This is my latest headphone amp built this last summer. The casing is your run of the mill Radio Shack project box. I pulled the volume pot and knob from an old broken boom box. Inside is a Chu Moy based off of the Burr Brown OPA2132. The dual power supply is simply resistively divided from the 9V input with caps on each rail. Nothing special except the convenience and size. Sounds pretty good.
Here's my second headphone amp built about two years ago. I used a Radio Shack 4AAA battery case. It uses two lm386 amps (I know they are jellybean chips but at low output currents they still sound pretty good). I added a low pass filter on the input for added bass (I was going through a period where I really liked powerful bass). I've added a max756 to step up the 1.5V to 5V (battery life is pretty good at around 15+ hours). I was planning on adding a digital volume control (ds1802, and hence the two buttons on the side) but ran out of room. My next revision for a pocket amp will be in the same case, but will feature a custom etched smt pcb and digital volume control.
This one was my first headphone amp I ever built (made it freshman year). The case is a recycled plastic hair pin container that I "borrowed" from my mom. It uses the same chip as my desktop amp above, runs off of a nine volt, and has an analog volume control. The power supply splits the single source using a resistive divider and buffers and stabilizes the output with the help of a opamp. Not my best work, but it sounds pretty good and I learned a lot making this.
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