the effect of adding capacitance linear? For example, I have a 6 uf cap
> in the circuit and the lfo runs at speed x.  If I insert an additional 6 uf
> cap, will the speed be x/2?
>
> - Is there some way that I can calibrate a circuit like this to get close to
> certain tempo ranges? I'd like to be able to get some degree of accuracy for
> tempo matched sweeps.
>
> Ok, I think that's it for now. I'm off to RS to get some more stuff :)
>
> Modding is rather addictive, isn't it?
>
> Thanks for the help
>
> Duane

--
Scott Bernardi
sbernardi at attbi.com