transformer impedance

Just best all around parameters. The only things that seperate a real transformer from an ideal transformer are resistance, inductance and capacitance. As you might've guessed from the pattern of wires crossing, it's very complex but can be approximated with three lumped constants: DC resistance of the windings, inductance of the winding in question and bulk capacitance. From there, you specify parameters to meet specifications, namely loss and frequency range. A transformer with 5% loss from 20Hz to

10kHz at 1kohm is the exact same as 10Hz to 20kHz, at 500 ohms. The loss doubles to 10% because the DCR remains constant but you are using it at half the impedance.

When you add a magnetic core, you add three more parameters: saturation at low frequency and hysteresis and eddy current losses at high frequency. Saturation acts similar to inductance but worse: instead of mere reduced impedance at low frequency, you get a reduced *inductance* at a certain field strength. (Gapping the core reduces the "core-ness" (i.e., reduced mu and nonlinearity) of the core by adding some air core back in...(kinda like using a small amount of negative feedback in an amplifier, where gain and distortion are reduced)). This stabs an absolute power x frequency limit on the transformer.

...So to answer your question, I have no idea! It simply depends too much.

The driving signal can handle a 300 ohm transformer so that would be the best option. It'll also drive the 10k load solidly (good "regulation" you could say). But since the load is "only" 10k, you could use a 10k:10k transformer and accept a little more loss (the 300 ohm source would rather put more current through that primary!) for extended frequency response (although both transformers should be wound so frequency doesn't come into it too much, keep this in mind).

The best of both worlds, particularly if you are transferring power, is to match source to load. But you knew that! ;-)

Tim

-- Deep Fryer: a very philosophical monk. Website:

The specified impedance is not the impedance of the transformer, itself, but the source and load impedance that makes the transformer operate most ideally over the specified frequency range. For example, the 600 ohm transformer approaches an open load inductive impedance of about 600 ohms at the lower end of the specified frequency range, and the leakage inductance approaches 600 ohms at the upper end of that range. Between those extremes, the transformer is essentially invisible (shunt inductance parallels an acceptable impedance much higher than the load, and the leakage inductance in series consumes an acceptable fraction of the total signal voltage). If you use a different frequency range, the optimum source and load impedances would shift.

Nope, now the pro audio is defined in dBu, which means "unspecified" impedance. The output might range up to 500R and the input impedance is usually greater than 10k. Microphone inputs are balanced with more than 4k impedance. Here 600R would result in unspecified behaviour of the mike.

It will tell you how much current the output stage has to provide. If the voltage is +/-12Vp, the current in 600R would be +/-20mApeak. into 10k the current would be +/-1.2mA only.

Transformers are hardly used any more, because of cost, size and weight. You can not easily measure its intended working impedance. What you can measure is its DC-resistance. It has to be *much* smaller, to not cause too much insertion loss.

The source impedance doesn't matter as long as it is small. A 1:1 Transformer has equal impedances on both sides (exept the losses), so if you connect the secondary you will measure 10k on the primary as well. The transformer itself has no impedance, but is intended to work with the specified impedance. If there is a big mismatch, the frequency response might suffer and/or saturating(distortion!) can occur, especially at very low frequencies.

--
ciao Ban
Bordighera, Italy

Nope, this is not recommended. It will reduce the level by 6dB each stage and waste a lot of heat unnecessarily. The same is true for audio amplifiers. Their output impedance should approach 0, otherwise the loudspeakers might develop additional errors in the frequency response.

--
ciao Ban
Bordighera, Italy

I read in sci.electronics.design that Walter Harley wrote (in ) about 'transformer impedance', on Wed, 17 Aug 2005:

It's a (somewhat convenient, but, as you've found, confusing) fiction, like loudspeaker 'power' ratings.

That is correct, if winding resistances are not too high for proper operation. Keep it in mind.

It means it's

***designed for use with a 600 ohm load, in circuits with the voltage levels associated with '600 ohm technology', i.e. from a few hundred millivolts to perhaps 20 V (+22 dBu)***.

The impedance isn't an electrical property of a winding or of the transformer as a whole.

You can't, because it's not a property of the transformer. You can only measure its properties and deduce what sort of circuits it's intended for, For example, if a winding has a resistance of 300 ohms, it's likely to be for '10 kohm' circuits than 600 ohm. If you apply 0 dBu( 0.775 V) to a winding and find that the transformer is saturated (output waveform distorted), it's probably a transformer for a microphone input of a mixer.

That's a bit high these days; 50 to 75 ohms is more usual.

1:1 voltage ratio means 1:1 turns ratio, if losses are negligible. Since you have a low-impedance source and a high impedance load, winding resistance losses are indeed minimal, so a precise 1:1 turns ratio will do.

It's not possible to tell, because what you call the 'nominal impedance' is actually irrelevant. If the winding resistances are low enough, and the transformer will accept the maximum voltage you want to apply without saturating, it will work.

--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
\'What is a Moebius strip?\'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I read in sci.electronics.design that John Popelish wrote (in ) about 'transformer impedance', on Wed, 17 Aug 2005:

All OK so far.

Only for crap transformers from Sum Yung Gai Inc. For decent transformers, the frequency range is specified for VERY SMALL values of input impedance reduction at the low frequency and insertion loss at the high frequency end.

Not really: any reasonable 'different' frequency range would overlap the specified range to a large extent (e.g. spec range 50 Hz to 16 kHz, 'different' range 40 Hz to 10 kHz), and the optimum impedances wouldn't change significantly. The winding resistances are involved, as well as the primary shunt and leakage inductances.

--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
\'What is a Moebius strip?\'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I read in sci.electronics.design that Tim Williams wrote (in ) about 'transformer impedance', on Wed, 17 Aug 2005:

NOT FOR AUDIO! Matching for maximum power transfer is never used in normal audio technology, for good and sufficient reasons. The rule is 'low-impedance source, high impedance load'.

--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
\'What is a Moebius strip?\'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Huh? Oh, you're talking Zo, not "best power output" impedance. I was talking load matching, obviously.

Tim

-- Deep Fryer: a very philosophical monk. Website:

Transformers are still widely used in pro audio - perhaps not in the "pro-sumer" category of gear, but in quite a lot of higher-end gear; just take a quick look through the ads in any pro audio magazine. This is partly because of audio engineers' habit of running millivolt-level signals through hundreds of feet of cable and still expecting S/N ratios of 80dB or better, and partly because of the mystique of successful recordings made with transformer gear in the past.

The problems with transformers, certainly, are that they are expensive and large. But the common-mode rejection, ESD and EMI handling, distortion, and noise specs of a good audio transformer are really quite hard to beat. (Bad ones, on the other hand, are lousy.)

Hello Ban,

That is what someone else in this forum also said (Martin?). But transformers are still used when there is no other viable option. These are mostly hardcore EMI and other interference situations.

Regards, Joerg

I dont thik it was me. But generally I agree. There was a thread many months ago about relays, me Vs Win, more or less. Cant remember who "won" though. Probably Win

For audio a good transformer , say Jensen JE16 or Sowter equivalent, in conjunction with star quad cable is incredible robust in terms of CMRR. generally for mic levels. The bog standard single or triple opamp circuitis usually adequate for line level signals.

One test I have always meant to do is a CM 20KHz to100MHz AM, 50% mod. sweep of opamps micamps (INA103 etc)to see how good they are under difficult conditions, ie 60dB gain. This AFAIK is never published in the spec sheets

martin

I read in sci.electronics.design that Walter Harley wrote (in ) about 'transformer impedance', on Fri, 19 Aug 2005:

Is right, wot 'e say. I very rarely 'me, too', but this is exactly what I would have written if WH hadn't.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Hello Martin,

This one is interesting for high CMRR if audio had to be transferred across very long data lines like CAT-5, but seems not suited for very low frequency:

It got a friend out of a pickle once. They also make XLR pro stuff.

You might be better off using differential video amps that have to perform up to several MHz. My favorite was the uA733.

Regards, Joerg

I read in sci.electronics.design that Joerg wrote (in ) about 'transformer impedance', on Sat, 20 Aug 2005:

This item in the spec worries me:

Maximum Input Level 1.1Vp-p (+20 dBu, unbalanced < 1% THD)

It isn't just a typo. 1.1 Vp-p is -6 dBu.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Here's my understanding:

Suppose you have a source with fixed output impedance Zs, and you want to deliver as much power as possible to a load. What load impedance will work best? Zero resistance won't work, because all the voltage will be dropped across Zs, so delivered power will be P = EI = 0*I = 0. Infinite resistance won't work, because there will be no current, so delivered power will be P = E*0 = 0. If you do the math, you discover that the optimum load is the same as Zs, at which point half the power is being dropped across the load and half across the source.

On the other hand, suppose you have a *load* with fixed impedance Zl, and you want to deliver as much power as possible to it. What source impedance will work best? Zero resistance works great: the full voltage of the source is delivered to the load, so power is P = EI = E^2/Zl. Any higher source resistance just reduces the amount of current you can deliver, as well as the voltage across the load. Note, though, that you want Zl to be non-zero - otherwise it can't consume any power.

But that's all about optimizing power transfer. In audio-land, we could care less about power transfer; what we want is accuracy, despite variations in cable capacitance and load impedance, coupling of noise currents and voltages, and so forth. For that goal too, zero source impedance and moderate load impedance work well.

That's how I understand things - if I'm wrong, I'd be glad to be set right.

-walter

I read in sci.electronics.design that Terry Given wrote (in ) about 'transformer impedance', on Mon, 22 Aug 2005:

... usually achieved with a pickaxe or digger bucket.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I read in sci.electronics.design that Walter Harley wrote (in ) about 'power transfer (was: transformer impedance)', on Sun, 21 Aug 2005:

You are right.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Reductio ad absurdum: maximum power transfer applied to mains distribution :)

Cheers Terry