Silicon Chip – April 2019

siliconchip.com.au Australia’s electronics magazine April 2019 39

with ]Bass & Treble Controls

]Motorised Volume Control

]Infrared Remote or Manual Control

]Relay input switching and isolation

]Suits practically ANY amplifier modules!

110 x 85mm.

Start by fitting the resistors where

shown. We published the resistor col-

our codes last month but it’s always

best to check the values with a DMM

set to measure resistance to make sure

they’re going in the right places.

Follow with diodes D1-D3, ensur-

ing that their cathode stripes face as

shown, then feed some resistor lead

offcuts through the ferrite beads and

solder them in place.

We recommend that you solder IC4

directly to the board, although you

can use a socket if you really want to.

Either way, make sure its pin

1 dot/notch faces

to the left, as shown.

Fit the MKT/MKP/ce-

ramic capacitors next.

We explained in detail last month

why there are three different options

for the 470pF capacitors, and that if

you use ceramics, they must be NP0/

C0G types for good performance.

We used MKTs on our prototype.

Mount them, plus the two 100nF

MKTs now.

Next solder the four transistors, not-

ing that Q5-Q7 are BC327s while Q8

is a BC337. The two electrolytics can

then go in, with the longer positive

leads through the holes marked “+”,

followed by the 10-way and 14-way

header sockets, CON8 and CON9.

These sockets must be installed with

their slotted key-ways towards the top.

Finally, complete the as-

sembly by installing the

relays, the three stereo

RCA input sockets and the two verti-

cal RCA output sockets.

Note the left and right labelling for

the output sockets – this is not a mis-

take and arranging them this way gives

the optimum layout for the PCB.

Front panel pushbutton board

assembly

There just four parts on the push-

button board – the three pushbutton

switches on one side and the 14-way

IDC header socket on the other (see

Fig.11 above). The board is coded

01111113 and it measures 66 x 25mm.

The three pushbuttons can go on

first but note that they must be in-

stalled the right way around.

These have “kinked” pins at each

corner plus two straight pins for the

01111112 T INPULIFIER MP PREA100n

F

100n

F

Q8

2.2k

Ω

2.2k

Ω

10k

Ω

10k

Ω

100kΩ

100kΩ

470pF 470pF 100kΩ

10 μF 10 μF

Q7

Q6

Q5

D1 D2 D3

CON9

2.2kΩ

2.2kΩ 2.2kΩ

2.2kΩ

2.2kΩ 2.2kΩ

2.2kΩ 2.2kΩ

2.2kΩ

100

Ω

100

Ω

100

Ω

100

Ω

100

Ω

100

Ω

CON8

CON1 5 CON1 4

RE

LA

Y1

RE

LA

Y2

RE

LA

Y2

BE

AD

BE

AD

CON1 1 CON21 CON1 3

9 1

10 2

13 1

14 2

IC4

LM 393

IC4

LM 393

4004 4004 4004

INPUT

1

INPUT

2

INPUT

3

RIGHT

OUTPUT

LEFT

OUTPUT

ÓSC 20 1 9

TO CONO 9 N INPUT SELECTOR BOARD

CON1 0

(ON BACK)

14 2

13 1

S+ 11 LED S2+LED2 S+ 33 LED

Fig.10 (left): follow this diagram to build the

input selector PCB. Make sure that the two

header sockets are correctly orientated and

note that Q5-Q7 are BC327 PNP transistors

while Q8 is a BC337 NPN transistor.

Fig.11 (below): the three switches are

mounted on the front of the pushbutton

board while the header socket goes on the

back (key-way towards S2). Take care with

the switch orientation (see text): the six pins

for each switch are for the switch contacts

themselves (four) plus two for the integral

LEDs.

These views show the completed

input selector and (at right) both sides of

the pushbutton board assemblies. Note the orientation

of the header sockets on the two modules – check that

these sockets, the relays, the RCA sockets and the

button switches are all sitting flush against their respective

PCBs before soldering their leads.