Zero autoformers -- how to measure impedance?

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

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. I have one pair of speakers connect to it but will measure again taking everything off. Thanks!

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?

I've ran the zeros with the Parasound amp using the NAD as a pre amp for over 5 hours last night with movies and music and neither unit gets hot or enter into protection mode. The speakers sounds much better than before. Unlike using a 3ohm in-line resistor, they no longer sound like someone speaking in a box. The 1.5 ohm resistor definitely helped preventing the 4 ohm woofer from sounding too loud.

I would definitely call this a success. Thanks to all for your help completing this project!! God bless you all

Thanks,
Ryan Huang

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??