It takes the voltages from the five sources and adds them together. The gain of the summer is set by
the input resistors and R44 and the V/OCT trimmer. For the two 1V/octave inputs the gain is
roughly -0.7. The V/OCT trimmer is adjusted to give a rise of one octave in output frequency when
the keyboard input voltage goes up by 1V. The output of the CV summer is then fed to the
exponential convertor via R29 and R30.
The exponential convertor is based around on half of U3 (5, 6, 7) and the quad NPN array of U4. Its
output is a current that is proportional to the exponent of the voltage applied at the base of the left
hand transistor of U4. The circuit gives the VCO a sensitivity of roughly -18mV/octave, so R29 and
R30 reduce the output of the CV summer to this level. However, it is worth noting that R30 is a
temperature sensitive resistor. The resistance of R30 will go up with temperature at a rate of 0.33%
for every degree Celsius. This should counteract the temperature effects produced by the
semiconductor junctions in U4. To get the best temperature stability R30 is mounted right on top of
U4. This way the temperature of the two devices should be the same.
It is possible to get an exponential response from a single transistor, but that has problems as Vbe,
the junction voltage, changes with ambient temperature. The ‘temp co’ resistor cannot compensate
for this change in the transistor’s operating current. So the now classic circuit with two perfectly
matched transistors and an op-amp, U4 and U3, is used. Changes in the Vbe for one half of the
transistor pair are mirrored in the other. The op-amp then matches the current in the first transistor
with the same current in the other one. So the collector current in the first transistor will effectively
control the collector current in the second. And it is the current drawn by the second transistor that
controls the frequency of the VCO.
The op-amp method also has another bonus, it allows an additional current to be injected into the
inverting pin of U3. This current will directly control the output current of the exponential
convertor. R15 allows an input CV to control this current. We now have a linear frequency
modulation input, whose sensitivity is set by R15. Connecting this input to the wiper of a pot,
allows you to control the sensitivity of this input directly from the VCO’s front panel.
Note that this input is not a true linear input control. This VCO, and most other modular VCOs, do
not have that capability. What we have here is a constant modulation index input. This is linear
modulation, but not linear control. The precise description of this statement is probably beyond the
scope of this document, but in a nut shell it means this: The linear input will not act with a strict
volt per hertz relationship. That is, for every 1V, the output frequency will not rise by a fixed
amount. But it will act so that audio rate FM will create a constant depth of modulation on any
given frequency controlled by the usual 1V/octave inputs. This means that the for audio rate FM
effects (clangs, bells and the like) you can get very good results with the linear FM input.
R3 has been chosen to set the operating frequency of the VCO at approximately 1kHz when the
voltage at the base of the left hand transistor is zero. The exponential convertor works at its most
accurate when the voltage across the two bases is zero. And since the human ear is particularly
sensitive to frequency changes at around 1kHz it is good to make the most accurate part of the VCO
at this frequency.
The output of the exponential convertor is a current. It is this current that controls the core of the
oscillator. Contrary to many people’s ideas, the core of a VCO is typically a linear CCO. That is a
current controlled oscillator. A doubling of current to (or from in this case) the CCO will produce a
doubling of output frequency.
6
Summary of Contents for VCO 5U
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