The tube headphone amplifiers on the market all use female output as the final power amplifier. This type of circuit structure should be said to be very reasonable. Therefore, it is adopted by most manufacturers. However, this circuit has a high output impedance and can only be equipped with headphones above 200 and 300 ohms, and the higher the impedance, the smaller the distortion.
The final stage of the transistor headphone amplifier uses an OTL circuit, which does not have strict requirements on impedance, and the lower the impedance, the greater the output current and the greater the power. Therefore, transistor amplifiers have high, medium, and low resistance, but it is better to push low and medium impedance headphones.
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