Hybrid Headphone Amplifier

Posted on Feb 6, 2014

Potentially, headphone listening can be technically superior since room reflections are eliminated and the intimate contact between transducer and ear mean that only tiny amounts of power are required. The small power requirement means that transducers can be operated at a small fraction of their full excursion capabilities thus reducing THD and o

Hybrid Headphone Amplifier
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ther non-linear distortions. This design of a dedicated headphones amplifier is potentially controversial in that it has unity voltage gain and employs valves and transistors in the same design. Normal headphones have an impedance of 32R per channel. The usual standard line output of 775 mV to which all quality equipment aspires will generate a power of U2 / R = 0. 7752 / 32 = 18 mW per channel across a headphone of this impedance. An examination of available headphones at well known high street emporiums revealed that the sensitivity varied from 96 dB to 103db/mW! So, in practice the circuit will only require unity gain to reach deafening levels. As a unity gain design is required it is quite possible to employ a low distortion output stage. The obvious choice is an emitter follower. This has nearly unity gain combined with a large amount of local feedback. Unfortunately the output impedance of an emitter follower is dependent upon the source impedance. With a volume control, or even with different signal sources this will vary and could produce small but audible changes in sound quality. To prevent this, the output stage is driven by a cathode follower, based around an ECC82 valve (US equivalent: 12AU7). This device, as opposed to a transistor configuration, enables the output stage to be driven with a constant value, low impedance. In other words, the signal from the low impedance point is used to drive the high...

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