A tiny headphone-amplifier which is powered from two 9V batteries. It uses 2 opamp's for its operation. At the input-side a NE5532 is used for voltage amplification. The NE5532 is a really good and effortable opamp. This amplification is determinant by the resistors 2k2 ohms and 920 ohms, which results into an amplification-factor of about 3.4 (=2k2/0.92+1). The second opamp is used as a buffer and therefor should be able to drive low impedances. On the pcb its labeled JRC4558. This is a cheap opamp which could drive 150 ohms headphones. A better alternative is the slightly more expensive NJM4556. The input capacitors are next to the label Audio_IN. They are labeled 2u2. If you use capacitors which won't fit on the pcb you can solder them directly on the inputs and bridge the pins on the pcb.