Haible_Juergen's comments on VCO
sscprick at horus.sara.nl
Fri Jun 9 11:40:10 CEST 1995
(This is a msg by Jurgen I forward to the list - Rick Jansen)
Now some things about details:
(1) The sawtooth buffer amp must be connected to the *left*
side of the 2k2 trimmer; otherwise the HFT trimm would have
(2) The electronic switch that discharges the cap is controlled
by 0 ...+5V, but it switches signals in the +5 ... +10V range.
Are You sure the switch You use is capable of that?
(3) Personally, I would avoid coupling capacitors where ever
possible. You have so many components already, why not
adding a little level-shifting instead of just AC-coupling?
Every VCO can be a very useful LFO ...
(4) I would replace the current source by a simple resistor.
I'd invest in a good on-board voltage regulator instead.
(5) You could implement linear FM by simply adding one
resistor ... but You surely know that!
(6) The heater, the heater!!! No I know what someone recently
meant with "flametorch"! You have such a good heat regulated
circuit in Your VCF - based on this omnipresent application note -
Why not use it for the VCO? I have used (nearly) the same thing
for my old VCO's and I never had tuning problems. Had to tune after
half a day or so. But I admit I never carried the thing outdoors.
And I used some Siemens transistor arrays (pin-compatible to
the 3046; I couldn't get the 3046 back then). But I don't think they
are produced anymore. But they seemed to be *very* similar to
(7) You have been much inspired by FORMANT. (Spaced sawtooth:
Fine!). Unfortunately, You also have used the same tuning scheme
as in FORMANT. Offset tuning and scale tuning are very, very dependant
on each other, and this almost drove me crazy. Let me explain:
When You start adjusting, the scale trimmer is just "somewhere", i.e.
Youi have the wrong scale factor. You can set what offset (i.e. initial
"C" or "A" or anything else) You like - it will be fucked up by
any slight turn of the scale trimmer.
You could sum the CV's first, with a scale trimmer in the opamp
feedback path, and then add the offset trim in a second summing stage.
Works much better, IMO. (If I remember right, the "newer" Minimoogs
did it that way!)
The triangle converter looks interesting. Where do You have the circuit
How does it work, exactly? (must be a resistor to gnd at the input, btw)
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