perfect ‘textbook’ triangle waveform. SHP-T is a trimmer that allows precise enough adjustment in
the operating point to eliminate these glitches.
The created rectified wave is filtered before amplifying and level shifting by U6. The SYM-T
trimmer sets the DC offset of the wave output to match that of the sawtooth output. The final output
is again 10V p-p.
The sine wave shaper circuit, shown on page two of the schematics, acts upon the triangle wave
output. The rounded peaks of the sine wave are created by deliberately overdriving the inputs of one
half of U8, a dual operational transconductance amplifier (OTA). These devices are normally used
as VCAs, but in this VCO it is merely acting as a soft clipping unit. The non linearities of the
OTA’s input stage being utilised to squash the top and bottom peaks of the triangle wave input.
SHP-S adjusts the amount of overdrive. Since U8 gives a current output, it must be turned into a
voltage and this is done by U9 (pins 5, 6, 7).
R18 provides a special negative feedback path within the shaper circuit. This increases the non-
linearity of the whole stage at the critical peaks of the output waveform.
SYM-S provides compensation for the OTA’s own offset voltage. This offset voltage means that
the OTA will soft clip asymmetrically. By adding a small voltage of the opposite polarity to one of
the input pins, we can cancel the effects of the offset.
The sine wave created by this process does not give us a perfect textbook example of a sine wave.
However, the wave is very low in harmonics and for musical uses it is adequate.
The final output, and the final circuit block is the pulse conditioner. This is also found on page two
of the schematics. The first section of this is the CV summer and is based around U9 (pins 1, 2, 3).
This circuit simply sums together the PWM (pulse width modulation) CV input and the voltage
from the wiper of the Width pot. The output of the CV summer will go from around -5V to +5V
when being controlled by the Width pot alone.
The pulse wave generator itself is based around the second half of the comparator, U7 (pins 5, 6 &
7). This compares either the triangle wave or sawtooth wave output with a voltage set by the output
of the CV summer. If the waveform is higher than the voltage from the CV summer, the
comparator’s output goes low. If it is lower, the output goes high. Thus, the output is either low or
high, and spends very little time in between. This creates a rectangular waveform, where the
proportion of time spent high or low is controlled by the WIDTH pot and/or the external CV. If the
width pot is set to its middle position, and no input CV is applied, the output waveform should be a
square wave. The output of the comparator swings from -15V to ground (or zero volts).
The output of the comparator is out of phase with the audio input. Q2 inverts this signal, so that the
generated pulse wave will go high when the saw or triangle wave is high. It would be possible to
generate the correct phase by simply wiring the comparator’s inputs pins differently. Thus you
wouldn’t have to invert the output since it would already be the right way around. Earlier issues of
the VCO, in fact, did this. However, this caused a less than perfect output waveform. The positive
feedback provided by R40, which is needed for good comparator action, would cause the sawtooth
or triangle wave to be superposed to the output waveform. This was considered not to be a problem,
but with the new issue I decided to do things differently. And perhaps more correctly.
What follows after the comparator is, I think, new to modular VCO design. The inverted output of
9
Summary of Contents for VCO 5U
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