Re: Finally oscillatin'/VCO stuff
1999-05-25 by Paul Schreiber
>From: "J. Larry Hendry" <jlarryh@...>Wonders of algebra.
>
>Well, I won't say I'm the last to get my VCOs going, but I didn't get in
>any hurry. I got them both powered up tonight and they seem to be doing
>what they are supposed to do. I set the V/Oct trimmer at 11.66K ohms as
>Paul indicated that was the "calculated" amount for correct V/Oct
>performance (yes, that was close to 12 turns from max). It is very close,
>and the tracking is already great before I have it completely dialed in.
>I'll get to that tomorrow.
>Usually it's better than 500uV over 4 octaves.
>I am also very pleased with the tracking of the Kenton Pro2000 that I am
>using for my control of my MOTM. looking at about 7 octaves I found that
>the V/Oct actual voltage according to my Fluke 77 is within .02 volts
>across the range.
>This is in the manual (sigh). It is due to the protection network of R39 and
>I have not had an opportunity to try all the modulation stuff, but I have
>noticed a couple of things I will toss up for discussion or consideration.
>
>1 - My FM1 switch position has some effect on pitch with the FM1 pot turned
>all the way down and nothing plugged into FM1 input. In the DC position,
>flipping the switch from EXP to LIN causes a shift in pitch. The same is
>true in the AC position, BUT to a much lesser degree. Actually, unless you
>have two oscillators fine tuned to the same frequency, you might not be
>able to hear the difference in the AC position. I certainly do not see
>this as a problem, since these switches will be left in one particular
>position if nothing is used in the FM1 input. However, I wondered if
>other's VCOs did the same (ie: am I normal?).
R40
to keep the $23.00 MAT-02EH from blowing up. A small price to pay.
>Yep, leads are switched, I bet.
>2 - I admit my ignorance to VCO sync. However, My hard/soft sync switch
>seems to be operating backward to my thinking (yes, I will check the wires
>again following sleep). In the soft position, my VCOs are dead locked. In
>the hard position, I get a VERY loose sync. In fact, if one oscillator is
>slightly de-tuned while I have a sync like this, the sync breaks. Unless I
>am missing the obvious, the only connection required is one cable from sync
>i/o of one oscillator to the other. I hope to learn more tomorrow when I
>hook up the scope to set the sine symmetry.
****NOTE****
The BEST way to HARD sync 2 VCOs is to put the MASTER VCO switch on SOFT and
the
SLAVE on HARD. Patch between the SYNC I/O jacks.
The reason is that the sync input circuit is AC-coupled by network R12/C4/D6
to make a "glitch"
to trigger reset of the cap C15. Therefore, the HARD sync *wants to see
narrow pulses*. The SINE
and TRI waveforms are *not* good sources of sync (this is also in the
manual). Since the SOFT sync
pulse is a controlled, narrow pulse, it is PERFECT for the job!
Note that SOFT sync only works if:
a) the inital freqs are within about 2 1/2 octaves
b) the freqs of the VCO are within 5-10% of each other.
If the freqs are wider apart, you will get "over-synching" and amplitudes of
one VCO
will start to decrease rapidly. When detuned, the sync does "break off":
it's supposed to!
This brings up a point!
Dave B. reported soft sync concerns. I haven't had any VCOs to confirm this
(cobbler's children have no shoes).
Everyone test SOFT sync!
Again, set BOTH VCOs to soft, and patch SYNC I/O together. Tune the VCOs
*close* to each other. As
you slowly detune one (use the FINE pot) you sould hear the VCOs go in and
out of sync.
The way this works is that the 2 sync pulses are OR'd together. The resistor
divider of R21 to R26 sets the "trip point".
Now, in order for THAT to work, the impedance looking back into the FET
buffer needs to be dominated by R26,
without the input Z of U3 getting in the way.
It may be that some VCOs work perfectly (Larry's appears to, the first 3 of
mine I built did) but part-to-part variations
may pervent it.
There may be no easy fix. This was a "freebie" based on the EMu VCO design.
If it is "flakey" there may be little I can do about it.
I am building up 8 (!) VCOs for people. Next week I can test them and see
for myself.
I think the "general wonderfulness" of the VCO's other features still makes
it a great module. Maybe we have a "quirk" or
in Microsoft parlance "undocumented feature".
>You should then hear THAT output through the '410 filter! Whee Haaa! Just
>
>BTW, two VCOs tuned a 5th apart, both fed into the MOTM-120 on "cross" mode
>is one awesome fat nasty waveform.
>
like
Alan Parsons "I Robot" LP with the Mutron Bi-Phase all over it.
Paul S.