Power supply requirements
The design requires plus and minus 15V supplies. The power supply should be adequately
regulated. The current consumption is about 30mA for each rail. Power is routed onto the PCB by a
four way 0.156” MTA156 type connector or the special five way Synthesizers.com MTA100
header. You could, of course, wire up the board by soldering on wires directly. The four pins on the
four way header are +15V, ground, earth/panel ground, -15V. The earth/panel connection allows
you to connect the metal front panel to the power supply’s ground without it sharing the modules’
ground line. More about this later.
Circuit Description
The VCO circuit can be thought of as several little subsections connected together. Let us look at
these little sections separately.
The first thing to look at is the power supply. This is shown on the bottom of the second sheet of the
schematics. The VCO could have been powered by +/-15V, but I wanted the VCO to sound like a
minimoog. To do this, I have used the same voltage levels within the VCO core as the original.
Thus we need to generate 10V and +5V supplies. I still used the +/-15V supply for most of
the op-amps as this would not have any affect on the waveforms.
Power enters the board on a 4-way MTA156 connector (PWR) or modified six way MTA100
header (PWR2). It is filtered initially by two small ferrite inductors, and decoupled by C20 to C23.
These prevent stray high frequency pulses from entering or leaving the unit by the power supply
lines. A LM723 voltage regulator IC is used to provide the 10V. This is an old IC design, some 30
years, but it is a superb device and has very low noise. The only problem is that you have to set the
output manually. PSU is a trimmer to set the voltage to be exactly 10.00V. The PCB has been laid
out for a 6mm cermet trimmer for long term stability.
Additional power supply decoupling is provided to each set of op-amps and ICs. This prevents any
reset pulses from travelling down the power supplies and soft synching the other VCOs in your
system.
To find the next part of the power supply we need to look at the first sheet of the schematics. Two
sections on this page generate the 5V and -10V required by the VCO.
U2 (pins 5, 6, 7) provides the +5V supply, by simply buffering half the +10V rail. An LT1013 is
specified to provide a very stable output.
U3 (pins 1, 2, 3) is a simple inverter circuit that produces a very stable -10.00V supply for the tune
pots and trimmers. Together with the 10.00V supply, this prevents any perturbations in the
+/-15V supply rails from affecting the tuning of the VCO. In reality, the tolerance of the 22K
resistors may mean that the output is not exactly -10.00V. This is perfectly OK, since we are not
interested in the actual value so much, but the stability. Those 22K resistors are metal film and their
value will not change appreciably over the years or at differing temperatures.
The VCO’s pitch is determined by a variety of sources. Two 1V/octave CV inputs, and two pots on
the front panel, and a trimmer to set initial frequency. U2 (pins 1, 2, 3) is built as a voltage summer.
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Содержание VCO 5U
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