In article ,
Andy wrote:
I want to use an audio transformer to see if it helps to reduce the
ffect of an earth loop.

Correct way to do it.

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.

Not that I've heard of, but I'm from the UK.

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.

DC measurements can give a guide to the turns ratio but not the impedance.

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.".

Right. That's an inter stage driver much used on early portable radios to
drive the push pull output stage. There'd likely be a matching output one
for a low impedance speaker - although some used approx 40 ohm ones driven
direct.

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

Have a read of a book on basic AC theory or do a Google on it.

(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.

No - it is an audio transformer, but will drop the level somewhat. This
may not matter in practice. If from a small transistor portable may not
have a very good spec.

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

600- 600 ohm transformers are used on balanced pro equipment. Or more
likely once were in valve days. Most balanced audio these days is low
impedance out high in - same as domestic.

Ground isolating transformers these days will be usually 10k in and out.

An easy solution is to buy a line level transformer from a good car audio
shop used to isolate the ground to an add on power amp. It will be stereo
too. In the UK they cost about 5 gbp so if things are normal $5 or less in
the US. Make sure it is a 1:1 though as some are designed to match off the
speaker outputs in the head unit.

--
*The severity of the itch is proportional to the reach *

Dave Plowman London SW
To e-mail, change noise into sound.