The impedance of the earphone is the sum of the DC resistance of the coil and the inductive reactance of the coil in ohms (Ω). Generally speaking, 16-200 Ω is called low-impedance earphone, 1-4 kΩ is called high-impedance earphone, and the usual high-impedance earphone is 800 Ω. Sensitivity, also known as sound pressure level, refers to the earphone plus 50 Hz sinusoidal signal. Its size is the sound pressure level generated in the coupling cavity when the voltage is equivalent to 1 mW of electric power. The unit is dB/mW. Headphone sensitivity is mostly at 88-122 dB/mW. Most people may think that the smaller the impedance, the easier it is for the earphone to sound and the easier it is to drive. In fact, the factor that determines whether a pair of headphones is easy to push, the sensitivity is more important than the impedance, even if the impedance of a pair of headphones is very low. If the sensitivity is not high, it is also difficult to drive. There are two common ways to access the earphone circuit.
(1) High-impedance earphone; serially connected in the collector circuit of the final amplifier circuit.
(2) The low-impedance earphone is connected to the output transformer secondary; the output of the OTL circuit; serially connected in the emitter circuit of the power amplifier tube. The general principle is: the input and output impedance of the circuit is matched, the power of the amplifier, and the current strength meet the performance requirements of the earphone. The relationship between impedance and sensitivity: the working power of the general earphone is below 250 mW, the best working power is 30 mW, and the output power of the Walkman is about 15 mW, and earplugs are often used. Impedance determines in many cases whether a headset can be pushed, and sensitivity determines whether it can be pushed well.
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