If you built our 20W Class-A Stereo Amplifier described last
year, you will be aware that it lacks a headphone socket. Similarly, many hifi
valve amplifiers also lack a headphone socket, the assumption being that a true
hifi enthusiast will want to listen via good-quality loudspeakers.
A headphone output was not included in the Class-A Stereo
Amplifier because it would degrade its superb audio performance. Both the wiring
paths and the general circuit layout are critical factors in the design and any
changes, however slight, can cause big changes in the signal-to-noise ratio and
harmonic distortion figures of the amplifier.
If you do want to listen via headphones, a far better option is
to build the simple Stereo Headphone Adaptor presented here. It connects
directly to the amplifier’s speaker terminals and switches the loudspeakers and
stereo headphone sockets using two DPDT (double-pole, double-throw) relays, so
there’s no chance of it degrading the audio performance.
As mentioned in the introduction, you can connect up to two
sets of stereo headphones. These can be switched on or off at the touch of a
button and the volume of each can be individually controlled. In addition, the
loudspeakers can be switched on or off and there’s also a Mute switch which
turns everything off.
