[EMAIL PROTECTED] wrote:
>Try an old style 2500 style single line phone...I can't tell you why, but I
>bet it will be an improvement over the electronic SL phone.
>
>Kenny
>
>
The 500/2500 is really quite a design. It was the first and leading set
to compensate for a wide range of loop resistance.
There are two thermistors in the transmitter and sidetone network that
are affected by the loop current (which is a good indication of the loop
resistance, or distance from the central office)
For a long loop, which will have low current, and lots of cable loss,
the thermistors cause the sidetone network to pass all of the audio it
gets from the line, to the receiver. They also maximize the transmitter
coupling to the line while also optimizing the network balance. So, all
the audio from the CO goes to the receiver, as much transmitter audio as
possible it sent back to the CO. But balancing the network has a novel
effect, it cancels out a lot of the transmitter audio that is sent to
the line from the receiver. The customer hears very little of his voice
in his receiver, which makes it easier to hear the distant party. Since
the customer "can't hear himself" - he talks LOUDER, just what the CO
needs to make up for the loss of the long cable run!
The other end of the scale for the 500/2500 is a customer next door to
the CO. Here, the short loop, high current, small loss causes the
thermistors to load everything! Reduce the transmitter output to the
line, reduce the incoming audio to the receiver and unbalance the
network. The unbalance sends a lot of the customers voice back to his
receiver, turning him down a little too.
This is tech for the non amplified local office calls, and worked very
well, much better than the earlier 400 series sets. Old COs had racks of
resistors added to local customer lines to make them electrically appear
much farther away from the CO. All this went away with the introduction
of the 500/2500.
So what? If you know how these sets work, you can make them work for
you. You can make a 500/2500 a real "hot" phone by adding
resistors/caps to reduce the line current. Resistors drop current, and
with parallel caps you can limit the loss of your new gain in the set.
Some ksu's use thermistors like the 500/2500 and can be "adjusted" for
poor line conditions just like the 500/2500. NEC, mitel, att... are a
few that had this loop compensation, unfortunately Pano had limited loop
comp. But these tricks can be used on an xdp port with a 500/2500. The
$5 drugstore set will not compensate like a 500/2500, but better
designed sets do.
-larry
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