Zero autoformers -- how to measure impedance?

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??

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??

Yes it does, but the number is so small I wouldn't worry about it. 22.6 mv into 8 ohms is 0.000064 watts!

Also, perhaps it is actually some small amount of low frequency noise, such as a ground loop might cause, which the meter is somehow partially rectifying and indicating as DC, even though it is AC at some frequency or combination of frequencies. Just speculating.

So I don't know why the NAD would have gone into protection when its speaker-level outputs were connected to the Zero, and through it to the speaker. I listed three possibilities in my first post in this thread, but none seem applicable at this point.

Ryan, congratulations and thanks for the nice words!!

Best regards,
-- Al

Ryan, congratulations and thanks for the nice words!!

agree. thanks for nice words.

Yes it does, but the number is so small I wouldn't worry about it. 22.6 mv into 8 ohms is 0.000064 watts!

This is an old thread but needs some correcting. The user who has -22 mV of offset on his amp needs a little clarification.

First, offset can be negative or positive. We would like it to be zero but 22 mv is quite acceptable. However when we connect a Zero to this,whose DC resistance is almost ZERO we can have significant current flowing which can saturate the core and make for some very bad bass and IM distortion. The impedance selected matters not.

The Zero is wound with very heavy wire and I do not know its DCR nor is that easy to measure with a DMM. Actually you measure low resistances by putting a known current throught the wire and seeing what the resultant voltage is. This is absoultely accurate and is not affected by probe resistances. Say the wire resistance was 0.2 ohms, which is quite likely then the current is 100 mA and I would expect quite a bit of core offset. 

Good point, Roger!  Thanks.

Best regards,
-- Al