Silicon Chip – April 2019

40 Silicon chip Australia’s electronics magazine siliconchip.com.au

integral blue LED. The anode pin is the
longer of the two and this must go in
the hole marked “A” on the PCB (to-
wards the header).
Once the pins are in, push the but-
tons all the way down so that they sit
flush against the PCB before soldering
their leads.
The IDC header socket can then
be installed on the other side of the
board, with its key-way notch towards
the bottom.

Choice of power supply

If you are building this preamp as
part of a full amplifier, the chances are
you will already have a suitable power
supply which produces the required
±15V DC rails. Otherwise, we men-
tioned a few different suitable power
supply boards last month.
That includes the March 2011 Uni-
versal Regulator (siliconchip.com.
au/Article/930) [available as a Jay-
car kit, Cat KC5501] and the Ultra-LD
Mk.2/3/4 power supply board, last de-
scribed in the September 2011 issue
(siliconchip.com.au/Article/1160). In
case you don’t have those magazines,
we’ll quickly cover building both of
those supplies here.
The Universal Regulator is a good
choice if you’re building a standalone
preamplifier, or building the preamp
into an amplifier which already has a
power supply but doesn’t have ±15V
DC rails.
The Ultra-LD power supply is best

if you are building the preamp into a

complete amplifier that you’re making

from scratch.

Building the universal regulator

Fig.12 shows the circuit of the Uni-

versal Regulator while Fig.13 is the

PCB overlay.

You can power it from a 30V centre-

tapped transformer secondary (15-0-

15V) or a single 15V winding.

The centre-tapped option is better

if you can swing it, since it results in

a lower ripple at the regulator inputs.

The AC output of the transformer is

rectified by a bridge formed by diodes

D1-D4 and filtered by a pair of 2200μF

capacitors.

It’s then regulated to +15V by REG1

and -15V by REG2. These regulat-

ed rails are available from terminal

block CON2, which is then wired to

the preamp.

It’s built on a board coded 18103111

which measures 71 x 35.5mm. You can

get this from the SILICON CHIP ONLINE

GND

IN OUT

GND

IN OUT

A

K

A

l K

l

D5

D6

100nF

100nF

100 μF

25V

100 μF

25V

C1

2200 μF

25V

C2

2200 μF

25V

LED1

LED2

1.5kW

1.5kW

SC^2011

D1

D2 D3

D4

REG1 7815

REG2 7915

CON1 CON2

INPUT OUTPUT

+15V

0V

–15V

T1

15V

15V

20.5V

20.5V

230V

A

N

0V

A A

A A

A

A

K K

K K K

K

UNIVERSAL REGULATORTAPPED TRANSFORMER SECONDARY, DUAL OUTPUT CONFIGURAT ION

A K

LEDS

A

K

GND OUT

GND

IN

7815

IN

OUT

GND

IN

7591

D1-D 6: 1N 4004

1

2

3

3

2

1

Fig.12: the Universal Regulator circuit generates ±15V rails. Diodes D1-D4 form a bridge rectifier, while capacitors C1 & C2

filter the rectified AC. Regulators REG1 & REG2 provide a steady output voltage while LED1 and LED2 indicate operation.

You can also use a transformer with a single secondary (or a plugpack) connected between pins 1 & 2 or 2 & 3 of CON1.

The “Universal” power

supply board can handle

a wide range of inputs and

outputs.With a 15-0-15V AC transformer

you will get a regulated +15, 0V and -15V

DC supply, perfect for the Ultra Low Noise

Preamplifer (and many other projects!).