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.
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 the 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
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