My new VCO

[jh]

Hi,

I have attatched the schematics of my new VCO. 
It's "Poly Modular Compatible" (if anybody is still interested),
i.e. V/Hz keyboard input (from a MUX'ed expo converter etc.),
and it has a tempco-less exponential current-in summing node for
exponential Modulation / Detuning etc.

But I've actually built this VCO for different reasons, namely
a little performance synth inspired by the old Korg Monosynths.

I've been bringing up the topic of thyristor-based VCOs on the
list some time ago, and I remember there were quite some
critical comments. But I wondered why Korg used them over
such a long span of time, in so many different products from their
earliest stuff up to the Trident. I thought they cannot be *that* bad
then. 
Though they were also used in true wide range VCOs (MS-20),
I guess that they really have their biggest advantage in the
upper frequency range: Very short discharge time. There is no
delay from a high impedance buffer, no comparator and monoflop,
just the two complementary transistors that immediately discharge
the capacitor when the voltage across them gets over the threshold.
So it's probably the best idea to use them in a configuration
like the 700 (Minikorg), 800DV etc.: Run them in the highest
footage (2'), and use a digital divider to generate the lower
octaves. This is no crappy staircase stuff, mind you. A SAW
wave is reconstructed for every footage by adding just the right
amount of the top octave SAW to the staircase. Apart from
*very* short glitches (inaudible ?) you get very nice waveforms
for all octaves. A bunch of cheap 1%  resistors do the job. (Sorry
Jorgen B. - no E12 values; even with E24 I still needed some 
combinations.)
With the VCO running at rather high frequencies, a (possible)
leakage of the thyristor (is there any ? Martin ?) would not be
harmful either.
Remains the infamous Interlock problem. Everybody who played
an MS-20 knows how bad this can be. (Maybe it's part of the
MS-20's "thin" sound ?). In order to avoid the use of the unregulated
voltage for the buffer FET (MS-20) or a negative reference voltage
for the exponential converter (Polysix), I chose to generate two
positive auxiliary voltages, +5V and +10V, anyway. Now I
carefully designed that part to get a precise DC voltage, and very
strong AC decoupling at the same time: A slow dual opamp is used
for the reference voltages, with a tantal across the outputs to provide
the HF current spikes as good as possible. (the cap is large enough
to form a dominant pole with the opams' output resistance in order
to preserve stability.) In case this decoupling was not enough, I
have RC-decoupled *both*, the GND and +15V supplies of the
dual opamp. Only the resistor divider for the reference voltage is
connected directly to the rails to provide exact DC values. 
I can proudly report that this prevents any interlock. I've built two
of these circuits in close proximity on a single Eurocard (16x10cm),
and I have very pleasant fluctuations of slow beat rates in all
octaves, but no unwanted sync. Needless to say that a pair of these
VCOs has a built in "Linear Detune" feature without extra tricks:
You can set the constant term of the beat rate with the Offset trimmers,
and the frequency proportional term with the Tune trimmers (or with
a tiny DC voltage at the Expo Mod input).

No waveform converters in the drawing - of course you need these
in addition, depending on the desired VCO waveforms.

(BTW, if you don't like V/Hz, just use a tempco resistor and a better
expo converter.)

More notes about the circuit, especially the components, in the
drawing.

Hope you'll like it,

JH.

 
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