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?

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

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.

you probably did not fry the Zero since the DC offset from the NAD must have been small.
But let me tell you - time has nothing to do with electronics breaking down or not. If it has to break, it'll break instantly - pretty much like an incandescent light bulb. Ever seen how those break when it's their time to fail? That's exactly how electronics breaks.
If the DC current was too high for the Zero, 2 hrs is a life-time to do damage to them. They would have been fried.

The fact that you can play music thru them thru the Parasound means they are functional.

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

1stly, did you follow Almarg's instructions to the T? Nothing connected to the amp means nothing connected to the amp.
2ndly, are you missing a "mV" after the -22.6? I.E. did you read the measurement correctly numbers & its units (Volts or milli-volts)?

DC offset can be positive or negative so a minus is definitely possible.

Just FYI: these amps have a plus DC rail (+20V or +30V, etc) & a minus rail (-20V, -30V, etc). So, when there is no music playing the red & black/white binding posts should sit a 0V (ground potential) + any DC offset (plus or minus DC offset).

if it's sitting at -22.6V then you have something driving that amp. Again, did you follow Almarg's instructions to the T?
Thanks.

other than the meter, and with the component driving the amp turned on but not providing a signal.

I re-read Almarg's post & he did say that the above in quotes. From my experience I've found ANYTHING connected to the amp while measuring the DC offset can influence this measurement adversely i.e. you are likely to not read the correct DC offset voltage. So, I like to ensure
(1) NOTHING is connected to the amp except a power cable. All inputs & outputs have no connections
(2) the amp is turned on & allowed to sit there for 25-30 minutes for the bias to stabilize before i measure the DC offset. When the amp 1st turns on the bias is very erratic & changing all the time as the components/devices run current, heat up & come to some sort of thermal equilibrium. No point measuring DC offset while all this happening - your reading will be all over the place & you might even fall off your chair if you actually believe that reading....

6-17-15: Angelgz2
Hi Al and Bombaywalla

Thanks for the inputs. I am glad that the zeros didn't fry. My bad, the -22.6 is in mV as indicated on the multimeter.

Whew!! glad that it was just missing the "mV" units....

6-17-15: Angelgz2
I did another measurement with everything disconnected. The unit has already been on for a while. The measurement read -2.0mV on the front channels and -1.9 on the surround channels. So I take this is normal since the voltage is practically zero?

looks like the NAD by itself is just fine. yes, those low mV numbers should be ignored like Almarg indicated in an earlier post.
But it looks like something upstream is feeding DC into your amp that, when amplified, creates a -22.6mV at the NAD output. What do you think, Almarg??

Ryan, congratulations and thanks for the nice words!!

agree. thanks for nice words.