I designed and built this headphone amp to use with my electric bass. It has a single musical instrument input and a stereo auxiliary input for a mp3 player, laptop computer, radio or other device and a stereo audio output to power nominal 32 ohm or higher impedance headphones.
The headphone amp has a volume control for the instrument input. The volume level for the auxiliary input is controlled by adjusting the volume level of the device that is plugged into the auxiliary input of the headphone amp. The auxiliary input is designed to be driven by the headphone output of a typical consumer audio device.
My headphone amp with auxiliary input now residing in a PACTEC model XP-9VB enclosure.
I used MPF102 FETs for the two musical instrument preamp stages and two LM386N-4 audio power amplifier ics for the headphone amp stereo audio output. These audio ics have a power supply voltage operating range of 5 to 18 volts and more than enough power output capability to drive a set of headphones.
The headphone amp current drain when powered by a 9 volt battery is about 15ma at idle, 16ma at low volume levels and 25 to 30ma at high volume levels.
Headphone amp built on a 2 1/2" square printed circuit board.