Low-noise, stereo-to-mono headphone amplifier. Pictures of the completed system can be seen below.
The amp consists of a set of input buffers, a summing amplifier, Baxandall volume stage, and finally a class AB output stage.
The PCB was designed in KiCAD.
It is rated for 9V but can be used with a supply voltage of up to 18V. Bandwidth (-3dB points) was designed to be from 20Hz to 20kHz when driving an 8 Ohm load. The amp can however drive larger loads easily, such as Beyerdynamic DT 880 Pro headphones at 250 Ohms.
The amplifier consists of four main sections:
- Power supply section: Reverse polarity protection, power supply filtering, bias voltage generation.
- Input buffers and summing: Simple NPN emitter followers (using standard BC547s) as high impedance buffers followed by a an approximately unity gain, op-amp summing amplifier (low-noise NE5532).
- Active volume control: This is a low-noise, Baxandall volume control (seen in Douglas Self’s book ‘Small Signal’) giving up to 17dB of gain.
- Output power amplifier: Finally, the output stage consisting of a unity gain op-amp buffer and a class AB power amplifier, capable of driving loads as small as 8 Ohms.