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Portable Headphone Amplifier

source: RED Free Circuit Designs


Circuit diagram:


R1 = 10K 1/4W Resistor
R2 = 100K 1/4W Resistor
R3 = 68K 1/4W Resistor (see notes)
R4 = 1K5 1/4W Resistor
R5 = 3K3 1/4W Resistor
R6 = 330R 1/4W Resistor
R7 = 4K7 1/4W Resistor
R8 = 2R2 1/4W Resistor

C1 = 1µF 63V Polyester Capacitor
C2 = 100µF 25V Electrolytic Capacitor
C3 = 470µF 25V Electrolytic Capacitor

Q1 = BC239C 25V 100mA NPN High-gain Low-noise Transistor
Q2 = BC337 45V 800mA NPN Transistor
Q3 = BC327 45V 800mA PNP Transistor

J1 = Stereo 3mm. Jack socket

SW1 = SPST Switch
B1 = 3V Battery (two 1.5V AA or C cells in series)


Can be directly connected to CD players, tuners and tape recorders.

Tested with several headphone models of different impedance: 32, 100, 245, 300, 600 & 2000 Ohms.

Schematic shows left channel only.

B1, SW1, J1 & C3 are common to both channels.

R3 value was calculated for headphone impedance up to 300 Ohms. Using 600 Ohms loads or higher, change R3 value to 100K.

Technical data:

Current drain: 35mA per channel with 32 Ohms impedance headphones. Much less with higher impedance loads

Output voltage: Above 2V peak-to-peak on all loads

Sensitivity: 90mV RMS input for 2V peak-to-peak output

Frequency response: Flat from 30Hz to 20KHz

Total harmonic distortion @ 1KHz & 10KHz: Below 0.05% on 32 to 600 Ohms load and up to 1.5V peak-to-peak output. Below 0.1% at maximum output

Unconditionally stable on capacitive loads

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