VCO Designs

[Stopp,Gene]

From: Harvey Devoe Thornburg

> So... I?m going to build my first "real"
> DIY-synth. Only problem is the VCO...
>
> In other words...
> Does someone know good VCO?s with saw/pulse
> waveforms and pw/pwm. With no Hard-to-get
> (read: Curtis)-ICs.
>

Sorry if this has been discussed before, but has anyone
tried the Harris ICL8038?  This is all I can find on the Web.

You can always add PWM with a comparator,
so no need to look for this specifically...

 --Harvey

The 8038 has been discussed before, but don't apologize - people come
and go on email lists all the time. Some questions are good enough to
answer over and over - depending on who's on the list at any given time,
you just may see something new!

Anyway, it is the consensus that linear VCO chips aren't good enough for
wide-range musical use in DIY circuits. This sounds like a bit of an
arrogant statement, but let me explain.

VCO chips that aren't specifically designed for musical use have linear
voltage-to-frequency response. VCO chips for music have exponential
voltage-to-frequency response. Linear chips include the CD4046, 8038,
566, EXAR 2206, and some others. Exponential chips are the SSM 2030 and
the CEM 3340 (are there any others, BTW?). Linear chips are easy to
find. Music chips are out of production (but can be found, just stick
with this list for a while).

Exponential response allows volts to become octaves. The intention
behind this relationship is due to the physics of musical vibration.
Using exponential VCO's (and VCF's, too) for pitched sounds makes it
real easy to transpose, modulate, and track musical intervals. Linear
response cannot produce musical intervals with the same ease of use.

Exponential VCO's have a very wide range, like .01 Hz to 50 kHz, with
one sweep of the control voltage. Linear VCO's have a much narrower
range, due to limited supply voltages and other factors.

Exponential VCO's require an exponential converter. This is a rather
finicky circuit, since the natural exponential current ratios of
semiconductor junctions are used, and they are quite sensitive to
ambient temperature and must be compensated for this. This subject has
been and always will be a hot topic in analog synthesis (pun intended,
ha ha). Linear VCO's have no such instability - they are rock solid.
However, when somebody says that linear VCO's are unstable, what they
mean is that linear VCO chips with external exponential converters are
less stable than VCO's that have exponential voltage-to-current
converters built in. The chip is stable; the converter is not.

It would be a mistake to say that linear VCO's are not musical, however.
Precise pitch control of musical intervals is only a subset of that
which can be considered musical. Sound effects and ambient sounds can be
created just as easily with both VCO types.

Some very nice synthesizers have been built around linear VCO's. The
Korg MS10 and MS20 are wonderful machines. Their VCO's are restricted to
operation within the limits of their architecture, but nonetheless they
are great little music-makers. The same goes for the old Yamaha CS
stuff, and the old PAIA kits.

The SSM 2030 VCO chip needs external temperature compensation, so it's
really not very popular anymore (except as replacement parts for
machines that use them). The CEM 3340 chip has internal temperature
compensation, and is a great, great VCO. Too bad they're not everywhere.
They are so good that they take the fun out of making your own VCO.

It is possible to make a very stable and highly accurate exponential VCO
using discrete parts. An exponential converter can be made from two
op-amps and a matched transistor pair. For best results you will also
need a tempco resistor, which is a bit difficult to come by. The rest of
the VCO can be made a few different ways:

1. Op-amp integrator with fast comparator and FET discharge
2. CA3080 integrator with CA3080 comparator
3. Op-amp integrator with charge-pump discharge

Design #1 results in a clean sawtooth of perfect shape and an extremely
wide range, typically 0.01 Hz to well over 100 kHz. Design #2 results in
a clean triangle and square waves, but the sawtooth must be pieced
together with a waveform converter and can have minor glitches in it.
This design can also use the newer LM13600 or LM13700 parts. Design #3
results in a less-than-perfect sawtooth, but has very few parts. Pulse
waves and pulse-width modulation are easily added to any design.

So, there you have it. Linear chips can be used, but you must constrain
yourself to sound effects and limited pitch intervals. Exponential has
superior range and ease of use, but the exponential converters require
less-than-common parts. CEM 3340's are expensive and of limited
availability. There's just no easy way out.

 - Gene

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