ith no surround sound decoding equipment at all,
sounds seem to come from places well outside the speakers.)
Now, what about controlling the balance between the front and rear
channels? There are two basic approaches to this:
One approach would be to buy a second stereo amplifier for the rear
channel. Simply wire the speakers to both + or red terminals (leaving no
connection to the - or black terminals); A second method would be to wire
in a stereo fader (available from Radio Shack). These faders are designed
for automobile stereo systems and can handle only 25 Watts per channel.
However, all you have to do is wire in the fader according to Figure 8.
Figure 8 includes a switch to switch between surround mode and regular
Figure 8: Wiring a stereo fader into the matrix
Balancing the System
In order for the above surround sound systems to work, the stereo balance
must be adjusted to null out the front channels in the rear speakers. To
do this: tune in an AM radio station (as mentioned above for checking the
phasing); disconnect the front speakers and adjust the speaker balance
control until the lowest possible sound level is heard in the rear speakers;
hook up the front speakers and you are ready to go. For the surround sound
to work properly this balance must be maintained.
Building an Active Decoder
Commercially made surround sound systems use an electronic circuit
called a differential amplifier to decode the surround sound. Since a differential
amplifier uses active electronic components, such as transistors, it is
The differential amplifier simply does, electronically, what the rear
speaker does in the passive decoder described above. The differential amplifier
rejects any signals that are identical in the front channels and pass any
differences to the rear channel. The output of the differential amplifier
is sent to the regular audio amplifier and then the rear speaker(s). One
advantage of this type of decoder is that you don't have to worry about
the null balance adjustment mentioned above. This adjustment is made in
the differential amplifier once and does not need to be readjusted.
If you want to take this route you can make a differential amplifier
with only four components, all available from Radio Shack (some Radio Shack
catalog numbers are given preceded with RS). With a few extra support components
a self-contained surround sound decoder can be built. The active decoder
is so simple that it can be built on a "breadboard" PC board also available
from Radio Shack. The full schematic is shown, including the pinout of
the IC, in figure 9.
Figure 9: An active surround sound decoder
The circuit is powered by a 9V DC converter. The IC, however, is designed
to operate from dual power supplies. To make it work with a single power
supply an artificial ground is used. The artificial ground is generated
by using the second amplifier in the LM1458 IC as a voltage follower. A
voltage divider (two 100k resistors) established the ground point at 4.5V
(half of the 9 Volt supply). In the decoder circuit, the artificial ground
should be considered as zero Volts, the V+ as +4.5 Volts and the V- as
-4.5 Volts. However, since the circuit is actually powered from a single
power supply, the power input is labeled as +V (9V+) and -V (9V-) representing
the + and - terminals of the DC converter. Bus connections, such as +V,
-V and GND are shown by symbols only. This keeps the schematic from looking
The input of the active decoder is to be connected to the "Tape Out"
of your stereo amplifier. The output of the decoder will go to another
amplifier to drive the rear speaker(s). The DPDT switch can remove the
decoder from the system for regular stereo in both the front and rear speakers.
The null adjustment is made as described above for the passive decoder
except instead of adjusting the speaker balance the 20k 15 turn trim-pot
Wireless Surround Sound System
One thing you can also do with the active decoder is attach it to a
Ramsey Electronics FM-10x Stereo FM Transmitter. With this combination
you can transmit the surround channel to any 