handle microphones. Most mics have
outputs in the range of about - 60
dbm (about 1 mv at 600 ohms).
This is quite a low level signal and
more care needs to be exercised in
order to amplify this level than I have
taken with this implementation (see
Refer to Schematic 3. The power
supply I implemented is powered
by a 20 VAC wall-wart. I strongly
recommend the use of a wall-wart
or external transformer in order to
keep AC mains outside of the box.
The supply circuit I built consists of
two half wave rectifiers and 12 volt
regulators. There is provision on the
board for a full wave rectifier if you
use a transformer with a center tap.
The value of the two series resistors —
10 ohms on my board — should
be selected based on the voltage
source. Just make sure that their
power rating is sufficient.
Ideally, the resistors should drop
enough voltage so that the regulators
are presented with about five volts
above their output voltage. This will
help minimize the regulators’ power
dissipation, as well as provide better
filtering of the rectified AC input. The
two audio boards draw a total of
about 100 mA.
Due to the symmetry of the
■ FIGURE 6