"Andy" wrote in message
...
I want to use an audio transformer to see if it helps to reduce the
ffect of an earth loop.

An old audio transformer I have come across is marked as "LT44" and I
that that LT44 is sometimes used to indicate a 1:1 (or 600-ohm:600-ohm)
transformer.

However when I measure the resistence of the tarnsformer I get 650-ohms
on the primary and 80-ohmns on the secondary which has a tap at 40-ohms.

That doesn't feel right.

A slip of paper with the transformer says LT.44 transistor driver
transformer and says "impediance ratio is 20K ohm to 1K ohm C.T.".

(1) Why is my measurement of DC resistemce so very different from the
impendence on the specifications?

(2) Is this LT44 useless as an audio transformer? I seem to remember
something about number of turns in the windings being important for an
audio coupling transformer as well as the impedences.

(3) Is it wrong to use "LT44" as an accepted shorthand for a 600-
ohm:600-ohm audio coupling transformer?

DC resistance is a very different animal from the AC resistance measurement
called impedance, which is a complex interaction of many factors, including
DC resistance, capacitive and inductive reactance, and frequency. Its value
varies considerably with frequency, and may change suddenly up or down, as
resonance is approached. Just as an example, a TV antenna might have a
folded dipole with a centre impedance of 300 ohms at the frequency of
interest. However, if you put your DC ohm-meter across there, that
folded-round piece of 1/4 inch metal rod, is going to read just about short
circuit, not 300 ohms. Either side of the design frequency, its impedance
will not be 300 ohms.

The transformer that you have is not "useless as an audio transformer", it's
just not the type of audio transformer that you were hoping for. What you
have there is as it says on your piece of paper - a "driver" transformer. It
is an inter-stage coupling transformer to go between an audio driver stage,
and a pair of transistors in a push-pull output stage. It serves the dual
purpose of transforming the high output impedance of the driver stage to a
lower one more suitable for driving the bases of the output pair, and also
phase splitting due to the centre tap on the secondary. The 1 : 1 600-600
transformer that you are looking for is more usually known as an isolating
transformer. The effect of placing it in an audio line should be virtually
zero, except to isolate the signal grounds from one another, which I guess
is what you are wanting to do.

According to my catalogues " LT44 " is exactly what you have got - a 20k to
1k CT driver transformer, so the term LT44 cannot be taken as shorthand for
a 600-600 isolation transformer.

Arfa