BSTL Speech Amplifier - Mod. 51
 
This BSTL speech amplifier fits the older style BSTL audio and video panels.
It provides full duplex speech between the front door panel and the telephone.
It is an encapsulated unit with a weather-resistant plastic cone loudspeaker for speech-out and a dynamic low frequency microphone for speech- in.
The volume of each amplifier can be individually adjusted with two continuously variable controls.
This  product can be powered from either 12V AC or 12V DC depending on the model of telephone used.

Dimensions: H:138mm W:82mm D:30mm