This amplifier can be directly connected to CD players, tuners and tape recorders. It is 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.
Circuit diagram
Portable Headphone Amplifier Circuit Diagram
Parts:
R1 = 10K
R2 = 100K
R3 = 68K
R4 = 1.5K
R5 = 3.3K
R6 = 330R
R7 = 4K7
R8 = 2.2R
C1 = 1µF-63V
C2 = 100µF-25V
C3 = 470µF-25V
Q1 = BC239C
Q2 = BC337
Q3 = BC327
J1,2 = Stereo 3mm. Jack socket
SW1 = SPST Switch
B1 = 3V Battery (two 1.5V AA or C cells in series, but see Notes)
Upgrade:
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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