Re: Telephone amplifier
the gain of the telephone equipment in the exchange can be set to "auto" this then works out the gain balance to use for optimum when the line is looped ( lifting handset).
try adding some resistance to the line, the voltage should increase as the exchange tries to maintain the line current, then s/cct the extra resistance the exchange should then maintain the same voltage thus increasing the line current and using the increased voltage to alter the gain balance setting in the exchange equipment.
this was part of the original design for digital exchanges but caused problems with manual switchboards that answered incoming calls (gain balance set) and then switched the call to an extension further away (more resistance so lower current )
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