TPA0212 Headphone Amplifier
Some time ago a friend of mine asked me whether
I could "tune" his computer headphones, one of these USB-Headsets.
Due to an european regulation (IMHO) headphones, especially with an integrated amplifier, may not exceed a certain audio power to prevent ear damage from listening to excessive loud music.
This resulted in a rather good headset with great sound but if someone spoke next to you this was nearly louder than the music you were listening to!
So I decided to built a little amplifier which should fit into the free space within the USB audio codec box attached to the cable of the headphones.
The main design limitations was the fact that
there should be no additional power source required than the USB connector (I
don't like battery powered equipment as batteries are always empty!).
This results in a 5V, 500mA max. power supply.
Building a bipolar push-pull transistor amplifier
using BC847 & BC857, I soon realized that I couldn't get good sound from
this supply voltage. After some different attempts I discovered the TPA0212
Headphone Amplifier integrated circuit manufactured by Texas Instruments.
The TPA0212 can deliver up to 2 Watts per channel into a 3 Ohm load at a nominal supply voltage of 5V. Due to the headphone's impedance of 33 Ohms the max. output power is reduced to about 250mW which is plenty of power compared to the 1,4 mW max. power measured in the unmodified state (212mV @ 33 Ohm).
Only one additional wire was required between the speakers and the power amplifier cause the TPA has a bridge tied load output as the original drive circuit has a ground referenced load output.
I think it wouldn't have been possible to built an amplifier with these specs using discrete parts, regarding power and quality as well as pcb space.
Here you can see the rather simple schematic
of this amplifer; all resistor/capacitor values are derived from the datasheet
and haven't been added to the eagle schematic cause it was a "one time"
You might add a decoupling capacitor as large as space permits; I've added one next to the pcb.
Eagle schematic and board download.
(c) Thomas Gulden, 03 / 2010