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How to build Pure Class-A Headphone Amplifier Schematic

2015-01-05 18:48  
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This design 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-A arrangement. Output power can reach 427mW RMS into a 32 Ohm load at a fixed standing current of 100mA. The single voltage gain stage allows the easy implementation of a shunt-feedback circuitry giving excellent frequency stability.

Tilt control:

The mentioned shunt-feedback configuration also allows the easy addition of frequency dependent networks in order to obtain an useful, unobtrusive, switchable Tilt control (optional). When SW1 is set in the first position a gentle, shelving bass lift and treble cut is obtained. The central position of SW1 allows a flat frequency response, whereas the third position of this switch enables a shelving treble lift and bass cut.

Circuit diagram:

Circuit diagram


P1 = 22K Dual gang Log Potentiometer (ready for Stereo)R1 = 15K 1/4W ResistorR2 = 220K 1/4W ResistorR3 = 100K 1/2W Trimmer CermetR4 = 33K 1/4W ResistorR5 = 68K 1/4W ResistorR6 = 50K 1/2W Trimmer CermetR7 = 10K 1/4W ResistorR8 = 47K 1/4W ResistorsR9 = 47K 1/4W ResistorsR10 = 2R2 1/4W ResistorsR11 = 2R2 1/4W ResistorsR12 = 4K7 1/4W ResistorR13 = 4R7 1/2W ResistorR14 = 1K2 1/4W ResistorR15 = 330K 1/4W Resistors (Optional)R16 = 680K 1/4W Resistor (Optional)R17 = 220K 1/4W Resistors (Optional)R18 = 330K 1/4W Resistors (Optional)R19 = 220K 1/4W Resistors (Optional)R20 = 22K 1/4W Resistors (Optional)R21 = 22K 1/4W Resistors (Optional)C1 = 10μF 25V Electrolytic CapacitorsC2 = 10μF 25V Electrolytic CapacitorsC3 = 10μF 25V Electrolytic CapacitorsC4 = 10μF 25V Electrolytic CapacitorsC5 = 220μF 25V Electrolytic CapacitorsC6 = 100nF 63V Polyester CapacitorsC7 = 220μF 25V Electrolytic CapacitorsC8 = 2200μF 25V Electrolytic CapacitorC9 = 1nF 63V Polyester Capacitors (Optional)C10 = 470pF 63V Polystyrene or Ceramic Capacitor (Optional)C13 = 15nF 63V Polyester Capacitor (Optional)C11 = 1nF 63V Polyester Capacitors (Optional)C12 = 1nF 63V Polyester Capacitors (Optional)D1 = 5mm. or 3mm. LEDD2 = 1N4002 100V 1A DiodesD3 = 1N4002 100V 1A DiodesQ1 = BC550C 45V 100mA Low noise High gain NPN TransistorsQ2 = BC550C 45V 100mA Low noise High gain NPN TransistorsQ3 = BC560C 45V 100mA Low noise High gain PNP TransistorQ4 = BD136 45V 1.5A PNP TransistorQ5 = BD135 45V 1.5A NPN TransistorIC1 = 7815 15V 1A Positive voltage regulator ICT1 = 220V Primary, 15 15V Secondary-5VA Mains transformerSW1 = 4 poles 3 ways rotary Switch (ready for Stereo)SW2 = SPST slide or toggle SwitchJ1 = RCA audio input socketJ2 = 6mm. or 3mm. Stereo Jack socketPL1 = Male Mains plug


Q4, Q5 and IC1 must be fitted with a small U-shaped heatsink.For a Stereo version of this circuit, all parts must be doubled except P1, IC1, R14, D1, D2, D3, C8, T1, SW1, SW2, J2 and PL1.If the Tilt Control is not needed, omit SW1, all resistors from R15 onwards and all capacitors from C9 onwards. Connect the rightmost terminal of R1 to the Base of Q1.Before setting quiescent current rotate the volume control P1 to the minimum, Trimmer R6 to zero 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.Connect a Multimeter, set to measure about 10Vdc fsd, across the positive end of C5 and the negative ground.Switch on the supply and rotate R3 in order to read about 7.7-7.8V on the Multimeter display.Switch off the supply, disconnect the Multimeter and reconnect it, set to measure at least 200mA fsd, in series to the positive supply of the amplifier.Switch on the supply and rotate R6 slowly until a reading of about 100mA is displayed.Check again the voltage at the positive end of C5 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):

32 Ohm: 427mW RMS64 Ohm: 262mW RMS100 Ohm: 176mW RMS300 Ohm: 64mW RMS600 Ohm: 35mW RMS2000 Ohm: 10mW RMS


140mV input for 1V RMS output into 32 Ohm load (31mW)500mV input for 3.5V RMS output into 32 Ohm load (380mW)Total harmonic distortion into 32 Ohm load @ 1KHz:1V RMS 0.005% 3V RMS 0.015% 3.65V RMS (onset of clipping) 0.018%Total harmonic distortion into 32 Ohm load @ 10KHz:1V RMS 0.02% 3V RMS 0.055% 3.65V RMS (onset of clipping) 0.1%Unconditionally stable on capacitive loads

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