Introducing "4hp", the 2+2 headphone amplifier module in ... 4hp! Two separate channels each with their own volume knob, so you can share your bleep bloops with a friend! The input jacks can be either wired as "through" so that you don't need a mult to pass a signal to the headphones - or you can wire them for two separate headphone channels. Also, jacks should be normalled left to right so that a mono signal shows up on both ears. The amp is designed to "blow your ears away", the (dirt cheap) NJM4556 opamps used can deliver up to 200mA per ear. The amp has unity gain because your typical modular levels of 10Vpp are enough to drive all kinds of cans. If needed one can add gain by cutting some PCB traces and adding SMT resistors. In the standard BOM, the internal 68R balancing resistors limit the current to about 150mA per ear at 5V, but they can be reduced down to 1R if needed. See more about the NJM4556 here: http://nwavguy.blogspot.de/2011/07/o2-design-process.html You can even drive small ~8Ohm loudspeakers at normal "room" level If you want to reduce the gain you can wire a resistor from the jacks to the PCB, effectively limiting the potentiometer range. Depending on your headphones, you might want to do that in order to get a bit more range on the knob.