The core of this VCO is a traditional sawtooth oscillator. C14 is the timing capacitor that is charged
by the output of the exponential convertor. Since one end of C14 is tied to +5V, the voltage at the
other side of C14 moves towards zero volts as current is ‘sucked’ out of it. The higher the current
the faster the voltage drops. This voltage is ‘sniffed’ by one half of dual op-amp U5, which
produces a replica of this voltage at its output. Q4 aids the output of U5, as well as providing a
suitable offset for the triangle generator circuit (see later).
U7 (pins 1, 2, 3) is a comparator, and it is ‘watching’ the output of U5. When the voltage across
C14 reaches -5V, the output of U5 (pin1), normally at -15V, suddenly flies upward towards 10V.
This sudden level change passes through C13, and turns on JFET, Q3. This rapidly shorts out C14,
and the voltage across the capacitor drops to zero and both of its pins are at +5V once again. This
means that the voltage at the input of U8 is also at +5V, and the charging process begins again. C10
controls the time that the FET is on.
The sync input enables an external voltage to trip the comparator early. This will cause premature
shorting of the capacitor, locking the fundamental of the VCO to an incoming external sawtooth
signal. However, the fall time of the VCO’s saw waveform will still be set by the input CV to the
VCO. Thus, you will not get traditional sawtooth waveforms, but half completed sawteeth. The
sonic affects of these are marvellous, especially if you sweep your VCO with an envelope generator
whilst locked to another VCO at fixed frequency. The controlled VCO is often called the ‘slave’,
while the fixed VCO is called the ‘master’.
The incoming sync signal is first buffered by Q1, a simple emitter follower. This is shown on the
second page of the schematics. R11, C1 and C2 provide decoupling to prevent power supply noise
from accidental triggering of the VCO. The buffered signal is then passed to a simple differentiator
based around C9 (back on page one again). This part in conjunction with D1 and R24 only allow
only fast moving rising edges of the input waveform to reset the VCO core. In theory this should
allow any pulse wave to be used as sync waveforms, but the best sync sounds can be obtained when
a falling sawtooth master signal is used.
This last statement is very important if you want to use sync effectively. Use sawtooth outputs from
Oakley VCOs to sync other Oakley VCOs. MOTM VCOs produce rising ramps not falling
sawtooth waveforms. So you if you want to use a MOTM VCO as the master to sync the slave
Oakley VCO, then you must invert the MOTM’s output first. You could use an Oakley Multimix to
do the inversion.
The suggested layout of the Oakley VCO includes a depth control for the sync input. This will
allow you to create partial synching effects. This effect is very difficult to describe in words and has
to be heard to be believed. But very very complex timbres can be produced this way.
At high frequencies the VCO can go a little flat due to the finite time it takes to reset C14, and
errors in the exponential convertor. By lowering the maximum peak voltage of the sawtooth
waveform, the capacitor has less charge to loose before the output of U5 reaches zero. Thus the
frequency is higher than it would have been. Using a resistance in series with C14, we have a
voltage drop developed across the resistor that increases with frequency, due to the increased
current through the exponential convertor. This method was first postulated by Sergio Franco, and
is usually called ‘Franco’ compensation because of this. This works very well, but it does mean that
the amplitude of the sawtooth waveform decreases slightly with increasing frequency. In the first
two VCO issues I made this resistor variable. I was using the Franco resistance to compensate for
both reset time and errors in the exponential convertor. In the issue 5 VCO, this resistance is fixed
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Содержание VCO 5U
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