CONSTRUCTING
THE AMPLIFIER
Small transistors in Figure 1 with the flat to the left are
PNP 2N5401; the ones with the flat facing right are NPN
2N5551. These are available at more than one of the
sources listed here. I recommend purchasing a supply of
each of these types.
The voltage rating is much higher than needed here,
but they can be used in other higher power amplifier
designs, as well. I saw the NPN for about four cents each
in 100 lots. The PNP was more like five cents.
Shorting the output to ground won’t blow up the
amplifier. If the amplifier does suddenly output the full
power supply voltage, it won’t (in general) wreck a pair of
headphones, but don’t poke or probe at the amplifier with
the headphones connected anyway. That is simply asking
for trouble.
The overall amplifier is DC coupled. For safety, since a
device connected to the input might have a DC voltage
on its output, a coupling capacitor is used. One 22 µF
non-polar capacitor or a pair of regular polarized
capacitors back-to-back can be used (also
22 µF).
This input coupling capacitor (or capacitors) protect
the amplifier in case a substantial voltage is accidently
attached to the input. This makes the lower 3 DB response
frequency about 3 Hz. Since we can’t hear much below
20 Hz, this is more than adequate.
Construction is easy on a piece of perfboard. Place
the parts as they appear in Figure 2 for easy checking of
the wiring when you are done. I generally use component
leads to make interconnections on the back of the board
and wire-wrap wire for connections where the wires are
not long enough, or wires have to cross each other. I
always solder them. The wire is thin and has thin
insulation, making it easier to work on the board.
However, if you nick the conductor when stripping the
wire, it can break easily so be careful.
Observe the proper polarity of the LED. The one I’ve
used has one longer lead which is the positive one, i.e.,
the anode. You might want to test an LED and the 2.2K
resistor connected to a 12 volt power supply to be sure
you have the right polarity.
Check the voltage across the LED. It needs to be
about 2.1 volts. Different colored LEDs have different
forward voltages, so you might have to try a few different
types. These devices make very good voltage references.
You can place one on the front panel of a box if you
decide to build two channels and package them.
TESTING
Don’t connect the headset yet. Solid-state amplifiers
are comfortable with no load. DON’T just connect and
turn on the power when you are done. With a multimeter,
check the resistance from each power supply lead to
ground one at a time. Resistance less than 1K ohm or so
indicates a problem.
Trace your wiring carefully to catch omitted wires,
untrimmed resistor or capacitor leads, and/or accidental
solder bridges. If the board is laying on your workbench,
be sure there are no scraps of resistor leads or tools under
the board. Short circuits need to be avoided if the circuit
is to have a chance to work. I’ve had a circuit blow up
due to a resistor lead on the bench.
When you apply power, note that the LED lights. If
the output transistors get more than just slightly warm or if
the 100 ohm resistor in series with the collector of the
September 2013 31
■ FIGURE 3.
Power supply
schematic.
