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This is only a preview of the February 1996 issue of Silicon Chip. You can view 26 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
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Surround Sound
MIXER & DECODER
PART 2 – By JOHN CLARKE
Last month, we described the circuit details
for the Surround Sound Mixer & Decoder and
gave the parts layout for the main PC board.
This month, we complete the construction
and give the test procedure.
We’ll begin this month with the
display driver PC board assembly –
see Fig.5(a). Among other things, this
board carries the four display driver
ICs (IC12-IC15) plus four 11-way pin
header sockets which make the connections to the display board.
Begin by installing PC stakes at the
external wiring points. This done,
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install the wire links, resistors, capacitors and diodes D1-D4, taking care
with the diode orientation.
The four ICs (IC12-IC15) can now
be installed, followed by transistors
Q1-Q4. Trimpot VR1 (shown dotted)
must be mounted on the underside
of the board – see also Fig.6. Do not
insert the pin headers yet; that step
comes later when the display board
is mounted.
Fig.5(a) also shows the LED display board. Begin by soldering in the
four 150Ω resistors. This done, the
40 LEDs can all be mounted, taking
care to ensure that they are correctly
oriented (the anode lead is the longer
of the two – see Fig.3). Do not solder
the LED leads yet, since the LEDs must
all be later adjusted for height when
the board is fitted to the front panel.
Preparing the case
The Surround Sound Mixer and
Decoder is built into a cabinet with
a sloping front panel. This cabinet
measures 170 x 213 x 31 x 82mm and
is fitted with two self-adhesive labels.
Begin by affixing the appropriate
label to the rear panel, then drill pilot
holes for the DC power socket, the RCA
sockets and the 6.35mm stereo jack
sockets. These holes can then all be
Fig.5(a): install the parts on the display driver
board and the LED display board as shown here.
Do not solder the LED leads until after the board
has been fitted to the front panel.
carefully reamed to size. This done,
mount the DC socket and the three top
6.35mm sockets in position.
Once the rear panel is completed,
the front panel can be prepared in
similar fashion. Note that the holes
for the LEDs and the display board
Fig.5(b): these are the full size etching patterns
for the display driver and LED display PC boards.
Check the boards carefully for etching defects
before installing any of the parts, as this can
eliminate a lot of hassles later on.
mounting holes should all be drilled
to 3mm. The four mounting holes for
the lid can be cut out using a knife and
lightly finished using a reamer.
Final assembly
The major hardware items can now
be fitted to the recommended case.
Attach the small plastic feet to the
underside of the case first, then install the main PC board. This board is
simply slid into the bottom of the case
so that the input and output sockets
protrude through their matching holes
February 1996 61
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Fig.6: shielded audio cable is used for most of
the wiring from the main PC board to the front
panel controls and the input sockets. Check the
wiring carefully as it is installed, as it is easy
to make a mistake which would be difficult to
trace afterwards.
in the rear panel. Attach the nuts to
the 6.35mm sockets, then secure the
board to the integral standoffs in the
base using the self-tapping screws that
come with the case.
Cut each pot shaft to length before
mounting all the pots on the front panel. The pots should all be oriented so
that the markers on the knobs line up
correctly with the front-panel markings. The toggle switches all mount
towards the rear of the front panel.
Once all the hardware items are in
position, the LED display board can
be completed. First, mount the board
to the underside of the front panel
using four untapped 6mm spacers
and 12mm-long screws, with the four
9mm tapped spacers used as nuts to
hold the board in position. This done,
push each LED into its front panel
hole, check that the top surfaces are
all aligned and solder the leads.
Next, solder the 11-way pin headers
to the underside of the display board,
adjacent to each row of LEDs. Now
plug the display driver board into
these pin headers and secure this
board to the tapped spacers using
6mm-long screws. Complete the assembly by soldering the pin headers
to the driver board – see photo.
Fig.6 shows the internal wiring
details for the unit. Use medium-duty hook-up wire for the wiring to the
display driver board and for the power
supply connections. It’s a good idea to
use red wire for the +12V wiring, green
for the GND wiring, blue for the 0V
wiring, and yellow for the DC socket
and power switch (S7) wiring.
The remaining wiring must all be
run using shielded audio cable. This
includes the wiring between the main
board and all the pots, the top row of
6.35mm sockets and the remaining
toggle switches. Keep these leads as
short as possible while still allowing sufficient length for the lid to be
opened comfortably and use cable ties
to bundle them into neat looms – see
photo in Pt.1.
Test & adjustment
Before applying power, check
thoroughly for possible wiring errors.
This done, apply power and check the
voltages on the main PC board.
If all is well, there should be 12V
between the +12V and GND terminals,
while the GND terminal should be at
about 5.45V with respect to the 0V rail.
If these are incorrect, switch off power
February 1996 63
immediately and locate the fault before
proceeding.
Assuming that all is OK, check that
pin 8 of each LM833 IC (IC1-IC10) is
at +12V with respect to 0V. You can
also check the supply to the ICs on
the display driver board (IC12-IC15).
In each case, pin 3 should be at +12V.
All other voltages are measured
with respect to GND. Check the pin 1
and pin 7 outputs of IC1-IC11. These
should all be within 100mV of the GND
voltage. If all these voltage checks are
OK, set VR1 to midway, then rotate it
clockwise so that all the LEDs in each
display are just extinguished.
The mixer can now be tested with
a signal. However, before doing this,
it may be necessary to make up some
adaptor leads; eg, leads with an RCA
to mono 6.35mm plug and/or leads
with an XLR socket to stereo plug.
This latter adaptor is depicted in Fig.9.
The rear panel carries the six phono input sockets, four RCA output sockets (L,
R, C & S) and a power socket.
This view shows the completed display board assembly,
prior to installation on the front panel. Make sure that the
LEDs are all correctly oriented.
The LED display board in mounted on the driver board
using 12mm spacers and the connections made via four
pin header sockets.
Fig.7: this full-size artwork
can be used as a template
when drilling the rear panel.
+
12VAC
IN
A INPUT
L INPUT
C INPUT
+
+
+
L
OUT
+
+
C
OUT
B INPUT
R INPUT
S INPUT
R
OUT
+
+
S
OUT
+
+
+
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Fig.8: this is the
full-size etching
pattern for the main
PC board. Check the
board carefully for
etching defects by
comparing it against
this pattern before
installing any of the
parts.
To test the unit, first secure the lid
and rotate all level controls fully anticlockwise. This done, connect a signal
to the Left input and adjust the output
and left level pots so that a reading
appears on the left bargraph display.
Check that there is a 20dB change in
level when switching between the
LINE and MIC inputs.
If the Left channel checks out OK,
Fig.9: this diagram shows how to wire an adaptor cable with
an XLR socket on one end and a stereo phono plug on the
other. Be sure to use 2-core shielded audio cable.
February 1996 65
4-CHANNEL
+
L
+
C
MIC
LINE
+
A-CHANNEL
MIC
+
LINE
SURROUND
SOUND MIXER
& DECODER
B-CHANNEL
MIC
+
LINE
+
R
POWER
S
+
OUTPUT
INPUTS
+
C
C
+
+
3
0
L
LEFT
PAN
R
L
PAN
R
-3
-6
-9
+
-12
+
+
-15
-18
CENTRE
C
PAN
S
C
PAN
-21
S
-24
SIGNAL LEVEL (dB)
L
+
C
R
S
RIGHT
+
+
+
+
SURR
LEVEL
LEVEL
OUTPUT
LEVEL
Fig.10: this full-size artwork can be used as a drilling template for the front panel.
you can test the Centre, Right and
Surround channels in exactly the
same manner.
Now feed the signal source into the
A channel input and check the opera66 Silicon Chip
tion of the Pan and Level pots. Note the
interaction between each channel as
the knobs are rotated. This done, test
the B channel in the same way.
Assuming that everything checks
out, you are now ready for some
surround sound recording. You will
probably need some practice to get
everything just right but the results
SC
will be well worth the effort.
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