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COM
PUTER
PC Surround Sound
Power Amplifier
implemented as a six-channel plug-in card
Design by T. Giesberts
From an idea by M. Ghanem
This six-channel amplifier has been designed to fit into either an ISA or
PCI slot, with its supply derived from the PC. In conjunc-
tion with a set of (possibly home built) loudspeak-
ers this forms the basis of an inexpensive
yet good quality PC surround
sound system.
Although
most PCs have
come with a pair of
speakers for a long
time, the increasing pop-
ularity of DVD drives adds a
new dimension. When the PC
is provided with a five-channel
software player and a sound card
with five output channels, the sound
section can be upgraded to a complete sur-
round sound system.
It is of course possible to use standard
active PC loudspeakers. Most of these are
very cheap, but the quality is nowhere near
hi-fi. What other options are there? Connect-
ing a full set of living room loudspeakers to
the PC is somewhat over the top — both from
a financial and a spatial point of view. We
would think that most PC owners would
rather choose from some of the inexpensive
and compact hi-fi loudspeakers that are
Two integrated amplifiers
Since there is only a limited amount
of space available inside a PC case
and we don’t want to cause a big
load on its power supply, the design
of the six-channel amplifier has to be
kept a modest affair. A large power
output is not really necessary for this
application, since PC users normally
sit very close to the loudspeakers
and the loudness of sound follows an
inverse square law. At a distance of
5 metres you would need a power
output 25 times greater than that
required to produce the same sound
pressure at a distance of 1 metre. For
PC use it follows that 1 or 2 watts
per channel should be sufficient.
In this surround sound amplifier
we’ve decided to use two integrated
widely available in the mar-
ketplace, or possible even
build their own. These types
of loudspeakers are usually
not active, which creates another
problem: how can we drive such a
surround sound system in a simple
manner?
Our contribution to all interested
PC owners is an easily built plug-in
card, which contains six compact
power amplifiers, making it possible
to connect ordinary passive loud-
speakers to the PC.
46
Elektor Electronics
11/2001
COM
PUTER
+12V
C26
1000
µ
16V
C4
3
13
VCC
VCC
C5
L
L
C1
4
1
INFL
OUT1
470n
Front
Front
C6
R
R
5
2
INFR
OUT2
IC1
C7
C2
12
15
INRL
OUT3
470n
TDA7370
C8
C3
11
14
INRR
OUT4
470n
Centre
C9
7
10
JP1
STBY
CLIPDET
S
PW
Centre
6x
1000
R1
10k
SVR
GND
GND
on
µ
/ 16V
6
9
8
off
Sub
P1
P2
P3
R2
R3
R4
C10
C11
C12
FL
FR
Center
10
µ
63V
47
µ
25V
100n
10k
10k
10k
K1
+12V
+12V
+12V
0
0
C25
C27
100n
1000
µ
16V
C16
3
13
VCC
VCC
C17
L
L
C14
4
1
INFL
OUT1
470n
Rear
Rear
C18
R
R
C13
5
2
INFR
OUT2
470n
IC2
C19
C15
12
15
INRL
OUT3
470n
TDA7370
C20
11
14
INRR
OUT4
Sub
C21
7
10
JP2
STBY
CLIPDET
S
PW
R5
10k
SVR
GND
GND
6x
1000
on
µ
/ 16V
6
9
8
off
P5
P6
P4
R6
R7
R8
C22
C23
C24
RL
RR
Sub
10
µ
63V
47
µ
25V
100n
10k
10k
10k
010022 - 11
Figure 1. Considering that this is a six-channel power amplifier, the circuit is a model of simplicity.
‘quad power amplifiers’ (TDA7370B),
which were originally intended for
automotive use. This has the advan-
tage that they have been designed
for use with a supply voltage of 12 V,
and can deliver an output power of 3
watts per channel into 4 Ω or
1.5 watts into 8
already done all the hard design
work and that this type of IC incor-
porates a range of protection circuits.
They are therefore almost indestruc-
tible under normal circumstances
and they function happily with only
a minimal number of external com-
ponents. The last is a ‘must have’
when the design calls for the whole
circuit to fit on a PC plug-in card.
Just how simple the six-channel
integrated amplifier is becomes clear
when we look at
Figure 1
, which
shows the complete circuit diagram
of the design described here. Apart
from the two ICs and the inevitable
input and output decoupling capaci-
tors, there are only six presets that
are used to set the input level. These are
required to compensate for any differences in
the loudspeaker efficiencies as well as other
possible differences.
Each TDA7370B contains four amplifiers, of
which we only use three; after all, we only
need six in total. IC1 is used to amplify both
Front channels and the Centre channel; IC2
deals with the two Rear channels and the
Subwoofer channel.
When connecting the loudspeakers you
have to take care with the polarity. The
TDA7370B has been designed such that the
four amplifier sections can easily be wired up
as two bridge amplifiers. It is for this reason
that outputs OUT2 and OUT4 are inverted rel-
ative to OUT1 and OUT3. In our case this
means that the polarity of the Centre and
. This may seem a
bit on the low side, but in practice
most PC users will find that this is
sufficient to produce deafening
sound levels.
Ω
Simple circuit
The advantage of using integrated
amplifiers is that the manufacturer
(in this case SGS-Thomson) has
11/2001
Elektor Electronics
47
COM
PUTER
Specifications
Considering its size and cost, the performance of this mini
surround sound amplifier is truly astounding. When we first
connected a set of Hi-Fi loudspeakers nobody would
believe that such a puny PC card could produce so much
sound. The sound produced is very full and you never feel
that anything is lacking, neither quality nor volume.
To satisfy those who have a liking for figures, we’ve
taken a few measurements of the amplifier. Not too many
of course, since this design was never intended as a high-
end amplifier. Furthermore, the compact construction lim-
its the channel separation.
This short list of figures was measured at a supply volt-
age of 12 V and a load of 8
100
A
50
20
10
5
2
%
1
0.5
0.2
0.1
0.05
0.02
0.01
20
50
100
200
500
1k
2k
5k
10k
20k
Ω
:
Hz
010022 - A
+3
– input sensitivity:
0.38 V
rms.
B
+2
–P
max.
(THD+N = 0.1%):
1.8 W
+1
-0
– THD+N (1 kHz/1.5 W):
0.04%
-1
-2
(20 kHz/1.5 W):
0.4%
-3
– bandwidth (–3 dB):
17 Hz to 70 kHz
-4
-5
– quiescent current:
0.24 A
d
r
-6
-7
-8
For completeness we’ve added two graphs. Graph
A
shows the distortion as a function of frequency with a
bandwidth of 80 kHz. As you can see, it is a steady line
without any spikes. The internal protection circuits in these
types of IC always cause a slightly higher distortion figure,
but they are no cause for alarm.
Graph
B
shows the amplitude as a function of fre-
quency: it’s nice and flat with rolloffs occurring at 17 Hz
and 70 kHz. The high frequency rolloff is determined completely by the internal circuitry of the TDA7370B. The low frequency rolloff is
mainly affected by the input and output capacitors
-9
-10
-11
-12
-13
-14
-15
10
20
50
100
200
500
1k
2k
5k
10k
20k
50k
100k
200k
010022 - B
Hz
Subwoofer speakers is reversed compared to
the rest. Hence the minus symbols shown
next to the appropriate outputs.
Just a few more details. There are two rea-
sons for using two components in parallel for
the output capacitors. Firstly, this reduces the
series resistance and parasitic inductance.
But more importantly, the height of the card
should be kept as small as possible and the
diameter of 1000 µF electrolytics is much less
than that of 2200
out any popping sounds; there is no
reason to worry that you’ll damage
any connected loudspeakers.
sis mounted jack sockets, allowing
everything to fit in any PC. We’ve
kept the distance between the cen-
tres of the jack connectors to 10 mm.
The drawing in
Figure 3
shows the
wiring between the board and con-
nectors.
It is obviously not compulsory to
mount the amplifier board inside the
PC. There is no reason why it can’t
be turned into a stand-alone piece of
equipment with its own power sup-
ply and enclosure. This even has
some advantages. Firstly it gives the
freedom to use a slightly higher sup-
ply voltage, increasing the available
output power. With a supply of
14.4 V the TDA7370B delivers 5
watts into 4
PCB
The PCB shown in
Figure 2
has
been designed to fit easily into either
an ISA or PCI slot of a PC. Populating
the PCB is a piece of cake, because
all things considered there are very
few components. There are eight
wire links on the board, and it is best
to solder these in first. When it
comes to mounting the ICs you have
to follow the instructions carefully;
this will be explained in detail in the
‘cooling’ section.
The connectors have purposely
not been incorporated on the board
so that you can decide for yourself
how to implement the input and out-
put connections. In our prototype
we’ve used a blank expansion slot
cover and added seven 3.5-mm chas-
F ones. Resistors R2, R3
and R4 and R6, R7 and R8 have been added
to provide a load for the output capacitors,
which stops those irritating popping sounds
from occurring when loudspeakers are
plugged into the amplifier when it is already
powered up.
Finally, JP1 and JP2 are used to put both
TDAs into stand-by mode. This can be useful
when the amplifier board won’t be used for a
long period, since the stand-by current drain
reduces to only 1 µA per IC. In today’s energy
conscious world we would recommend that
the jumpers are replaced with a small switch.
Switching to and from stand-by mode is with-
µ
and at the maximum
supply voltage of 18 V that increases
to 8 watts. Furthermore, with the
board housed in its own enclosure
there are far fewer space restrictions
and the ICs can be provided with
normal heatsinks.
Ω
48
Elektor Electronics
11/2001
COM
PUTER
COMPONENTS LIST
C15
PCI
RR
RL
Resistors:
R1,R5 = 10k
Ω
R2,R3,R4,R6,R7,R8 = 470
P6
C27
IC2
RR
Ω
P5
RL
P1-P6 = 10k
Ω
preset
RR
C23
Capacitors:
C1,C2,C3,C13,C14,C15 = 470nF
C4-C9,C16-C21,C26,C27 =
1000
RL
C14
C19
C18
C17
C16
-
Sub
R8
P4
C22
JP1
F 16V axial, dia. 10mm,
lead pitch 28mm
C10,C22 = 10
µ
+12V
C21
R5
ON
ON
µ
F 63V axial
0
010022-1
H15
C11,C23 = 47
F 25V axial
C12,C24,C25 = 100nF
µ
R1
C25
C8
K1
C10
JP2
P3
Semiconductors:
IC1,IC2 = TDA7370B (ST Micro-
electronics)
R4
H14
Center
-
C4
C5
C7
C6
C2
FR
C11
FL
Miscellaneous:
JP1,JP2 = 3-way pinheader with
jumper
K1 = 4-way pinheader
7 off 3.5mm stereo jack socket,
PCB mount
PC expansion slot cover
P2
FR
FL
IC1
P1
H12
H4
C26
ISA
FL
C1
FR
Cooling
Since we are severely limited by the
space available on a PC plug-in card,
the heatsink for the ICs has to be
made to measure. To keep its size to
an acceptable level we’ve designed
it to cope with the dissipation
caused during typical music play-
back rather than the maximum
caused when all six channels are
fully driven by sinewaves — the last
is none too realistic anyway.
The photos in
Figure 4
show
pretty well what should be done.
We’ve used a 3 mm thick piece of
aluminium, measuring 15.5 by
9.5 cm, which the ICs are screwed
on to. The whole is screwed to the
edge of the board with the help of a
small piece of aluminium. The con-
struction itself is not very difficult,
but there are a few points that
require some attention.
To start with, the pins of the ICs
have to be bent backwards, so that
Figure 2. The PCB is designed as a PC plug-in card. The circuit is dominated by the large
number of electrolytic capacitors (board not available ready-made).
11/2001
Elektor Electronics
49
COM
PUTER
they can be mounted lying flat. The best way
is to bend the back row first with the help
with a straight piece of metal, followed by the
front row. The pins of the IC should be sup-
ported by a thin but strong piece of metal at
the point where they leave the IC, so the
body of the IC won’t be damaged while the
pins are bent. When this has been done prop-
erly the pins still stick far enough through the
board for them to be soldered. This soldering
should only be done
after
the heatsink and
ICs have been screwed to the board.
The parts of the edge of the board imme-
diately underneath the IC mounting tabs
should be cut off with a jigsaw, leaving only
the central part with three mounting holes.
It’s on to this that the previously mentioned
small piece of aluminium is screwed, provid-
ing for mechanical strength between the
heatsink and board. A piece measuring about
40 mm by 9 mm should be cut from the edge
of the heatsink, lining up with the piece of
board with the three mounting holes. This
way the heatsink fits right against the board.
Only when the heatsink has been fixed to
the board are the ICs screwed to it after
which they are soldered to the board.
Remember to put some heat transfer com-
pound between the ICs and heatsink, but iso-
lating washers aren’t required since the
mounting tab of the IC is connected internally
to ground and we also want the heatsink to
be at ground potential.
Front
L/R
FR
FL
Centre/
Sub
FR
FL
Rear
L/R
- Centre
Centre
Front
L/R
Sub
- Sub
Centre
RR
RL
Sub
RR
RL
Rear
L/R
010022- 12
Figure 3. The prototype uses 3.5mm jacks for external connections.
into 8
loudspeakers the average
current consumption is about 1.1 A.
It should be kept in mind that the
peak current drain can briefly rise to
4 A. This is normally not a problem
for a PC power supply, but where the
PC power supply is already fully
loaded it is possible to mount a sup-
Ω
ply connector on the expansion slot
cover and use an external power
supply. But we should reiterate that
this is rarely required.
Power supply
For the supply a small PC supply connector is
used (K1), which only uses the +12 V con-
nection. When all six channels are fully driven
(010022-1)
b
a
Figure 4. Mounting of the heatsinks, seen from above (a) and below (b).
50
Elektor Electronics
11/2001
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