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Portable Headphone Amp: Circuit & 3.7V Upgrade
Description
This amplifier can be seamlessly connected to CD players, tuners, and tape recorders. The design has undergone rigorous testing with a variety of headphone models exhibiting differing impedance values: 32, 100, 245, 300, 600, and 2000 Ohm. The schematic provided illustrates only the left channel. Components labeled B1, SW1, J1, and C3 are shared across both channels. The resistance value of R3 was carefully calculated to accommodate headphone impedances up to 300 Ohm. For operation with loads of 600 Ohm or higher, it's recommended to adjust R3 to a value of 100K to ensure optimal performance.
More about components
The amplifier utilizes three integrated circuits: BC239C, BC337, and BC327. These are general-purpose bipolar junction transistors (BJTs) widely employed in audio amplifier designs due to their good linearity and relatively high current gain. They provide amplification of the input signal, driving the load (headphones) with a larger voltage and current. Each transistor functions as a current source and voltage amplifier, facilitating efficient audio reproduction.
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 enhancement for this circuit was proposed by Mike Baum, NY USA. This involves incorporating a Lithium-Ion Prismatic Rechargeable 340948 Battery, characterized by a nominal voltage of 3.7V and a current capacity of 1200mAH.
- These batteries measure 38 grams and possess dimensions of 34.2mm x 8.5mm x 48mm. With a 3.7V supply and a 32 Ohm impedance headphone, the correct value for R3 should be 100K; under these conditions, the amplifier will generate approximately 3V peak-to-peak undistorted output.
- This configuration results in nearly doubled output power on a 32 Ohm load, relative to the 3V supply. Current consumption will increase to 40mA. When utilized in a stereo version of this amplifier, the battery is expected to provide approximately 15 hours of continuous operation.