Studio Series Stereo Headphone Amplifier

Here's a top-class headphone amplifier that can drive high or low impedance 'phones to full power levels, with very low noise and distortion. For best performance, it can be teamed with the Stereo Preamplifier described last month. Alternatively, it can be used as a standalone unit, requiring only a power supply and a volume control pot for use with any line-level signal source (CD/MP3 player etc). It even includes dual outputs, so you can listen with a friend!

Many of our high-power audio amplifier designs already provide an output for headphones. The additional circuitry required for headphone support is simple; just two resistors in series with the loudspeaker outputs to limit the drive current and protect the ’phones in the case of amplifier failure.

Considering its simplicity, this resistive limiting scheme works well, although it will cause distortion if the load is non-linear – a likely prospect with most headphones. Apart from eliminating this potential source of distortion, there are a number of other reasons why you might consider building a separate headphone amplifier.

For a start, not everyone owns a pair of top-rated headphones or even a high-performance power amplifier. After all, an amplifier that equals or betters the performance of this new headphone amplifier will set you back more than a few shekels!

Main Features:

1. High performance – very low noise & distortion
2. Drives high and low-impedance headphones
3. High output power (up to 200mW; into 8? and 32?)
4. Dual headphone sockets – can drive two pairs!
5. Works with a preamp or any line-level audio source

Measured Performance:

1. Frequency response.......................... flat from 10Hz to 20kHz (see graphs)
2. Rated output power........................... 200mW into 8? and 32?, 85mW into 600?
3. Max. output power (current or voltage limited)...............575mW into 8?, 700mW into 32?, 130mW into 600?
4. Harmonic distortion........................ typically .0005% (600? load),.001% (32? load) and .005% (8? load)
5. Signal-to-noise ratio (A-weighted)......................... -130dB (600?), -120dB (32?) and -111dB (8?) with respect to 100mW output power.
6. Channel crosstalk.................. better than -68dB from 20Hz-20kHz at 100m? output power (see graphs)
7. Input impedance.................................... ~47k? || 47pF
8. Output impedance..................... ~5?