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______________1µF 63V Polyester Capacitor C2____________100µF 25V Electrolytic Capacitor C3____________470µF 25V Electrolytic Capacitor Q1____________BC549C 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)
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.
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
