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Hybrid Headphone Amplifier

Category: Audio, Headphone Amplifiers

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 other 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 impedance of the output stage, a
situation which promotes low overall THD. At the modest output powers required
of the circuit, the only sensible choice is a Class A circuit. In this case the
much vaunted single-ended output stage is employed and that comprises of T3 and
constant current source T1-T2.

Circuit diagram:

Hybrid Headphone Amplifier Circuit
Diagram

The constant
current is set by the Vbe voltage of T1 applied across R5 With its value of 22R,
the current is set at 27 mA. T3 is used in the emitter follower mode with high
input impedance and low output impedance. Indeed the main problem of using a
valve at low voltages is that it’s fairly difficult to get any real current
drain. In order to prevent distortion the output stage shouldn’t be allowed to
load the valve. This is down to the choice of output device. A BC517 is used for
T3 because of its high current gain, 30,000 at 2 mA! Since we have a low
impedance output stage, the load may be capacitively coupled via C4.

Some
purists may baulk at the idea of using an electrolytic for this job but he fact
remains that distortion generated by capacitive coupling is at least two orders
of magnitude lower than transformer coupling. The rest of the circuitry is used
to condition the various voltages used by the circuit. In order to obtain a
linear output the valve grid needs to be biased at half the supply voltage. This
is the function of the voltage divider R4 and R2. Input signals are coupled into
the circuit via C1 and R1.

R1, connected between the voltage divider and
V1’s grid defines the input impedance of the circuit. C1 has sufficiently large
a value to ensure response down to 2 Hz. Although the circuit does a good job of
rejecting line noise on its own due to the high impedance of V1’s anode and T3’s
collector current, it needs a little help to obtain a silent background in the
absence of signal. The ‘help’ is in the form of the capacitance multiplier
circuit built around T5. Another BC517 is used here to avoid loading of the
filter comprising R7 and C5. In principle the capacitance of C5 is multiplied by
the gain of T5.

In practice the smooth dc applied to T5’s base appears at
low impedance at its emitter. An important added advantage is that the supply
voltage is applied slowly on powering up. This is of course due to the time
taken to fully charge C5 via R7. No trace of hum or ripple can be seen here on
the ‘scope. C2 is used to ensure stability at RF. The DC supply is also used to
run the valve heater. The ECC82 has an advantage here in that its heater can be
connected for operate from 12.6 V.

To run it T4 is used as a series pass
element. Base voltage is obtained from the emitter of T5. T4 has very low output
impedance, about 160 mR and this helps to prevent extraneous signals being
picked up from the heater wiring. Connecting the transistor base to C5 also lets
the valve heater warm up gently. A couple of volts only are lost across T4 and
although the device runs warm it doesn’t require a heat-sink.

Author:
Jeff Macaulay - Copyright: Elektor
Electronics

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