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427mW Class-A Headphone Amp

Description

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. Integrated circuits are crucial components in this design. The 7815 voltage regulator IC (IC1) provides a stable 15VDC supply, vital for optimal transistor operation. This ensures consistent performance across varying input levels. The BC550C NPN (Q1 & Q2) and BC560C PNP (Q3) transistors are key to the push-pull output, offering high gain and low noise characteristics. The BD136 PNP (Q4) and BD135 NPN (Q5) transistors further enhance the output stage’s robustness and current handling capabilities.

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. Integrated circuits are also crucial for the control features. The 7815 voltage regulator IC (IC1) supplies the necessary power to the control circuitry.

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. Integrated circuits are key to this feature, with the 7815 (IC1) supplying power to the associated circuitry.

Circuit diagram:

Circuit diagram

Parts:

  • P1 = 22K Dual gang Log Potentiometer (ready for Stereo)
  • R1 = 15K 1/4W Resistor
  • R2 = 220K 1/4W Resistor
  • R3 = 100K 1/2W Trimmer Cermet
  • R4 = 33K 1/4W Resistor
  • R5 = 68K 1/4W Resistor
  • R6 = 50K 1/2W Trimmer Cermet
  • R7 = 10K 1/4W Resistor
  • R8 = 47K 1/4W Resistors
  • R9 = 47K 1/4W Resistors
  • R10 = 2R2 1/4W Resistors
  • R11 = 2R2 1/4W Resistors
  • R12 = 4K7 1/4W Resistor
  • R13 = 4R7 1/2W Resistor
  • R14 = 1K2 1/4W Resistor
  • R15 = 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 Capacitors
  • C2 = 10µF 25V Electrolytic Capacitors
  • C3 = 10µF 25V Electrolytic Capacitors
  • C4 = 10µF 25V Electrolytic Capacitors
  • C5 = 220µF 25V Electrolytic Capacitors
  • C6 = 100nF 63V Polyester Capacitors
  • C7 = 220µF 25V Electrolytic Capacitors
  • C8 = 2200µF 25V Electrolytic Capacitor
  • C9 = 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. LED
  • D2 = 1N4002 100V 1A Diodes
  • D3 = 1N4002 100V 1A Diodes
  • Q1 = BC550C 45V 100mA Low noise High gain NPN Transistors
  • Q2 = BC550C 45V 100mA Low noise High gain NPN Transistors
  • Q3 = BC560C 45V 100mA Low noise High gain PNP Transistor
  • Q4 = BD136 45V 1.5A PNP Transistor
  • Q5 = BD135 45V 1.5A NPN Transistor
  • IC1 = 7815 15V 1A Positive voltage regulator IC
  • T1 = 220V Primary, 15+15V Secondary-5VA Mains transformer
  • SW1 = 4 poles 3 ways rotary Switch (ready for Stereo)
  • SW2 = SPST slide or toggle Switch
  • J1 = RCA audio input socket
  • J2 = 6mm. or 3mm. Stereo Jack socket
  • PL1 = Male Mains plug

Notes:

  1. Q4, Q5 and IC1 must be fitted with a small U-shaped heatsink.
  2. 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.
  3. 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.
  4. 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.
  5. Connect a suitable headphone set or, better, a 33 Ohm 1/2W resistor to the amplifier output.
  6. Connect a Multimeter, set to measure about 10Vdc fsd, across the positive end of C5 and the negative ground.
  7. Switch on the supply and rotate R3 in order to read about 7.7-7.8V on the Multimeter display.
  8. 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.
  9. Switch on the supply and rotate R6 slowly until a reading of about 100mA is displayed.
  10. Check again the voltage at the positive end of C5 and readjust R3 if necessary.
  11. Wait about 15 minutes, watch if the current is varying and readjust if necessary.
  12. 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.
circuit from http://www.extremecircuits.net/2009/12/pure-class-headphone-amplifier.html