Zero autoformers -- how to measure impedance?

Ryan, yes, most multimeters can only measure impedance at a frequency of 0 Hz, or in other words, DC resistance. And given that, the numbers you cited seem reasonable, regardless of whether or not a speaker is connected (although the DC resistance of a speaker, if one is conected, will have a slight effect on those readings).

But as you are aware the amplifier should see a considerably higher impedance at audible frequencies, essentially corresponding to the speaker impedance x the impedance transformation ratio the Zero is connected to provide. The impedance of the Zero by itself, at audible frequencies when it is not connected to a speaker and an amplifier, is pretty much meaningless.

It does seem conceivable that if the amplifier is putting out significant amounts of DC, perhaps due to a fault, the amplifier could sense the presence of a near short at 0 Hz, and enter protection mode. You should be able to measure the DC output of the amp with the multimeter. Do that with the outputs of the amp connected to nothing, other than the meter, and with the component driving the amp turned on but not providing a signal. If you measure just say a few millivolts, I wouldn't worry about it.

The other possibility that occurs to me is that if you are running the amp with the Zero connected but no speaker connected to the Zero's output, the amp could shut down, or conceivably even be damaged, as a result of what is called "inductive kickback." Especially if the amp is processing a signal when connected under that condition.

Finally, of course, as we discussed in your other recent thread connecting the Zero backwards, so that it reduces the load impedance seen by the amp rather than increasing it, could obviously cause the amp to enter protection mode. Although I presume based on our previous discussions that you are not doing that.

Not sure what else to suggest at this point. Good luck. Regards,
-- Al

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

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

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

Good point, Roger!  Thanks.

Best regards,
-- Al