Impedance Matching
Audio transformers work in the audio frequency range over about 9 octaves (20-20,000Hz)
Useful for impedance matching and converting between balanced and unbalanced lines.
Disadvantages of transformers are weight and cost.
Audio Matching Transformers
Devices that use coils eg: microphones, guitar or piano pickups, have a low impedance 50-600 ohms. The impedance of an amplifier will usually be in the kilo ohms to mega ohms range. The mismatch results in loss of power which has to be compensated for with higher gain in the amplifier which will also amplify the noise.
Still used in professional audio applications where long leads from microphone to mixing board
Valve transformers
Used to match the high impedance (Kilo ohms) of a valve output to a lower impedance speaker (1 to 2000 ohms, usually 4-16)
Hybrid Coil
A 3 coil transformer that seperates the microphone and earphone signals in a telephone. Converts between 2 wire and 4 wire working which has to be done to amplify a telephone circuit.