AMP & LS No. 12A
Issue 1, Mar 1981
The Amplifier and Loudspeaker No. 12A, sales name HARMONY, is a loudspeaking telephone attachment which can be added to existing handset telephone installations, (except Internal Telephone System) to provide full, voice-switched, loudspeaking telephone facilities. Colours: dark brown, light brown and mid grey.
The circuitry of HARMONY is based on that of the DORIC (Amplifier and Loudspeaker unit No. 11A), which it will eventually replace. The case has been styled to be compatible with the Herald call connect system and the Ambassador telephone range.
The HARMONY incorporates a line connect relay which operates when the "on" button is pressed, this connects the unit across the line pair and disconnects the telephone instrument. The relay has an additional break unit which is brought out onto terminals T9 and T10 for use on C wire signalling PMBX extensions.
Controls on the unit consist of:-
1. Drum Wheel Volume Control To adjust the incoming speech level.
2. A red light emitting diode (LED) indicator, which is illuminated when the HARMONY is switched on.
3. Three Press Button Keys Their function from left to right is as follows:-
METHOD 1 METHOD 2 Used when connected to:- Used when connected to:- Plan extensions Nos. 2 Plan extensions Nos. 1, 2A and 105/107 (main) 1A, 4 and 105/107 (extn). Keymasters 3 and 4. Exclusive and shared service exchange lines. Two wire and C wire signalling PBX extensions. MAKING A CALL Pick up the telephone Pick up the telephone handset and call the handset and call the desired numbers desired number. Press the "on" key. The Press the "on" key. The indicator will light and indicator will light and remain illuminated. remain illuminated. The telephone set The telephone handset will be disconnected but will be disconnected and must not be replaced on may, if desired, be its cradle. replaced on its cradle. Adjust the loudspeaker Adjust the loudspeaker volume to the level you volume to the level you prefer. prefer. ANSWERING A CALL The call must be answered Press the "on" key. The normally by handset and indicator will light and then transferred to the remain illuminated unit. Adjust the loudspeaker volume to the level you prefer. the telephone dial or press button unit must be allowed to finish pulsing out to line before the HARMONY "on" button is pressed. METHOD 1 METHOD 2 TRANSFERRING FROM Press the "on" key. The Press the "on" key. The HANDSET TO UNIT indicator will light and indicator will light and remain illuminated. remain illuminated. The telephone handset The telephone handset may, will be disconnected but if desired, be replaced must not be replaced on on its cradle. its cradle. Adjust the loudspeaker Adjust the loudspeaker volume to the level you volume to the level you prefer. prefer. TRANSFERRING FROM Press the "off" key. Lift the telephone handset UNIT TO HANDSET Continue your conversation from its cradle and then using the telephone press the "off" key. handset. Replacing the Continue your conversation handset will terminate using the telephone handset. the call. Replacing the handset will terminate the call.
Harmony loudspeaking unit
The circuitry is such that only two adjustments are needed during production. One ensures that the output voltage of the power supply is set to 1 2V, and the other is to balance the send and receive switching
levels. It is not intended that these adjustments should be carried out in the field.
There are two circuit problems with voice-switching LST's. The first is the time taken to decide the direction of transmission and to switch the unit to that direction. This can result in the first part of a word being lost
- known as clipping.
Harmony attempts to improve these characteristics by using a technique known as ‘soft-switching’. The on-off channel switching is carried out by variable attenuators rather than electronic switches as in the LST4. The unit is designed so that there is always 40 dB total attenuation between the microphone and loudspeaker to prevent acoustic feedback. This attenuation is divided between the send and receive channels as required thus enabling the unit to have at least three states of voice switching:-
When neither party is speaking, the unit will be in the centre state. This helps to speed up the unit switching when one of the parties speaks as it is already halfway between the send and receive states. As neither channel is attenuated completely, the first part of the sentence is audible, although at reduced volume.
The room noise problem is improved by special circuitry which discriminates between speech and noise and performs two functions:-
During the development of Harmony it was felt desirable to avoid the normal BT power units which were large and usually required fixing to a wall. The transformer unit 17A
- a plug-mounted transformer providing 20 volts ac nominal at about 250mA was selected.
Connexion to the power unit is by screw terminals which are shrouded for safety on the underside. Protection is provided by a standard replaceable mains fuse and a
non-replaceable thermal fuse which operates if the internal temperature rises above 125 degrees C.
Last revised: September 27, 2010