Precision Headphone Amplifier

Designs for good-quality headphone amplifiers abound, but this one has a few special features that make it stand out from the crowd. We start with a reasonably conventional input stage in the form of a differential amplifier constructed from dual FET T2/T3. A particular point here is that in the drain of T3, where the amplified signal appears, we do not have a conventional current source or a simple resistor. T1 does indeed form a current source, but the signal is coupled out to the base of T5 not from the drain of T3 but from the source of T1. Notwithstanding the action of the current source this is a low impedance point for AC signals in the differential amplifier.

Precision Headphone Amplifier Circuit Diagram

Measurements show that this trick by itself results in a reduction in harmonic distortion to considerably less than –80 dB (much less than 0.01 %) at 1 kHz. T5 is connected as an emitter follower and provides a low impedance drive to the gate of T6: the gate capacitance of HEXFETs is far from negligible. IC1, a volt-age regulator configured as a current sink, is in the load of T6. The quiescent current of 62 mA (determined by R11) is suitable for  an output power of 60 mWeff into an impedance of 32 Ω, a value typical of high-quality headphones, which provides plenty of volume.

Using higher-impedance headphones, say of 300 Ω, considerably more than 100 mW can be achieved. The gain is set to a useful 21 dB (a factor of 11) by the negative feedback circuit involving R10 and R8. It is not straightforward to change the gain because of the single-sided supply: this voltage divider also affects the operating point of the amplifier. The advantage is that excellent audio quality can be achieved even using a simple unregulated mains supply.  Given the relatively low power output the power supply is considerably overspecified. Noise and hum thus remain more than 90 dB below the signal (less than 0.003 %), and the supply can also power two amplifiers for stereo operation.

The bandwidth achievable with this design is from 5 Hz to 300 kHz into 300 Ω, with an output voltage of 10 Vpp. The damping factor is greater than 800 between 100 Hz and 10 kHz. A couple of further things to note: some-what better DC stability can be achieved by replacing D1 and D2 by low-current red LEDs (connected with the right polarity!). R12 prevents a click from the discharge of C6 when headphones are plugged in after power is applied. T6 and IC1 dissipate about 1.2 W of power each as heat, and so cooling is needed. For low impedance headphones the current through IC1 should be increased. To deliver 100 mW into 8 Ω, around 160 mA is required, and R11 will need to be 7.8 Ω (use two 15 Ω resistors in parallel).

To keep heat dissipation to a reasonable level, it is recommended to reduce the power supply volt-age to around 18 V (using a transformer with two 6 V secondaries). This also means an adjustment to the operating point of the amplifier: we will need about 9V between the positive end of C6 and ground. R4 should be changed to 100 Ω, and R8 to 680 Ω. The gain will now be approximately 6 (15 dB). The final dot on the ‘i’ is to increase C7 by connecting another 4700 µF electrolytic in parallel with it, since an 8 Ω load will draw higher currents.

Source:   http://www.ecircuitslab.com/2012/02/precision-headphone-amplifier.html

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Modular Headphone Amplifier

140mW into 32 Ohm loads, Ultra-low Distortion

Those wanting private listening to their music program should add this Headphone Amplifier to the Modular Preamplifier chain. The circuit was kept as simple as possible compatibly with a High Quality performance. This goal was achieved by using two NE5532 Op-Amps in a circuit where IC1B is the "master" amplifier wired in the common non-inverting configuration already used in the Control Center Line amplifier. IC1A is the "slave" amplifier and is configured as a unity-gain buffer: parallel amplifiers increase output current capability of the circuit. Two Headphone outputs are provided by J3 and J4. The ac gain of the amplifier was kept deliberately low because this module is intended to be connected after the Control Center module, which provides the gain sufficient to drive the power amplifier.

If you intend to use this Headphone Amplifier as a stand-alone device, a higher ac gain could be necessary in order to cope with a CD player or Tuner output. This is accomplished by lowering the value of R1 to 1K5. In this way an ac gain of 9 is obtained, more than sufficient for the purpose. Contrary to the two 15V positive and negative regulator ICs used in other modules of this preamp, two 9V devices were employed instead. This because the NE5532 automatically limits its output voltage into very low loads as 32 Ohm in such a way that the output amplitude of the amplified signal remains the same, either the circuit is powered at ±9V or ±15V. The choice of a ±9V supply allows less power dissipation and better performance of the amplifier close to the clipping point.

The input socket of this amplifier must be connected to the Main Out socket of the Control Center Module. As this output is usually reserved to drive the power amplifier, a second socket (J2) wired in parallel to J1 is provided for this purpose. As with the other modules of this series, each electronic board can be fitted into a standard enclosure: Hammond extruded aluminum cases are well suited to host the boards of this preamp. In particular, the cases sized 16 x 10.3 x 5.3 cm or 22 x 10.3 x 5.3 cm have a very good look when stacked. See below an example of the possible arrangement of the front and rear panels of this module.

Parts:

P1______________47K Log. Potentiometer (twin concentric-spindle dual gang for stereo)
R1_______________4K7 1/4W Resistor
R2______________12K 1/4W Resistor
R3,R4___________33R 1/4W Resistors
R5,R6____________4R7 1/4W Resistors
C1_______________1µF 63V Polyester Capacitor
C2,C5__________100nF 63V Polyester Capacitors
C3,C6___________22µF 25V Electrolytic Capacitors
C4,C7_________2200µF 25V Electrolytic Capacitors
IC1__________NE5532 Low noise Dual Op-amp
IC2___________78L09 9V 100mA Positive Regulator IC
IC3___________79L09 9V 100mA Negative Regulator IC
D1,D2________1N4002 200V 1A Diodes
J1,J2__________RCA audio input sockets
J3,J4__________6mm. or 3mm. Stereo Jack sockets
J5_____________Mini DC Power Socket

Notes:

• The circuit diagram shows the Left channel only and the power supply.
• Some parts are in common to both channels and must not be doubled. These parts are: P1 (if a twin concentric-spindle dual gang potentiometer is used), IC2, IC3, C2, C3, C4, C5, C6, C7, D1, D2, J3, J4 and J5.
• This module requires an external 15 - 18V ac (100mA minimum) Power Supply Adaptor.

Technical data:

Output power (1KHz sinewave):
32 Ohm: 140mW RMS
Sensitivity:
275mV input for 1V RMS output into 32 Ohm load (31mW)
584mV input for 2.12V RMS output into 32 Ohm load (140mW)
Frequency response @ 2V RMS:
Flat from 15Hz to 23KHz
Total harmonic distortion into 32 Ohm load @ 1KHz:
1V RMS and 2V RMS 0.0012%
Total harmonic distortion into 32 Ohm load @ 10KHz:
1V RMS and 2V RMS 0.0008%

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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.

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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Battery powered Headphone Amplifier

Low distortion Class-B circuitry 6V Battery Supply

Some enthusiasts of High Fidelity headphone listening want the utilization of battery energyed headphone amplifiers, not just for transportable gadgets but additionally for home \"table\" softwares. This design is intended to fulfil their needs and its topology is derived from the Portable Headphone Amplifier that contains an NPN/PNP compound pair emitter follower output stage. An superior output using functionality is achieveed via making this a push-pull Class-B association. Output energy can reach one hundredmW RMS into a 16 Ohm load at 6V supply with low standing and mean present consumption, allowing lengthy battery duration. The single voltage gain stage permits the easy implementation of a shunt-feedback circuitry giving superb frequency steadiness.

Circuit diagram :

Battery-powered Headphone Amplifier Circuit diagram

Notes:

• For a Stereo model of this circuit, all parts should be doubled aside from P1, SW1, J2 and B1.
• Before setting quiescent current rotate the volume regulate P1 to the minimum, Trimmer R6 to most resistance and Trimmer R3 to in regards to the middle of its shuttle.
• Connect an acceptable headphone set or, better, a 33 Ohm 1/2W resistor to the amplifier output.
• Switch on the provision and measure the battery voltage with a Multimeter set to about 10Vdc fsd.
• Connect the Multimeter throughout the positive end of C4 and the poor floor.
• Rotate R3 with a view to learn on the Multimeter show precisely half of of the battery voltage in the past measured.
• Switch off the availability, disconnect the Multimeter and reconnect it, set to measure about 10mA fsd, in sequence to the certain supply of the amplifier.
• Switch on the provide and rotate R6 slowly except a studying of about 3mA is displayed.
• Check once more the voltage on the positive finish of C4 and learnjust R3 if essential.
• Wait about quarter-hour, watch if the present is various and readjust if important.
• Those fortunate sufficient to achieve an oscilloscope and a 1KHz sine wave generator, can power the amplifier to the utmost output power and alter R3 with a goal to get hold of a symmetrical clipping of the sine wave showed.

Technical data:

Output power (1KHz sinewave):
    16 Ohm: 100mW RMS
    32 Ohm: 60mW RMS
    sixty four Ohm: 35mW RMS
    100 Ohm: 22.5mW RMS
    300 Ohm: eight.5mW RMS
Sensitivity:
    160mV enter for 1V RMS output into 32 Ohm load (31mW)
    200mV input for 1.27V RMS output into 32 Ohm load (50mW)
Frequency response @ 1V RMS:
    flat from 45Hz to 20KHz, -1dB @ 35Hz, -2dB @ 24Hz
Total harmonic distortion into 16 Ohm load @ 1KHz:
    1V RMS (62mW) zero.015% 1.27V RMS (onset of clipping, a hundredmW) 0.04%
Total harmonic distortion into 16 Ohm load @ 10KHz:
    1V RMS (62mW) 0.05% 1.27V RMS (onset of clipping, one hundredmW) 0.1%
Unconditionally secure on capacitive loads

Source : red circuits

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