Some lovers of High Fidelity headphone listening prefer the use of battery powered headphone amplifiers, not only for portable units but also for home table applications. This design is intended to fulfil their needs and its topology is derived from the Portable Headphone Amplifier featuring an NPN PNP compound pair emitter follower output stage.
An improved output driving capability is gained by making this a push-pull Class-B arrangement. Output power can reach 100mW RMS into a 16 Ohm load at 6V supply with low standing and mean current consumption, allowing long battery duration.
The single voltage gain stage allows the easy implementation of a shunt-feedback circuitry giving excellent frequency stability.
Notes: For a Stereo version of this circuit, all parts must be doubled except P1, SW1, J2 and B1. .
Before setting quiescent current rotate the volume control P1 to the minimum, Trimmer R6 to maximum resistance and Trimmer R3 to about the middle of its travel. .
Connect a suitable headphone set or, better, a 33 Ohm 1/2W resistor to the amplifier output. .
Switch on the supply and measure the battery voltage with a Multimeter set to about 10Vdc fsd. .
Connect the Multimeter across the positive end of C4 and the negative ground. .
Rotate R3 in order to read on the Multimeter display exactly half of the battery voltage previously measured. .
Switch off the supply, disconnect the Multimeter and reconnect it, set to measure about 10mA fsd, in series to the positive supply of the amplifier. .
Switch on the supply and rotate R6 slowly until a reading of about 3mA is displayed. .
Check again the voltage at the positive end of C4 and readjust R3 if necessary. .
Wait about 15 minutes, watch if the current is varying and readjust if necessary. .
Those lucky enough to reach an oscilloscope and a 1KHz sine wave generator, can drive the amplifier to the maximum output power and adjust R3 in order to obtain a symmetrical clipping of the sine wave displayed.
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