VCOs:
As Elvis would say "Oh man!!" I just finished tweaking my two 300 VCOs and
WOW! The tracking is absolutely incredible. I got mine to within one
cycle at the top of an 8 octave range. Through the first 7 octaves, they
are dead on. And the adjustment was SO EASY. I found myself wishing the
"fine tune" was a little more fine during the tuning process. Talk about
tweaking !
First off, thanks to Dave B for his original tips of May 14 on how to tune
the VCOs. I found this to be an excellent starting point to get very
close, very fast. Following that I took my first oscillator and did some
further tweaking compared to an external DCO as a reference. I still used
the method described by Dave to determine the direction of adjustment.
However, after you get started, that becomes second nature.
After I completed, I went back and made some final very small adjustments
to tweak the two 300s exactly together using the same CV from the Kenton.
This eliminated any thought of error introduced by the Kenton (or other
MIDI2CV you may be using) as I beat the two oscillators together at each
octave.
When I first started, I tried something else which might be helpful to
some. When comparing the low note of the drone oscillator to the higher
note of the oscillator under control, I found it somewhat difficult to hear
the beating until I got them close enough for the beating to be in a LOW
range (under 20 cycles). During that first rough adjustment phase, here is
a trick you can use to help. I plugged the oscillator under adjustment
into my MOTM 120. When I was comparing it (3 or 4 octaves up) to the
reference oscillator (still low) I used the subdivider as my comparison
signal. At the lower frequencies it was much easier to hear the beat until
you get close. Once you get close, you are better off tweaking at the high
end (disconnecting the 120), but this helped me get right into range very
quickly.
Sync:
OK, here is how mine are behaving after checking them against Paul's
suggestions earlier today:
For hard sync - I was messing up by placing both switches to hard. After I
did what Paul said (master to soft and slave to hard) that magic hard sync
jumped out and just about made me have a sexual experience. MAN, that is
one awesome sound. I found that using audio out of the master oscillator
to the sync of the slave (set to hard) produced the same awesome hard-sync
sound.
For soft sync - I hooked up as Paul suggested and watched with the o-scope
since I had just finished my sine wave adjustment. With both switches on
soft, the oscillators were locked together and would not break sync.
Timber changed as one oscillator was forced to comply. I found that the
oscillator with the highest frequency setting acted like the master. If
one was adjusted to go higher than the other it took over as master.
However, at no time did the oscillators break apart. BUT, when they were
far apart, they went into over-synching just as Paul described and both VCO
amplitudes decreased significantly (watched on scope).
I have no idea whether that is what you modular-savvy guys would consider
right or wrong for soft sync. I am only reporting my particular
performance. What my soft sync would not do is sync two oscillators an
octave or other interval apart. Seems like Dave B might have suggested
that soft sync should do this. But, I might be dreaming.
OK, I've rambled on long enough. GREAT module Paul. Easy to build and
adjust. Tracking is just incredible.
Larry Hendry
-------------------------------
Paul wrote:
> ∗∗∗∗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".