The DY-1 is a dynamic transmitter allows the telephone to function as a microphone for high intelligibility paging. Replace carbon transmitter with DY-1 during installation. The dynamic transmitter is the low-impedance type, suitable for input to the Lo-Z microphone input of the amplifier. Assembly replaces the carbon transmitter with a moving coil (dynamic, 500 ohm at 1kHz with a response of 250Hz to 5kHz) transmitter that has an impedance of 500 ohm at 1kHz.