Zero autoformers -- how to measure 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

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.