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
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.
