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Thursday, November 21, 2013

Hybrid Headphone Amplifier

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.

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.

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Posted by
ryante dut at
4:00 AM

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