## Wednesday, August 26, 2009

### Battery-powered Headphone Amplifier

Some lovers of High Fidelity headphone listening prefer the use of battery powered headphone amplifiers, not only for portable units but also for home "table" applications.

Parts:

P1_____________22K Dual gang Log Potentiometer (ready for Stereo)

R1_____________15K 1/4W Resistor

R2____________100K 1/4W Resistor

R3____________100K 1/2W Trimmer Cermet

R4_____________47K 1/4W Resistor

R5____________470R 1/4W Resistor

R6____________500R 1/2W Trimmer Cermet

R7______________1K 1/4W Resistor

R8,R9__________18K 1/4W Resistors

R10,R11_________2R2 1/4W Resistors

R12____________33R 1/4W Resistor

R13_____________4K7 1/4W Resistor

C1,C2__________10µF 25V Electrolytic Capacitors

C3,C5_________100nF 63V Polyester Capacitors

C4,C6_________220µF 25V Electrolytic Capacitors

Q1,Q2,Q5______BC560C 45V 100mA Low noise High gain PNP Transistors

Q3,Q4_________BC550C 45V 100mA Low noise High gain NPN Transistor

Q6____________BC327 45V 800mA PNP Transistor

Q7____________BC337 45V 800mA NPN Transistor

SW1____________SPST slide or toggle Switch

J1_____________RCA audio input socket

J2_____________6mm. or 3mm. Stereo Jack socket

B1_____________6V Battery (4xAA or AAA Alkaline or rechargeable cells, etc.)

Output power can reach 100mW RMS into a 16 Ohm load at 6V supply with low standing and mean current consumption, allowing long battery duration.

The single voltage gain stage allows the easy implementation of a shunt-feedback circuitry giving excellent frequency stability.

Notes:

* For a Stereo version of this circuit, all parts must be doubled except P1, SW1, J2 and B1.

* Before setting quiescent current rotate the volume control P1 to the minimum, Trimmer R6 to maximum resistance and Trimmer R3 to about the middle of its travel.

* Connect a suitable headphone set or, better, a 33 Ohm 1/2W resistor to the amplifier output.

* Switch on the supply and measure the battery voltage with a Multimeter set to about 10Vdc fsd.

* Connect the Multimeter across the positive end of C4 and the negative ground.

* Rotate R3 in order to read on the Multimeter display exactly half of the battery voltage previously measured.

* Switch off the supply, disconnect the Multimeter and reconnect it, set to measure about 10mA fsd, in series to the positive supply of the amplifier.

* Switch on the supply and rotate R6 slowly until a reading of about 3mA is displayed.

* Check again the voltage at the positive end of C4 and readjust R3 if necessary.

* Wait about 15 minutes, watch if the current is varying and readjust if necessary.

* Those lucky enough to reach an oscilloscope and a 1KHz sine wave generator, can drive the amplifier to the maximum output power and adjust R3 in order to obtain a symmetrical clipping of the sine wave displayed.

Technical data:

Output power (1KHz sinewave):

16 Ohm: 100mW RMS

32 Ohm: 60mW RMS

64 Ohm: 35mW RMS

100 Ohm: 22.5mW RMS

300 Ohm: 8.5mW RMS

Sensitivity:

160mV input 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) 0.015% 1.27V RMS (onset of clipping, 100mW) 0.04%

Total harmonic distortion into 16 Ohm load @ 10KHz:

1V RMS (62mW) 0.05% 1.27V RMS (onset of clipping, 100mW) 0.1%

Unconditionally stable on capacitive loads

Parts:

P1_____________22K Dual gang Log Potentiometer (ready for Stereo)

R1_____________15K 1/4W Resistor

R2____________100K 1/4W Resistor

R3____________100K 1/2W Trimmer Cermet

R4_____________47K 1/4W Resistor

R5____________470R 1/4W Resistor

R6____________500R 1/2W Trimmer Cermet

R7______________1K 1/4W Resistor

R8,R9__________18K 1/4W Resistors

R10,R11_________2R2 1/4W Resistors

R12____________33R 1/4W Resistor

R13_____________4K7 1/4W Resistor

C1,C2__________10µF 25V Electrolytic Capacitors

C3,C5_________100nF 63V Polyester Capacitors

C4,C6_________220µF 25V Electrolytic Capacitors

Q1,Q2,Q5______BC560C 45V 100mA Low noise High gain PNP Transistors

Q3,Q4_________BC550C 45V 100mA Low noise High gain NPN Transistor

Q6____________BC327 45V 800mA PNP Transistor

Q7____________BC337 45V 800mA NPN Transistor

SW1____________SPST slide or toggle Switch

J1_____________RCA audio input socket

J2_____________6mm. or 3mm. Stereo Jack socket

B1_____________6V Battery (4xAA or AAA Alkaline or rechargeable cells, etc.)

Output power can reach 100mW RMS into a 16 Ohm load at 6V supply with low standing and mean current consumption, allowing long battery duration.

The single voltage gain stage allows the easy implementation of a shunt-feedback circuitry giving excellent frequency stability.

Notes:

* For a Stereo version of this circuit, all parts must be doubled except P1, SW1, J2 and B1.

* Before setting quiescent current rotate the volume control P1 to the minimum, Trimmer R6 to maximum resistance and Trimmer R3 to about the middle of its travel.

* Connect a suitable headphone set or, better, a 33 Ohm 1/2W resistor to the amplifier output.

* Switch on the supply and measure the battery voltage with a Multimeter set to about 10Vdc fsd.

* Connect the Multimeter across the positive end of C4 and the negative ground.

* Rotate R3 in order to read on the Multimeter display exactly half of the battery voltage previously measured.

* Switch off the supply, disconnect the Multimeter and reconnect it, set to measure about 10mA fsd, in series to the positive supply of the amplifier.

* Switch on the supply and rotate R6 slowly until a reading of about 3mA is displayed.

* Check again the voltage at the positive end of C4 and readjust R3 if necessary.

* Wait about 15 minutes, watch if the current is varying and readjust if necessary.

* Those lucky enough to reach an oscilloscope and a 1KHz sine wave generator, can drive the amplifier to the maximum output power and adjust R3 in order to obtain a symmetrical clipping of the sine wave displayed.

Technical data:

Output power (1KHz sinewave):

16 Ohm: 100mW RMS

32 Ohm: 60mW RMS

64 Ohm: 35mW RMS

100 Ohm: 22.5mW RMS

300 Ohm: 8.5mW RMS

Sensitivity:

160mV input 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) 0.015% 1.27V RMS (onset of clipping, 100mW) 0.04%

Total harmonic distortion into 16 Ohm load @ 10KHz:

1V RMS (62mW) 0.05% 1.27V RMS (onset of clipping, 100mW) 0.1%

Unconditionally stable on capacitive loads

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