EI7IS
  Radio Experimenter Theory Notes

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The Magnetic Microphone

Microphone This microphone is very like a loudspeaker. It consists of a very light diaphram which is connected to a coil. This coil is suspended in a magnetic field from a permanent magnet. The sound waves are converted to an electrical signal by the movement of the coil in the magnetic field.

This works in a similar way to a dynamo or generator, but instead of generating a constant stable sine-wave output it generates an output in sympathy with the audio signal vibrating the diaphram.

Microphone Input Circuit Typical Microphone Input Circuit.

Input is typically via a 47uF capacitor - this is the DC blocking capacitor and the value is chosen to allow AC at audio frequencies (AF). The output will also have a DC blocking capacitor. This is effectively a pre-amplifier and the output is further fed to an amplifier.

Loudspeaker

Loudspeaker The loudspeaker is driven by an amplifier. The output from the amplifier is changed to a magnetic signal by the coil. The interaction of this magnetic force with that of the permanent magnet causes the coil to vibrate in and out and this is what drives the cone.

This is the reverse action to the microphone and works in a similar fashion to an electric motor. The signal, which represents the audio alternating in the coil causes movement with the magnet and vibrates the diaphram in sympathy with the input signal.

Class A amplifier The amplifier (left) operating in class A is typical of an amplifier used to drive a loudspeaker in many small applications. A more efficient configuration of audio amplifier is the push-pull circuit. This configuration is used for class B amplification as only half the waveform is amplified in class B, each half of the push-pull circuit amplifies a half cycle of the wave. One transistor deals with the positive half-cycle while the other deals with the negative half-cycle.

The Class B audio output stage

Class B amplifier The audio input enters via T1. Each transistor amplifies one half of the cycle. T2 is used to match the impedance of the output transistors to that of the loudspeaker.