capacitor. This reduces the size, normally the height, of the capacitors used, making the whole
PSU assembly physically less high.
The voltage across the smoothing capacitors is fairly constant but it is not stable enough to
drive a modular synth. Modular synths need to have a regulated supply so that they stay in
tune and behave in a predictable fashion whatever patches are selected. The regulators smooth
out the voltage across the big capacitors even more and will produce a constant, low ripple
+15V and -15V output.
Both of the regulators work in a similar way. You can think of them like a person controlling
the speed of a water wheel. If the wheel runs too fast, the water flowing over the wheel is cut
back by closing the sluice gate. If it runs too slowly, then the water flow is increased by
opening the gate. The person uses his own eyes to monitor the speed of the wheel, and will
make adjustments to the sluice accordingly. Now the wheel speed will depend on two things
primarily, firstly the water rate, but also the load on the wheel itself. If the wheel needs to do
more work it will slow down unless the water rate is increased.
The regulated power supply must also control its output with changes of load (ie. the number
of synth modules in your system) and the input voltage (ie. fluctuations in the line lump’s
output).
Before we talk about the circuit in more detail, I ought to say why I haven’t used the more
common and I might add cheaper 7815 and 7915 regulators. The main reason is down to
controllable current limiting. The 78XX and 79XX series do not feature this. In fact, if you
were to short out a 7815 to ground, it would take around 1.7A from the supply and would
heat up until the inbuilt thermal limiter kicked in. 1.7A may well be high enough to damage
any connected transformer or line lump, to say nothing of the rectifiers and smoothing
capacitors that would have to supply this current. By using the LM723 and a few external
components we put the control back in the hands of the builders of the circuit.
Let us look at the positive regulator first since this actually also controls the negative rail too.
The circuit is built around the venerable LM723 chip, U2. It contains pretty much everything
you need to make a small regulator, including precision voltage reference, feedback control
systems and a current limiter. However, it doesn’t have a very big output current capacity so
to use it for anything greater than 100mA, one needs to use an external pass transistor, Q1.
The pass transistor is analogous to our sluice gate. The resistance of the pass transistor is
effectively changed by the voltage at its base (pin 1). Decrease the voltage at its base with
respect to its emitter (pin 3) and the transistor will increase its effective resistance and the
output voltage of the PSU will drop.
The base voltage is controlled by the internal electronics of the LM723, but this in turn
responds to the feedback from the output of the power supply. The feedback path is
analogous to the sluice gate man’s visual record of the water wheel’s motion. In this case a
fraction of the output signal is fed back into the LM723 at pin 4. The electronics will
determine whether to make the base drive for Q1 higher or lower depending on what it ‘sees’.
The ADJ trimmer adjusts the fraction of the output voltage pin 4 will see and thus controls the
overall output voltage.
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