 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 FM radio for a wireless surround
You can save some effort by wiring the active decoder right into the
FM transmitter. Power is derived from the transmitter's power supply (a
9 Volt battery if the transmitter is not modified). The RCA connectors
and the ground generator are eliminated, reducing the circuit to the four
components actually needed for the differential amplifier. The stereo/surround
switch could also be added to the transmitter.
Figure 10: Decoder to add to Ramsey FM-10 stereo transmitter
Since I am unfamiliar with the newer versions of the FM-10x transmitters
I cannot give a wiring diagram. You will have to cut the circuit board
traces where the signals enter the transmitter and wire the decoder in
there. Pin 11 of the BA1404 IC acts as an artificial ground and is where
the GND connection of the decoder goes.
One More Thing
One unadvertised thing a surround sound decoder will do is reduce the
volume of the lead vocal in most popular music. Reduce the volume in the
front speakers and you have an instant karaoki machine.