I forgot my impedance meter - duh, but I am troubleshooting a system that is down. The back of the amp shows all eight channels set to 70 V operation. Just for grins, checking all the speaker lines with my Fluke ohmmeter, they read about four ohms, every one. Expecting to read transformers under an ohm.

Most of these rooms have four speakers, and I see them likely home run to the amp location and wire nutted in parallel. This reinforces my thought that two 8 ohm speakers are in parallel reading 4 ohms.

What would you expect the input side of a 70v transformer to read?

One amp channel LED shows fault so the amp is down. Want to set the replacement up correctly. Thanks!

Don't forget that you are reading wire resistance too. I usually see about 4.6 Ohms DC resistance for an 8-Ohm speaker. If this is a 70V system, there are likely transformers at each speaker and you will be measuring the transformer primary resistance which will vary by the power tap.

That was my question - the DC resistance of the primary.

Just checked a 70v transformer I had in my garage - on the 10 watt tap, reads about 7 ohms. Jumps up with each successive tap to 65 ohms on the 0.625 watt tap. I guessed wrong in the field.

Up on a ladder, I was able to verify that these JBL's were control 25's, and not 25T. No switch for the various power taps. So they were in fact 8 ohm speakers, the amplifier was set up in the 70 V position. Client complained of poor sound. I wonder why?

Sorry I was not closer to my box of transformers or I could have given more precise resistance estimates. This also explains the amplifier's fault light.

I guess that the original installer was visually oriented. He matched pictures of a commercial installation he saw somewhere, but missed the point about that little "T" on the end.

BTW, the fault light was only on channel 1 out of 8 channels. All others were clear. I put a new Crown amp in and had no sound yet so there is also a preamp failure - aaaahhhh!

Thanks for your input nonetheless!

A fact that will bend a few minds is that an 8 ohm speaker is electrically identical to a 70 volt speaker set to 612.5 watts. There are no such transformers, but still -- the impedance of a transformer primary that would deliver 612.5 watts at 70 volts is eight ohms.

A 70V amplifier output will work just fine directly into 8 ohm speakers as long as the volume is not turned up far enough to damage the amp.

DON'T DO IT.

But if you see it and it hasn't blown up, well, it's not outside the range of possibility.