=20
Normal=20
headphones have an impedance of 32R per channel. The usual standard=20 line
output of 775 mV to which all quality equipment aspires will = generate=20 a
power of U2 / R =3D 0.7752 / 32 =3D 18 mW per channel across a = headphone
of=20 this impedance. An examination of available headphones at well = known
high=20 street emporiums revealed that the sensitivity varied from 96 dB
to=20 103db/mW! So, in practice the circuit will only require unity gain =
to reach=20 deafening levels. As a unity gain design is required it is
quite = possible to=20 employ a low distortion output stage.
The
obvious choice is = an=20 emitter follower. This has nearly unity gain
combined with a large = amount of=20 local feedback. Unfortunately the
output impedance of an emitter = follower is=20 dependent upon the source
impedance. With a volume control, or even = with=20 different signal sources
this will vary and could produce small but = audible=20 changes in sound
quality. To prevent this, the output stage is = driven by a=20 cathode
follower,based around an ECC82 valve (US equivalent:=20 12AU7).
This
device, as opposed to a transistor configuration, = enables=20 the output
stage to be driven with a constant value, low impedance. = In other=20
words, the signal from the low impedance point is used to drive the =
high=20 impedance of the output stage, a situation which promotes low =
overall THD.=20 At the modest output powers required of the circuit, the
only = sensible=20 choice is a Class A circuit. In this case the much
vaunted = single-ended=20 output stage is employed and that comprises of T3
and constant = current=20 source T1-T2.
= The
constant=20 current is set by the Vbe voltage of T1 applied across R5
With its = value of=20 22R, the current is set at 27 mA. T3 is used in the
emitter follower = mode=20 with high input impedance and low output
impedance. Indeed the main = problem=20 of using a valve at low voltages is
that it=E2=80=99s fairly = difficult to get any=20 real current drain. In order to
prevent distortion the output stage=20 shouldn=E2=80=99t be allowed to load the
valve. This is down to the = choice of output=20 device. A BC517 is used for
T3 because of its high current gain, = 30,000 at 2=20 mA! Since we have a
low impedance output stage, the load may be = capacitively=20 coupled via
C4.
Some purists may baulk at the idea of using = an=20 electrolytic
for this job but he fact remains that distortion = generated by=20
capacitive coupling is at least two orders of magnitude lower than=20
transformer coupling. The rest of the circuitry is used to condition =
the=20 various voltages used by the circuit. In order to obtain a linear =
output the=20 valve grid needs to be biased at half the supply voltage.
This is = the=20 function of the voltage divider R4 and R2. Input signals
are coupled = into=20 the circuit via C1 and R1.
R1, connected
between the voltage = divider=20 and V1=E2=80=99s grid defines the input impedance
of the circuit. C1 = has=20 sufficiently large a value to ensure response
down to 2 Hz. Although = the=20 circuit does a good job of rejecting line
noise on its own due to = the high=20 impedance of V1=E2=80=99s anode and T3=E2=80=99s
collector current, = it needs a little help=20 to obtain a silent background
in the absence of signal. The = =E2=80=98help=E2=80=99 is in the=20 form of the capacitance
multiplier circuit built around T5. Another = BC517 is=20 used here to
avoid loading of the filter comprising R7 and C5. In = principle=20 the
capacitance of C5 is multiplied by the gain of T5.
In = practice=20
the smooth dc applied to T5=E2=80=99s base appears at low impedance = at its=20
emitter. An important added advantage is that the supply voltage is=20
applied slowly on powering up. This is of course due to the time = taken=20
to fully charge C5 via R7. No trace of hum or ripple can be seen = here=20 on
the =E2=80=98scope. C2 is used to ensure stability at RF. The DC = supply is=20
also used to run the valve heater. The ECC82 has an advantage here = in=20
that its heater can be connected for operate from 12.6 V.
To=20 run
it T4 is used as a series pass element. Base voltage is obtained = from=20
the emitter of T5. T4 has very low output impedance, about 160 mR = and=20
this helps to prevent extraneous signals being picked up from the =
heater=20 wiring. Connecting the transistor base to C5 also lets the = valve
heater warm=20 up gently. A couple of volts only are lost across T4 and
although = the device=20 runs warm it doesn=E2=80=99t require a heat-sink.
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