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100mW Headphone Amplifier Schematic Build Guide

Description

High-fidelity headphone listeners frequently employ battery-powered headphone amplifiers, particularly for portable applications, but also for home listening setups. This design fulfills those needs by providing an enhanced output driving capability through a push-pull Class-B arrangement. The resulting power output reaches 100mW RMS into a 16 Ohm load at a 6V supply, coupled with low standing and mean current consumption, facilitating extended battery life.

Circuit diagram:

Circuit diagram

Parts:

  • P1 = 22K Potentiometer
  • R1 = 15K Resistor
  • R2 = 100K Resistor
  • R3 = 100K Resistor
  • R4 = 47K Resistor
  • R5 = 470 Ohm Resistor
  • R6 = 500 Ohm Resistor
  • R7 = 1K Resistor
  • R8 = 18K Resistor
  • R9 = 18K Resistor
  • R10 = 2.2 Ohm Resistor
  • R11 = 2.2 Ohm Resistor
  • R12 = 33 Ohm Resistor
  • R13 = 4.7K Resistor
  • C1 = 10uF-25V Capacitors
  • C2 = 10uF-25V Capacitors
  • C3 = 100nF-63V (PF)
  • C4 = 220uF-25V Capacitors
  • C5 = 100nF-63V (PF)
  • C6 = 220uF-25V Capacitors
  • Q1 = BC560C PNP Transistor
  • Q2 = BC560C PNP Transistor
  • Q3 = BC550C NPN Transistor
  • Q4 = BC550C NPN Transistor
  • Q5 = BC560C PNP Transistor
  • Q6 = BC327 PNP Transistor
  • Q7 = BC337 NPN Transistor
  • J1 = RCA Audio Input Socket
  • J2 = 3mm Stereo Jack Socket
  • B1 = 6V Battery Rechargeable
  • SW1=SPST Slide or Toggle Switch

Notes:

  1. For a stereo version of this circuit, all components must be duplicated except P1, SW1, J2 and B1.
  2. Prior to establishing the quiescent current, rotate the volume control potentiometer (P1) to its minimum setting, the trimmer resistor R6 to its maximum resistance value, and the trimmer resistor R3 to approximately the middle of its travel.
  3. Connect a suitable headphone set or, preferably, a 33 Ohm 1/2W resistor to the amplifier's output.
  4. Activate the power supply and measure the battery voltage with a multimeter, configured to display approximately 10Vdc at full scale (FS).
  5. Connect the multimeter across the positive terminal of capacitor C4 and the negative ground.
  6. Rotate resistor R3 to achieve an output reading on the multimeter display that precisely matches half of the previously measured battery voltage.
  7. Deactivate the power supply, disconnect the multimeter, and reconnect it, configured to measure approximately 10mA at full scale (FS), in series with the positive supply line to the amplifier.
  8. Activate the power supply and slowly rotate resistor R6 until a reading of about 3mA is displayed on the multimeter.
  9. Again, check the voltage at the positive terminal of capacitor C4 and make any necessary adjustments to resistor R3.
  10. Allow approximately 15 minutes for the current to stabilize, observing its fluctuations and making further adjustments to resistor R3 if required.
  11. Individuals with access to an oscilloscope and a 1 KHz sine wave generator can drive the amplifier to its maximum output power, adjusting resistor R3 to achieve a symmetrical clipping of the sine wave, as displayed on the oscilloscope.

Technical data:

  • Output power (1 KHz sine wave):
  • 16 Ohm: 100mW RMS
  • 32 Ohm: 60mW RMS
  • 64 Ohm: 35mW RMS
  • 100 Ohm: 22.5mW RMS
  • 300 Ohm: 8.5mW RMS

Sensitivity:

  1. 160mV input for 1V RMS output into 32 Ohm load (31mW)
  2. 200mV input for 1.27V RMS output into 32 Ohm load (50mW)

Frequency response @ 1V RMS:

  1. Flat from 45Hz to 20 KHz, -1dB @ 35Hz, -2dB @ 24Hz

Total harmonic distortion into 16 Ohm load @ 1 KHz:

  1. 1V RMS (62mW) 0.015% 1.27V RMS (onset of clipping, 100mW) 0.04%

Total harmonic distortion into 16 Ohm load @ 10 KHz:

  1. 1V RMS (62mW) 0.05% 1.27V RMS (onset of clipping, 100mW) 0.1%
  2. Unconditionally stable on capacitive loads
circuit from http://www.extremecircuits.net/2009/12/hiqh-quality-headphone-amplifier.html