Is my LT44 transformer suitable for audio (de)coupling?

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

Correct way to do it.

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

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

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.

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

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.

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.

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    Dave Plowman        dave@davenoise.co.uk           London SW
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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.

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ciao Ban
Apricale, Italy

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

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

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.

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

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

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Pearce Consulting
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No - the phantom 'stand off' resistors don't go in series with the signal path. They are from the plus side of the phantom supply to each leg of the balanced input - very simply.

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

That's right - but from the point of view of the balanced signal, they are in series, from one side to the other.

d

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

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.

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

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.

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

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.

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

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

Bob

True - but this would just effectively reduce the impedance of the circuit.

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

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

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.

sound the two resistores in series are parallel with the input to me

Bye. Jasen

The resistors are in series with each other, that combination is in parallel with the input. Because the resistors are 6.2k, the combination applies 12.4k as the maximum possible input impedance of a microphone preamp.

d

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Pearce Consulting
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