An amplifier to drive low to medium impedance headphones built using discrete components.
Both halves of the circuit are identical. Both inputs have a dc path to ground via the input
47k control which should be a dual log type potentiometer. The balance control is a single 47k
linear potentiometer, which at center adjustment prevents even attenuation to both left and
right input signals. If the balance control is moved towards the left side, the left input track
has less resistance than the right track and the left channel is reduced more than the right side
and vice versa. The preceding 10k resitors ensure that neither input can be "shorted" to earth.
Amplification of the audio signal is provided by a single stage common emitter amplifier and
then via a direct coupled emitter follower. Overall gain is less than 10 but the final emitter
follower stage will directly drive 8 ohm headphones. Higher impedance headphones will work equally
well. Note the final 2k2 resistor at each output. This removes the dc potential from the 2200u
coupling capacitors and prevents any "thump" being heard when headphones are plugged in. The
circuit is self biasing and designed to work with any power supply from 6 to 20 Volts DC.