"Don Pearce" wrote in message
news:4a58fe67.1496071125@localhost...
On Tue, 30 Jun 2009 12:55:25 +0100, "David Looser"
wrote:

"Don Pearce" wrote in message
news:4a55f0af.1492558562@localhost...
On Tue, 30 Jun 2009 11:52:24 +0100, "Serge Auckland"
wrote:


I wonder if I can explain without pictures - probably not. The
transformer has a single winding for the line (and yes, it really is
balanced both ways).

snip to save bandwidth

Did that make any sense?


That's only one implementation, and not one that's been used on an
standard
UK telephone. Since pre-electronic CB telephones had carbon microphones
powered from the line current it was necessary for the microphone to be
directly in the line as far as DC is concerned. One simple implementation
that was used here in the 200 and 300 series telephones used a 3- winding
transformer. The microphone was wired in series with winding (a) across
the
line. In parallel with the microphone was a series combination of a 2
microfarad capacitor, winding (b), winding (c) and the earpiece. Finally a
30 ohm resistor was connected from the junction of windings (b) and (c) to
the junction of the microphone and earpiece.

When the user spoke current was induced in winding (c) which circulated
through the earpiece and 30 ohm resistor and which cancelled out the
speech
currents flowing in the whole series circuit.

The later 700 series used a slightly different variation on the same
theme.

Higher performance hybrid transformers were used to split the go and
return
paths at each end of a 4-wire amplified circuit. Higher performance was
necessary here to stop the whole 4-wire circuit "singing".

David.


Yup - I was just trying to describe the simplest implementation. The
more complex ones tend to hurt my head a bit - I can't see how they
work just by looking at them, I have to draw little arrows all over
them to see how the phases add and subtract at the various ports.

d

The one David describes is the one that hurt my head when trying to
understand it. Yours was much easier to understand. All a bit academic now
though, but still a hole in my education I'd like to fill..

S.



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