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.)
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. This design is intended to fulfil their needs and its topology is derived from the Portable Headphone Amplifier featuring an NPN/PNP compound pair emitter follower output stage. An improved output driving capability is gained by making this a push-pull Class-B arrangement. 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.
- 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.
- 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
- 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