Transformer Z?

What determines transformer impedence? If a transformer is 1:1, does that mean that the reflected impedence must also be 1:1?? If so, then why are some transformers rated 600:600 or 10k:10k (ohms)? Aren't they both the same 1:1 transformer?

I'm guessing that maybe the way the transformer is wound/constructed/etc sorta makes it better for hi or lo impedence
situations.

Also, is it possible to have a turns ratio of 1:1, but a impedence ratio that is different? My main question would be: Is impedence ratio entirely dependent on turns ratio, or is something else going on??

Thanks,
Ian

> the way the transformer is wound/constructed/etc sorta makes it better for hi or lo impedence

Right. To get up to 10K impedance at the bass frequencies, you need high inductance. But that causes losses and treble problems when you try to use it at 600 ohms.

It isn't an exact value: a "600 ohm" winding will work fine at 500 ohms or 1,000 ohms. But go to far and the loss and frequency response shift too much for complete happiness.

> is it possible to have a turns ratio of 1:1, but a impedence ratio that is different? My main question would be: Is impedence ratio entirely dependent on turns ratio, or is something else going on??

The impedance ratio is never exactly the square of the turns ratio, because of losses. But losses at nominal impedance and frequency range are usually small, so the "error" is negligible.

You can always load the secondary with a different impedance than rated, or than the primary impedance times the turns ratio. In fact we normally use hi-Z loading.

Thanks PRR! Your post answered my questions perfectly! I've got one more tho: Is it possible to have a transformer that has a voltage gain of 1:1, but an impedence ratio of say, 5:1 or 10:1?

Ian

My understanding is that the impedance ration is the square of the turns ratio, i.e. a 2:1 turns ration is a 4:1 impedance ratio... since voltage doubles, current must halve, and for current to halve at twice the voltage, the impedance must be a quarter...

Likewise the square of a 3:1 turns ratio would represent a 9:1 impedance ratio... repeat and apply as required.

Keith

I know that some transformers marked as 600:600 don't have the same DC resistance on both windings...

> Is it possible to have a transformer that has a voltage gain of 1:1, but an impedence ratio of say, 5:1 or 10:1?

Remember how a transformer works. It has zero power gain, and normally insignificant power loss.

So if it is wound 1:2, the output voltage is twice the input voltage, and the output current is twice the input current. So the output impedance is 2V/0.5V= 4 times the input impedance. Any other answer violates the same-power in/out fact. (Since there ARE losses, the impedance isn't exactly the square of ratio. The output impedance is a little high due to copper resistance, a little low due to iron losses. Both losses are small in a well designed transformer, and pull in opposite directions, so the output Z is V^2 for most practical purposes.

What is your real question? Do you have a practical need for different impedances across a 1:1 transformer? A specific example may be clearer than abstractions.

I don't really have a specific application or anything. Just curious! Thanks a lot for all the responses!

Ian

Impedance, in this case inductive reactance, is given by 6.28 f L.
So you can see that impedance will very with frequency.
So you can see that we don't really care about impedance at the high end.
We need a minimum inductance at the low end to meet the 600 ohm (actually a enviromental description) spec with good frequency response.
DC resistance plays a part here too.
You wouldn't want a 10K:10K transformer with a winding resistance of 1 ohm. It would load down the source too much.

So a 10K:10K will have the same turns ratio, but more turns and more dcr to match the source. And more inductance also.

> we don't really care about impedance at the high end.

It matters a lot, but gets complicated. The bass end is complicated enough.

> Impedance, in this case inductive reactance, is given by 6.28 f L

Or a worked example:

Say you have a core and winding that is 1 Henry. At 10Hz: 6.28 * 10 * 1H = 63 ohms; at 50Hz: 6.28 * 50 * 1H = 314 ohms; at 100Hz: 6.28 * 100 * 1H = 628 ohms; at 1KHz: 6.28 * 1K * 1H = 6,280 ohms, and so forth.

That is just the inductance. With real iron and copper you also have pure resistance, which acts in series with the inductance. Without defining "Q", just assume that the Q of an iron/copper inductor at 50Hz is generally no more than about 5 or 10. The inductive reactance of 1H at 50Hz is 314 ohms, so the resistance is probably 63 to 31 ohms. This is often called "DC Resistance", though it actually affects the low-end too.

So with no load on the secondary, the input impedance of a 1H iron-core transformer is 30 to 60 ohms from DC to about 5Hz or 10Hz, then rising to 314 ohms at 50Hz, 6K ohms at 1KHz.

This impedance "shorts" the source, so we have to ask what source impedance we will use. And as far as secondary voltage is concerned, the DC Resistance adds to the source impedance. Assume 30 ohms DCR. If we used a 315 ohm source, 345 ohms total source impedance, the response would be down -3dB at 55Hz and falling below that, but flat above that. Adequate voice quality; for music we might try a 100 ohm source, total source impedance now 130 ohms, -3dB at 22Hz. So we might call this a 100 ohm winding.

> You wouldn't want a 10K:10K transformer with a winding resistance of 1 ohm

You can't MAKE an iron-core inductance that you can call "10K ohms over the audio band" that has a 1 ohm resistance. One ohm of copper wire that fits on any core found in an audio studio won't have 10K worth of impedance until around 50KHz.

And usually the problem, especially in transformers that have to deliver power (output transformers), is that DCR wants to be too high. Like my headphone amp with 10K nominal load impedance and 4K ohms of DC resistance wasting most of the power. It is lucky (?) that you can usually manage to keep DCR down to 1/10th of working impedance far into the music-bass area.

Good info indeed, thanks.

Then this:

How would the Prim vs Sec fit into all this ?

I mean, I've seen several cases of TXs being used backwards
seemingly without problems, but still Edcor has both a
10k:600 AND a 600:10k type.



And then this #2:

How easy/reliable is it to characterize unmarked TXs ?
Turns ratio is easy, but w.r.t their impedance is it OK enough to draw
conclusions from the estimated Q (5 to 10) and the measured DC-resistance ?

I'll be reading (RDH chapter 5, TXs) - and as a last resort there's of
course always the 'just build it & measure'-approach, but any advice welcomed.


Thanks,

Peter

> is it OK enough to draw conclusions from the estimated Q (5 to 10) and the measured DC-resistance?

That's a starting point. 30 ohm DCR is more likely to be about-600 than about-50K.

For a next step, I would pad-out a signal generator with the estimated impedance, and check the frequency response of the secondary. If the source impedance is too high, the bass will not be good. You can try rating it at a lower impedance, but for cheap transformers you may run into the DCR before you get to 20Hz. This also limits any transformer when used with a zero-ohm source such as a feedback amp.

You should also check the high frequency response. But this is very dependent on load impedance. So you need to pick a specific application and try those values. In general you will get "most" high frequency with very low capacitance and very high resistance. But this usually (for higher-Z windings) gives a peak. The peak will be reduced if you decrease load resistance and increase source resistance, but that can reduce midband level and you usually do not want to lose that. The next step is to use an R-C network to load and damp the treble ring without loading the midrange. But the added C always lowers the ring frequency. On very hi-Z or sloppy-design transformers, there may not be a good compromise.

Thanks PRR for responding.

This also limits any transformer when used with a zero-ohm source such as a feedback amp.

Click to expand...

Ehh, such TXs were exactly the most nearby unknown ones :?

I measured these TXs for their DCR. They were the ones that were following the Ampex SE540 circuit that was previously discussed here & you've previously helped me with.

This Ampex-TX is assumed to be 1:2.5 step up (the schematic goes from right to left).






DCR is much lower so they're likely other kind of beasts - maybe like the Lundahl LL5402 as used in the Gyraf G9 & G1176 - but then backwards.

Measured 11 Ohms DCR between the output terminals (3-4) and some 2 Ohms between the input terminals (1-6). The (2-5) winding is 1 Ohm (monitoring/meter drive).

I'll do some AC-measuring as well.

You can try rating it at a lower impedance, but for cheap transformers you may run into the DCR before you get to 20Hz

Click to expand...

If I understand this correctly: is it that the DCR may become a problem for such low source-resistances because it starts to dominate the total effective source impedance ?
And then with the lower corner freq determined by (Rout_amp + DCR)/ 2*pi*f*L (?) there's a limit - without any further possible influence of the lower corner from lowering Rout_amp, correct ?

The next step is to use an R-C network to load and damp the treble ring without loading the midrange.

Click to expand...

I'll check for peaking. The R-C looks easy to add, but you're right, it may affect the band of interest.


Thanks,

Peter

Hi,

This TX-Z has become more clear (if my previous post didn't contain too much errors...)

... but I'm still wondering what the differences could be between say a 10k:600 and a 600:10k transformer.

Can't they just be used backwards ?

Thanks,

Peter

I remember Bill Whitlock of Jensen talking about that.
He mentioned something about an electrostatic shield being required on the in or output, but I can'r remember which!
These UTC inputs I have been taking apart do not have any e-shields.

from CJ:

I remember Bill Whitlock of Jensen talking about that.
He mentioned something about an electrostatic shield being required on the in or output, but I can'r remember which!

Click to expand...

I've been reading a bit on the Lundahl-site, like differences between driving a TX balanced & unbalanced - this consequently requiring adapted winding structures etc. Didn't make much sense when looking at an ideal TX - but the 'why' soon became understandable.

I guess it'll be an alike mechanism w.r.t. TXs backwards or not - since everything seems to matter once you depart from the ideal TX... :roll:

So for now, I still don't know the reason for backward vs non-backward differences, but feel there could at least be one. That's progress...

Peter

> what the differences could be between say a 10k:600 and a 600:10k transformer.

I think you mentioned a specific brand, Edcor? They may be the same transformers with different pin-out or markings, so low-pay workers can't stuff them backward.

And yes, there can be subtle differences "backward". For low-price iron, they may even work better the other way; high-class iron should work best the "right" way but maybe fine the other way.

The differences will be largest for hi-Z (over 10K) and probably small for 600 ohms or less.

from PRR:

> what the differences could be between say a 10k:600 and a 600:10k transformer.

I think you mentioned a specific brand, Edcor? They may be the same transformers with different pin-out or markings, so low-pay workers can't stuff them backward.

Click to expand...

Edcor indeed. Hard to say if there are pinning differences from the info at their site, but defin. appreciating that you never overlook to mention practical reasons for certain things - unlikely or unneeded from the theoretical side, but very understandable because of some simple practical reason.

And yes, there can be subtle differences "backward". For low-price iron, they may even work better the other way; high-class iron should work best the "right" way but maybe fine the other way.

Click to expand...

Sorry, how do you mean 'low-price iron ever better' when actually used in the wrong direction ?


Thanks,

Peter