siliconchip.com.au Australia’s electronics magazine May 2019 71
Fig.6: if you have a transformer with a centre-tapped
secondary, use this overlay.This photo of the PCB actually has ALL the power supply
components shown in the four overlays. Some are obvi-
ously not necessary, depending on the version you build.
(Use the component overlay for your version).
Fig.4: the power supply can be much simpler when the unit
is only to be operated from a single-ended DC supply. Some
components are omitted while others have their values
changed. The negative supply rail for dual op amp IC1 is
connected to 0V via a wire link, while the signal ground is
biased to half supply by a pair of resistors and a capacitor.
Fig.8: this overlay is for the single-ended DC supply, as
shown in the circuit diagram at left.Fig.5 shows the components required for a transformer
with a single secondary (including most AC plugpacks) and
Fig.6 for a transformer with two secondaries connected in
series, or a single centre-tapped secondary.
Fig.6 also applies if you have a transformer with individ-
ual secondaries (eg, two 9V or two 12V secondaries). In this
case, the phases of the windings need to be correct: the end
of one secondary is connected to the start of the other sec-
ondary to effectively form a centre-tapped winding.
You need to be careful with this connection – measure
across the two windings (ignoring the centre tap) to ensure
you have twice the individual winding voltage.
If you get 0V (or close to it) you have connected the two
windings incorrectly.
Where a split DC supply is called for (eg, +15-0-15V) use
the overlay shown in Fig.7, whereas a single DC supply uses
the overlay shown in Fig.8
So let’s get building!
Start by fitting the smaller resistors. While these have col-
our-coded bands indicating their values (as shown in the
parts list), because certain colours can look similar depend-
ing on your lighting, it’s much safer to measure their values
with a multimeter before installing them. Use the appropriate
overlay diagram as a guide as to which resistors go where.
Next, fit whichever of diodes D1-D4 are required for your
particular configuration, followed by zener diodes ZD1 and
(if needed) ZD2. In each case, ensure that the cathode stripe
Fig.5: here’s the overlay for a single AC supply, from either
a 9-16V transformer or plugpack, plugged in to CON5.Fig.7 if you have a split DC supply (eg, ±15V & 0V) you will
connect it to CON5 and omit some components.