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