Gainclone Headphone Amplifier
The LM4881 is a low power stereo chip designed by National Semiconductor. It is part of their Boomer amplifier series. The Boomer series are chips that have less than 3W output at less than 6V supply voltage range. This particular chip is a dual 200mW headphone amplifier chip. It features a shutdown mode to turn the chip off or on without pops.
Schematic: (Copyright National Semiconductor)
Use a 1000uf or higher value capactior for the output. Otherwise, this circuit will have poor or no bass response.
This is the original PCB. I highly recommend not using it. It is designed for the LM4881N DIP-8 chip. It also has a spot for a 5V regulator, diode and Ni-Cd battery. Also, many of the lines are touching that shouldn't. Finally, it is designed to put the chip into shutdown mode rather than completely off, and all that does is waste battery power.
I then designed this PCB. It is MUCH smaller and uses the surface mount chip and 5 surface mount caps. It also charges a 3.6V Ni-Cd battery with a LM78L05 regulator, diode and transformer. If you want to power the circut from a 9V battery and not have it rechargeable, then you need to connect the regulatoir still (since 9V is over the max voltage for the chip) but connect a jumper where the surface mount diode goes (well actually a though hole diode surface mounted). Finally, just leave the traces for battery alone.If your source voltage is under 5V, then just connect it where the rechargeable battery would go.
PCB COMING SOON!!
Useage / Testing:
When I buit the first one, I learned the hard way that the Co is required on the output. Luckily I didn't try it with my headphones, and instead a speaker. Once I got the amplifier working, it sounds great. I already have two people who have heard it and want to buy one from me.