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?

"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

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.

"Dave Plowman (News)"
Andy

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.


** Maplin sell them ......



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.


** That is quite wrong.

With small to medium audio transformers, the rated impedance of a winding =
10 to 20 times the DC resistance.

This fact derives directly from the *efficiency* of such transformers being
around 80% to 90 %.

The LT44 complies.




......... Phil

"Phil Allison" wrote in message
...

"Dave Plowman (News)"
Andy

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.


** Maplin sell them ......



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.


** That is quite wrong.

With small to medium audio transformers, the rated impedance of a winding
= 10 to 20 times the DC resistance.

This fact derives directly from the *efficiency* of such transformers
being around 80% to 90 %.

The LT44 complies.




........ Phil



Maplin sell them (and the LT700 PP O/P transformer that goes with it), but I
think the actual manufacturer might possibly be Eagle Electronics.

In article ,
Phil Allison wrote:
DC measurements can give a guide to the turns ratio but not the
impedance.


** That is quite wrong.

With small to medium audio transformers, the rated impedance of a
winding = 10 to 20 times the DC resistance.

This fact derives directly from the *efficiency* of such transformers
being around 80% to 90 %.

The LT44 complies.

It might well do. But it's not a rule of thumb I'd rely on for all
transformers.

--
*If a thing is worth doing, wouldn't it have been done already?

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

"Dave Plowman (News)"
Phil Allison
DC measurements can give a guide to the turns ratio but not the
impedance.


** That is quite wrong.

With small to medium audio transformers, the rated impedance of a
winding = 10 to 20 times the DC resistance.

This fact derives directly from the *efficiency* of such transformers
being around 80% to 90 %.

The LT44 complies.

It might well do. But it's not a rule of thumb I'd rely on for all
transformers.


** Silly answer.

Contrary to your false claim above - it is a quite reliable guide.





........ Phil

On 25 Apr 2006, Dave Plowman wrote:

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.


Wow, now that's got me confused!

I am not using old tube equipment and I'm not using pro stuff either.
Instead of 600-600 ohm, you seem to be saying that I need 10K-10K ohm.

Can you or someone else explain this a little please.

10K-10K seems very different from the 600-600 which I was about to get!

"Andy" wrote in message
...
On 25 Apr 2006, Dave Plowman wrote:

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.


Wow, now that's got me confused!

I am not using old tube equipment and I'm not using pro stuff either.
Instead of 600-600 ohm, you seem to be saying that I need 10K-10K ohm.

Can you or someone else explain this a little please.

10K-10K seems very different from the 600-600 which I was about to get!



It's all starting to get a bit confused now, isn't it ? 600 ohms is the '
pro ' standard for audio transmission lines, and is usually, although not
always, a balanced line system. Mics with XLR connection systems are often
600 ohm matched, and balanced, although may be 10k if they don't contain a
600 ohm line matching transformer.

You don't say exactly what pieces of kit you are trying to isolate from one
another, but assuming it's a couple of items operating at ' standard ' line
levels - such as a CD player, or tuner, or cassette deck, feeding into the
corresponding input of an amplifier, or the aux in, then the likely
impedance both ends will be around 47k. Obviously, 600 ohms is not a
terribly good match to 47k, but it's probably about the best that you're
going to do with readily available
1 : 1 line matching transformers. I would suggest that you just give it a
try. There will almost certainly be some effect on the overall frequency
response of the system, but depending on what you listen to, or how critical
your ear is, this may not be an issue, and might possibly be largely
overcome-able by judicious use of the system tone and equalizer controls.

I have used 600 ohm 1 : 1 line matching transformers on several occasions,
to do what you are trying to accomplish, and have never had any real
problems with the finished audio.

Arfa

"Arfa Daily" wrote in message
...

"Andy" wrote in message
...
On 25 Apr 2006, Dave Plowman wrote:

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.


Wow, now that's got me confused!

I am not using old tube equipment and I'm not using pro stuff either.
Instead of 600-600 ohm, you seem to be saying that I need 10K-10K ohm.

Can you or someone else explain this a little please.

10K-10K seems very different from the 600-600 which I was about to get!



It's all starting to get a bit confused now, isn't it ? 600 ohms is the '
pro ' standard for audio transmission lines, and is usually, although not
always, a balanced line system. Mics with XLR connection systems are often
600 ohm matched, and balanced, although may be 10k if they don't contain a
600 ohm line matching transformer.

You don't say exactly what pieces of kit you are trying to isolate from
one another, but assuming it's a couple of items operating at ' standard '
line levels - such as a CD player, or tuner, or cassette deck, feeding
into the corresponding input of an amplifier, or the aux in, then the
likely impedance both ends will be around 47k. Obviously, 600 ohms is not
a terribly good match to 47k, but it's probably about the best that you're
going to do with readily available
1 : 1 line matching transformers. I would suggest that you just give it a
try. There will almost certainly be some effect on the overall frequency
response of the system, but depending on what you listen to, or how
critical your ear is, this may not be an issue, and might possibly be
largely overcome-able by judicious use of the system tone and equalizer
controls.

I have used 600 ohm 1 : 1 line matching transformers on several occasions,
to do what you are trying to accomplish, and have never had any real
problems with the finished audio.

Arfa

Actually, thinking about it, 22k is probably a more realistic figure for
line level inputs, as opposed to 47k for a phono input, but everything else
said, still applies.

Arfa

In article ,
Arfa Daily wrote:
Mics with XLR connection systems are often 600 ohm matched, and
balanced, although may be 10k if they don't contain a 600 ohm line
matching transformer.

In broadcasting the most common combination with balanced mics is an
approx 150-300 ohm output going into an about 1.2k input on the mixer.
Older STC ribbons were sometimes 30 ohms. Accurate 600 ohm matching only
really was used into very long lines. Like telephones. But most long lines
these days will be digital.

--
*Stable Relationships Are For Horses. *

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

On Wed, 26 Apr 2006 10:14:47 +0100, "Dave Plowman (News)"
wrote:

In article ,
Arfa Daily wrote:
Mics with XLR connection systems are often 600 ohm matched, and
balanced, although may be 10k if they don't contain a 600 ohm line
matching transformer.

In broadcasting the most common combination with balanced mics is an
approx 150-300 ohm output going into an about 1.2k input on the mixer.
Older STC ribbons were sometimes 30 ohms. Accurate 600 ohm matching only
really was used into very long lines. Like telephones. But most long lines
these days will be digital.

Shouldn't that be 12k in? A pair of 6.2k ohms in series feeding the
phantom power would give you that. Even with phantom switched off, the
resistors are still there, so the input impedance doesn't get any
higher for dynamics.

d

--
Pearce Consulting
http://www.pearce.uk.com

Arfa Daily wrote:
"Andy" wrote in message
...
On 25 Apr 2006, Dave Plowman wrote:

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.


assuming it's a couple of items operating at ' standard ' line
levels - such as a CD player, or tuner, or cassette deck, feeding into the
corresponding input of an amplifier, or the aux in, then the likely
impedance both ends will be around 47k. Obviously, 600 ohms is not a
terribly good match to 47k,

As others have pointed out audio distribution in modern sound systems
is not matched for maximum power transfer.

I have used 600 ohm 1 : 1 line matching transformers on several occasions,
to do what you are trying to accomplish, and have never had any real
problems with the finished audio.

A 1:1 transformer is not doing matching. They just help with common
mode noise and ground loops.

Bob

On Tue, 25 Apr 2006 22:05:51 +0100, Andy wrote:
On 25 Apr 2006, Dave Plowman wrote:
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.

Wow, now that's got me confused!

I am not using old tube equipment and I'm not using pro stuff either.
Instead of 600-600 ohm, you seem to be saying that I need 10K-10K ohm.

Can you or someone else explain this a little please.

10K-10K seems very different from the 600-600 which I was about to get!

Well, what exactly is it you're trying to accomplish? Your original post
was quite vague, and seems to have been lost in a sea of speculation -

What are you trying to connect to what?

And if you have a ground loop, the solution is to diagnose and repair
the loop itself, rather than apply band-aids.

What do you have now, and what are you trying to do?

Thanks,
Rich

Andy wrote:
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?

It is probably from an old microphone amplifier.
I think it will be quite suitable for the test you want to do. It will also
work the other way round, connect the 20k to your input and the 1k side with
a 3.3k to 10k resistor in series to the output of your setup. The resistor
will attenuate the signal and thus compensate for the gain of the
transformer.
--
ciao Ban
Apricale, Italy

"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-ohms 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.".

One thing is sure - this is a transformer with a turns ratio of something
like 4 to 1 to 8:1.

The secondary has a center tap. It might have been designed to be used as a
driver transformer in a transistor push-pull amplifier. I think these were
used in transistor radios in the 1960s.

The resistances have a ratio of about 8:1, but the specs of 20K to 1K
suggest a ratio of 4.5:1 since impedance ratio relates to the square of the
turns ratio.


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

It looks like you got the specifications wrong. No way is this a 1:1
transformer.

(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.

Whether the transformer is good for audio depends on facts not in evidence,
such as power handling levels and frequency response. However, what you say
does not suggest good audio performance.

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

Yes, this is no way a 1:1 or 600:600 transformer.

"Andy" wrote in message


I want to use an audio transformer to see if it helps to
reduce the ffect of an earth loop.


Go straight to Radio Shack and get their "Ground Isolator".

I've tested them on the bench, and they aren't all that bad.

I just did some bench tests of Radio Shack's "Ground Isolator" 270-054 using
test signals that maxed out around 2.5 v RMS. ZSource = 150 ohms, ZLoad =
5Kohms.

The measured performance was truely amazing for a pair of transformers case
and cables selling for only $16.65.


All IM, THD, and noise artifacts were at least 80 dB down with most in
the -100 dB range or better. Frequency response showed a 2 dB peak at 20 Hz
and then 10 dB down at 10 Hz. There was a 3 dB peak at about 51 KHz falling
to about 10 dB down around 100 KHz. +0.5 dB at 20 KHz.

I repeated the tests with the secondary loaded with 1.5K, and the peak at 51
Khz became well-damped with only about 0.6 dB rise.

In article , "Arny Krueger" wrote:
"Andy" wrote in message


I want to use an audio transformer to see if it helps to
reduce the ffect of an earth loop.


Go straight to Radio Shack and get their "Ground Isolator".

I've tested them on the bench, and they aren't all that bad.


I in the past have been using their 600 ohm 1:1 transformer
to make such a device, with much success. I still want
to measure it. Buying two transformers, a box, and wiring,
is much more complicated than purchashing this premade "Ground Isolator"
I do have reservations of having the unit attenuate the signal
voltage.

greg

"GregS" wrote in message

In article
, "Arny
Krueger" wrote:
"Andy" wrote in message


I want to use an audio transformer to see if it helps to
reduce the ffect of an earth loop.

Go straight to Radio Shack and get their "Ground
Isolator".

I've tested them on the bench, and they aren't all that
bad.

I in the past have been using their 600 ohm 1:1
transformer
to make such a device, with much success. I still want
to measure it. Buying two transformers, a box, and wiring,
is much more complicated than purchashing this premade
"Ground Isolator"
I do have reservations of having the unit attenuate the
signal
voltage.

There are no doubt some losses.

Primary DCR = 136 ohms
Secondary DCR = 175 ohms

Turns ratio probably follows this pattern, IOW a slight step up perhaps to
minimize apparent loss.

I would guess its working impedance at around 3 K ohms. This is the range
where the resonance is nicely damped and the signal handling ability allows
for good dynamic range.