Some time ago I bought a pair of in ear headphones and I am very pleased with them, but I don't have a decent headphone amplifier for them at home. So I searched the web and became very interested in building a headphone amplifier BUF634 and OPA627 combination. I decided to use the circuit in figure 5. from the BUF634 datasheet, in this configuration the amplifier operates in class A up to approx. 14-15 mW with a 32 ohm headphone, which is more than enough for me. We have finished the amplifier now and I am very pleased the sound, plenty of bass with in ear headphones and otherwise a clear and precise sound.
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This is the back of the finished amplifier, we choose to let the audio signal pass through the amplifier and have mounted it in the audio signal way between the preamplifier and the power amplifier. This gives the advantage of remote controlled signal source and volume. The volume potentiometer in the amplifier is used to adjust the signal level for the chosen in ear headphones. We use a 14 Volt AC, 500 mA wall outlet from Conrad to power the amplifier.