PORTABLE HEADPHONE AMPLIFIER

Circuit diagram:

Parts:

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______________1�F   63V Polyester Capacitor
C2____________100�F   25V Electrolytic Capacitor
C3____________470�F   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, but see Notes)

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Notes:

• 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 Ohm.
• Schematic shows left channel only.
• B1, SW1, J1 & C3 are common to both channels.
• R3 value was calculated for headphone impedance up to 300 Ohm. Using 600 Ohm loads or higher, change R3 value to 100K.
• An interesting upgrade for this circuit was suggested by Mike Baum, NY USA. This involves the use of a Lithium-Ion Prismatic Rechargeable 340948 Battery, featuring a nominal voltage of 3.7V and a current rating of 1200mAH. These weight 38 grams and are 34.2mm x 8.5mm x 48mm. Correct value of R3 when using a 3.7V supply and 32 Ohm impedance headphones will be 100K: under these conditions, the amplifier will deliver about 3V peak-to-peak undistorted output. This means that the output power on 32 Ohm load will be almost doubled in respect to 3V supply. Current drawing will raise to 40mA. When powering a stereo version of this amplifier, the battery will last about 15 hours.

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Technical data:

Current drain:

35mA per channel with 32 Ohm 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% into 32 to 600 Ohm loads and up to 1.5V peak-to-peak output.

Below 0.1% at maximum output

Unconditionally stable on capacitive loads

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