1999-09-18 by J. Larry Hendry
> From: John Speth <johns@...>
> 1. Tuning - has anybody been able to adjust the 1V/oct dead on? I know
> this is basically impossible but I'd think that over 5 octaves (which is
> the range I tune over), I should easily be able to tune so I get a slow
> phasing type beating between my reference oscillator (a digitial synth
> generating a sine) and the MOTM-300. The pot seems so touchy when you
zero
> in on that "perfect tune" point that actually nailing it with a slight
turn
> of the pot happens more by luck than by anything else - in other words
you
> can't turn it small enough which is what you need to do. Has anybody
else
> had this problem?
I tried the tuning against a digital synth tuning option and found it to be
a very bad idea. Especially if the digital synth is really a sample based
machine. I found that it was not very accurate at all.
I had best luck beating the VCO against another fixed oscillator that did
not change during the tuning. So, you beat the oscillator under tune to
the other oscillator at varied intervals (mostly octaves). And,
surprisingly enough, it is often easier to hear the beat from an octave or
two off rather than right at the same note.
Here's one idea I found useful. Tune your reference VCO (tuning not to
important) to an interval easy to beat against at the high end of your
scale range. Put that output into a MOTM-120 sub oct and you can use the
varied outputs (exact frequency divisions) for up to 5 different tuning
points to compare the oscillator under tune without one moving the
reference oscillator.
I also found some tracking error introduced by my Kenton. The error was
very small, BUT, it was enough to show up when comparing to a reference
oscillator. Out of curiosity, I compared the Kenton to my only V/OCT
control voltage synth (MicroMoog) and found the Kenton about 5 times as
accurate as the Moog.
When you get your oscillators tracking together, the error introduced by
your controller will be almost none since the error goes to both.
Lastly, I was not able to get both oscillators absolutely perfectly the
same. They are VERY close. Once you do get them tracking the same, any
error can be diminished by doing your initial beating together at the very
top end of your scale range. Errors as you go DOWN from there will be
reduced frequency as the overall frequency is lower. When you are soft
syncing, you will find this method works to eliminate any beating. Trying
to zero beat at the lower frequencies when making your initial patch tuning
will leave you disappointed.
On the subject of a touchy fine pot for zero beating (not to be confused
with scratchy), I do agree somewhat. This seems to be the by-product of
giving the fine tuning pot such a large range. I thought once or twice
that I might like the fine tuning pot to have a smaller range and be easier
to zero-beat (not so touchy). This should be an easy modification if you
are interested (slap me if I say anything stupid or incorrect Paul).
Increasing the size of resistor R3 should lessen the range of the fine
tuning control and make it less touchy. However, you must be careful to
use precision resistors as Paul did to eliminate drift. One easy way to
make a range switch would be to add another 3M32 resistor just like the one
on the board (so no complaints from Paul). Solder it from terminal to
terminal across the back of a SPST switch, so in one position the resistor
is shorted out. Connect the lead normally going to the # 2 pin of the fine
pot to one side of the switch. Connect the other side of the switch to the
# 2 pin on the fine pot. This arrangement would give "normal" fine range
to the control as Paul had intended it in one position and another switch
position for less touchy, smaller tuning range to the fine control. If I
were doing it however, I would size the added resistor so that in series
with the 3M32 it provided exactly 1/2 cent up and down on the fine control
in the "less touchy range."
Finally, I think since there is not room really left on the panel for a
switch, it would be SUPER cool (big dream here) if the fine tune pot
actually had a switch (push-pull) for the different ranges.
Of course, if you don't mind the overall reduction in range of the fine
pot, just putting an appropriately sized larger resistor on the board in
place of R3 makes the zero-beating tuning less touchy. Again, be sure to
use the same precision resistor specified by Paul.
Disclaimer: These are all the ideas of a totally unqualified Stooge
(Larry, that is). Please consult Paul before actually following any of my
stupid advice.
Larry Hendry