This mini amplifier
is based on the Thomson Type TEA2025. In this 16-pin DIL device hides a stereo
amplifier that with a supply voltage of 9 V will provide 1watt output per
channel into a 4-ohm loudspeaker. At full output, the input sensitivity is
about 25 mVpp. If this is too sensitive, a resistor R may be connected between
pin 6 and C7 and between pin 11and C2. The sensitivity then becomes (25+ Y2R)
mV, provided R> 1k. Furthermore, the supply voltage may lie between 3 V and
12 V. The operation of the IC cannot be discussed here, but for those
interested its internal circuit diagram is reproduced in Fig.
One useful
feature of the TEA2025 is that it has a soft-start circuit on board, thus
obviating annoying plops in the loudspeaker at switch-on. Construction of the amplifier is fairly simple, but has its
peculiarities. First, there is the earth, which in this case should not be of
wire, but rather consist of a metal earth plane (if you design your own PCB,
this would be of copper). If at all possible, pins 4
and 5 as well as pins 12and 13, should be connected to a (copper) area of not
less than 5 cm, The two areas should be connected in a suitable manner, and in
such a way that a heat sink is formed under the IC as shown in Fig. This
ensures both good heat conduction and a good
earth. Moreover, all other connections should, of course, be kept as short as
possible. This is particularly important in the case of the supply lines, which
should be decoupled by C11as close as possible to the IC. The negative terminal
of this capacitor should be soldered direct onto the earth plane; the positive
terminal is soldered in the normal manner to pin 16. Finally, the distortion'
for a power output of about 0.25W is roughly 0.3
per cent. 
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