Zero autoformers -- how to measure impedance?

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

Oh, I see! Thanks for this info, Almarg. I didn't know the OP had another paralller thread running.

Thanks Al and Bombaywalla. I figured this is a separate topic so I started a new thread. I hope I didn't fry the zeros. I have an amp and a receiver. The zeros were originally connected to a NAD T757 V2 and this unit kept going into protection. Yesterday, I rewired it to the Parasound new classic 5125 (connected to the front pre-outs of the T757) and no more protection mode and the amps are not hot at all.

While it was connected to the T757, I can feel a relatively high current on the binding posts where the zeros are connected to the receiver. When I put my fingers on these binding posts, the current "zaps" my fingers a bit but not that bad.

I hope I didn't fry the zeros did I? I mean, how can they be fried since they didn't get hot at all and I've only ran them on the NAD unit for no more than 2 hours. I'll get a reading on the DC output on the NAD unit today and hopefully we can come to a conclusion whether they are fried.

Hi Al,

I measured the DC output of the NAD T757 and found it to be -22.6. How can it be negative? Is that normal?

Thanks,
Ryan