Introducing "4hp", the 2+2 headphone amplifier module in ... 4hp!
Compact and skiff friendly, only 35mm deep.
I made this because I needed something to drive headphones for the Happy Knobbing 2016 modular meeting. 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-d ... ocess.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. Potentiometers are 9mm dual type, they are not as common but available from some sources like Smallbear:
http://smallbear-electronics.mybigcomme ... -pc-mount/
If you cannot get audio taper pots you can also use linear taper and install a resistor across to change the taper as explained here:
http://sound.westhost.com/project01.htm
The output is DC coupled so that there is no need for huge coupling capacitors, but the input is AC coupled to prevent frying your headphones with DC biased signals. You can also short/omit the coupling caps if you want to use this as a DC coupled CV driver for whatever you want to connect.
full kit:
