Zero autoformers -- how to measure impedance?

Found this video on the Net.

Transformer impedance
.

Ryan, yes, most multimeters can only measure impedance at a frequency of 0 Hz, or in other words, DC resistance. And given that, the numbers you cited seem reasonable, regardless of whether or not a speaker is connected (although the DC resistance of a speaker, if one is conected, will have a slight effect on those readings).

But as you are aware the amplifier should see a considerably higher impedance at audible frequencies, essentially corresponding to the speaker impedance x the impedance transformation ratio the Zero is connected to provide. The impedance of the Zero by itself, at audible frequencies when it is not connected to a speaker and an amplifier, is pretty much meaningless.

It does seem conceivable that if the amplifier is putting out significant amounts of DC, perhaps due to a fault, the amplifier could sense the presence of a near short at 0 Hz, and enter protection mode. You should be able to measure the DC output of the amp with the multimeter. Do that with the outputs of the amp connected to nothing, other than the meter, and with the component driving the amp turned on but not providing a signal. If you measure just say a few millivolts, I wouldn't worry about it.

The other possibility that occurs to me is that if you are running the amp with the Zero connected but no speaker connected to the Zero's output, the amp could shut down, or conceivably even be damaged, as a result of what is called "inductive kickback." Especially if the amp is processing a signal when connected under that condition.

Finally, of course, as we discussed in your other recent thread connecting the Zero backwards, so that it reduces the load impedance seen by the amp rather than increasing it, could obviously cause the amp to enter protection mode. Although I presume based on our previous discussions that you are not doing that.

Not sure what else to suggest at this point. Good luck. Regards,
-- Al

Assume nothing so here goes -
* how have you connected the Zero xformers? I.E. did you connect them up in division mode? Or, did you connect them up in multiplication mode? If you connected them in multiplication mode then the side that has only 2 binding posts should be connected to your speaker. The side that has 4 binding posts in a row should be connected to your amp.
* if you truly have the Zero xformer connected in multiplication mode, then, how is your amp going into protection mode?
Actually, it could. I'm thinking out aloud here so some of this could be gibberish - pardon me in advance.
If the Zero xformer is in multiplication mode - say 2X - and 8 ohms becomes 16 ohms, the voltage developed across the output stage of the amp might begin to clip as it reaches its plus/minus or both power rail supplies. That might inform the amp to go into protection mode due to excessive voltage. The current into 16 Ohms would be well below the max current limit of the amp i.e. the amp is not current limiting.

If you have a SS amp that has DC offset or excess DC offset, you risk frying that side of the Zero that the amp is connected to (because the coils of that side of the xformer are basically an inductor. And, an inductor is a short to DC voltages. So, having a DC offset in the SS amp means having that same DC voltage across the Zero windings. Since the winding resistance is usually very small, the DC current can be (very) high that could fry the windings of the Zero which is connected to the amp. Hence the caution from Paul Spelz.
Still that should not put your amp into protection mode - it'll destroy the Zero tho').

Check your connections.
(actually I'm not sure which mode to use the Zero in - multiplication mode might yield a too high voltage. Division mode might yield a too high current. Both modes could set off the protection mode depending on what the amp is capable of).
See if you can eliminate or nearly eliminate the DC offset.

Why are you using a Zero with a SS amp? Are you driving a 1-Ohm Apogee or something similar that you need the amp to "see" a higher speaker load impedance?

Bombaywalla, based on info the OP has provided in another recent thread he is driving a combination of four drivers in parallel. Three of them are 8 ohms, and the other is 4 ohms placed in series with a 1.5 ohm resistor. The purpose of the resistor is to cause the 4 ohm driver to receive approximately the same amount of power as the other drivers.

The overall impedance of that combination is approximately 1.8 ohms. He is using the Zero to step that up 4x, so that the amplifier will see a load of 7.2 ohms.

Best regards,
-- Al