This headphone amp is a Craig Anderton design from his Electronics Projects for Musicians book. I built the headphone amp to use with my electric bass. It is a simple LM380 based amp and works quite well.
I changed some of the component values, eliminated the second input and used a single input with selectable sensitivity. I also eliminated the built in output volume level control. The output level is now controlled by the toggle switch selected input sensitivity and your instrument volume control settings.
The power audio amp ic is a LM380. Powering the ic are two 9 volt batteries connected in series to create a 18 volt power supply.
Below is a schematic of the headphone amp as I built it.
See my electric bass / guitar headphone amp with stereo auxiliary input and stereo output.
I used an enclosure made by PAC-TEL, model HP. The enclosure dimensions are 5.7 x 3.6 x 1.1 inch
I modified the enclosure by cutting a piece of aluminum to fit in the
end of the case to mount the switch and jacks. I also cut the screw
posts flat on one side to accommodate the aluminum plate.
The batteries are held in place by the edge of the circuit board and with self adhesive weather strip. I like the way the amp turned out with the exception of the power switch. I would lean towards using a slider switch so that the power wouldn't be turned on by accident.