Kit 106. 50 Watt Audio Amplifier
Page
2
of
5
Testing :
Before applying power, check that all parts are
inserted in the correct locations. Make sure that the
electrolytic capacitors are the right way round.
Check the power supply voltages before connecting
the power supply to the amplifier. Then connect the
power supply leads with a current meter (or
multimeter set to read amps) in series with the
positive rail. Connect a load resistor to the speaker
output, and short out the input terminals. Make sure
there are links inserted for
Mute
and
Standby
.
Switch on the power and check that the current
settles down to between 20 and 60mA. There will be
a brief surge as the filter capacitors charge. Also
check the DC voltage at the output is not more than
200mV before connecting a speaker.
Note : Do not operate the module without a
heatsink.
You must have the heatsink fitted,
otherwise the thermal overload protection circuit will
cut in and switch the device off. The heatsink tab on
the TDA7294 IC is internally connected to the
negative supply rail.
Therefore If the heatsink is
connected to an earthed metal enclosure, the IC
must be insulated from the heatsink
.
If not, the
negative supply rail will be shorted to ground. A
silicon impregnated rubber washer and a plastic
insulating bush are provided for this purpose.
To minimize noise and distortion, keep the input
signal leads away from the power supply and output
leads. Use screened cable for input leads, and
suitably large cables for all DC power wires and
speaker connections. Remember the current may be
a few amps in each case. Make sure all earths
connect at a single point to eliminate hum caused by
ground loops.
Operation :
No input attenuation is provided, so if you are not
using a separate pre-amp then you may require a
volume control. This can be a 10k ohm logarithmic
potentiometer connected as shown. If you are using
two modules for stereo, then a dual gang pot will be
required. A CD player can be connected like this and
will drive the module to full power without a pre-
amp. Alternatively you might like to use one of our
stereo pre-amp or tone control kits.
The maximum continuous power you can obtain will
depend largely on the size of the power supply and
the size of the heatsink used. make sure both are
sufficiently large for the intended application. The
IC has internal thermal protection that causes the
mute to cut in at 145
°
C and switches the amplifier
into standby at 150
°
C. You should however avoid
operating at such high temperatures by using a
suitably large heatsink for the average power you
need. A heatsink fan is an alternative to a larger
heatsink, if fan noise is not a problem.
If it does not work :
Dry joints caused by poor soldering are the most
common reason that the circuit does not work. Check
all soldered joints carefully under a good light. Re-
solder any that look suspicious. Check that all
components are in their correct position on the PCB,
and the electrolytic capacitors in the right way round.
Make sure you have inserted the links for
Mute
and
Standby
. Remember that the metal tab on the IC is
internally connected to the negative supply rail, so
check that the negative supply rail is not shorted to
ground.
The data sheet on the TDA7294 IC is available from
the ST Electronics web site at :
http://www.st.com
or our web site at :
http://www.kitsrus.com
Parts List :
Resistors (1/4 W carbon)
150R (brown green brown)
R1
1
10K (brown black orange)
R2, R3, R5 3
680R (blue grey brown)
R4
1
22K (red red orange)
R6
1
Capacitors
1u5F or 1u MKT polyester
C1
1
2n7F MKT polyester
C2
1
22uF 63V electrolytic
C3, C6
2
10uF 63V electrolytic
C4, C5
2
2200uF 50V electrolytic
C7, C9
2
100nF MKT polyester
C8, C10
2
Semiconductors
TDA 7294
IC1
1
Miscellaneous
K106 PCB
1
Header, 2 pin
2
Jumper
2
Silicon impregnated insulating washer
1
Insulating bush
1
Screw, 3 x 12mm
1
Nut, 3mm
1
Washer, 3mm
1
