Zero autoformers -- how to measure impedance?

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.

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....

Hello Ryan & Bombaywalla,

The reason I suggested making the measurement "with the component driving the amp turned on but not providing a signal" is so that in the event that all of the electronics is DC coupled, and a large DC voltage is being generated due to a fault upstream of the amp, the measurement at the amp output would pick it up. (In this case, of course, it turns out that the NAD T757 that is being used is an A/V Receiver, rather than just a power amp, so what I said would apply to whatever source component is connected to it and selected).

Assuming that the -22.6 is volts, not millivolts (and I suspect it is volts given your mention of a mild shock sensation), that is definitely abnormal and indicative of a major fault, that could conceivably burn out any speaker or Zero that is connected. However given what has been said I suspect the Zero is ok.

If you haven't already done so, the next step would be to verify that the -22.6 volts is present at the output of the NAD when nothing is connected to it, except power. If anything close to that much voltage is present under that condition, the unit needs to be professionally repaired.

Good luck. Regards,
-- Al