# What impedance should I get when testing a volume control?

My newly purchased house is wired for audio in 7 rooms. There is a distribution hub in the basement, and an impedance matching volume control in each room. The first time I hooked up my amp to the speaker wires I blew a fuse (in the amp, not a breaker). So now I'm individually testing each room. It works fine with 1 room connected, and I measured the impedance of the speakers in that room independently of the volume control and it was approximately 7.8 Ohms (I assume they are 8 Ohm speakers which is the most common in the US.) Then I measured the impedance from the point of view of the amplifier running through the volume control with different impedance matching settings on the volume control, and this is what I saw:

1X = 3.8 Ohms
2X = 5.1 Ohms
4X = 6.5 Ohms
8X = 7.8 Ohms


I thought that was strange because I was expecting approximately:

1X = 8 Ohms
2X = 16 Ohms
4X = 32 Ohms
8X = 64 Ohms


Is my expectation wrong?

• How are you measuring the impedance? If you're using a DMM, you're measuring DC resistance, not impedance, and the values will be quite low. Jul 16, 2017 at 20:05
• OK. I am using a regular multimeter. Maybe I should swap the word "impedance" for "resistance" in my question?
– TTT
Jul 16, 2017 at 20:19
• No, you're asking about the right thing: the impedance is what matters. The DC resistance of speakers with the same impedance can vary widely, so DC resistance measurements aren't useful for sorting out details of what's going on in a speaker system. But since none of the measurements is 0 and none is infinite, it suggests that the speakers are okay. I know that's vague, but I don't know much about audio systems. Jul 16, 2017 at 20:26
• Are you sure the system isn't a 100 V line system? These expect transformers in the speakers. The local volume controls select taps on the transformers. Jul 16, 2017 at 20:26
• @PeteBecker - When I connect multiple speakers in series, using my DMM I can see the "resistance" change from 8 Ohm to 16 Ohm, and when I put them in parallel I seem them change to 4 Ohm. Which is why I was assuming the multiplier would somehow increase it up to 64 Ohm, so when 8 speakers are in parallel it would go back down to 8 Ohm.
– TTT
Jul 16, 2017 at 20:33

The purpose of the control is to present the correct impedance to your amplifier based on the number of connected speaker pairs while providing a local volume control for each pair of speakers.

Without the controls, paralleling multiple speakers would reduce the impedance by a factor equal to the number of speakers. For instance, four 8 ohm speakers connected in parallel would normally appear as a 2 ohm load to the amplifier. But the controller effectively multiplies the impedance of each speaker so that when they are connected in parallel, they present the correct impedance to the amp. So the 8 ohm speakers in the prior example would have their impedance multiplied times four. Now when four 32 ohm speakers are connected in parallel, their combined impedance is 8 ohms.

The impedance multiplication is simply selecting different taps on a transformer. Measuring resistance of a transformer winding is not a direct indication of impedance.

To troubleshoot this system, here are some steps I would recommend. Don't overlook the possibility there could be multiple problems.

Check that all controllers are configured to multiply their load by eight.

Disconnect the wires that run from each control back to the amplifier. Measure and compare the resistance of each controller where the wires were connected. Any substantial difference in resistance should be investigated further.

Hook a signal source to the still disconnected controller and audibly confirm that the local speaker pairs are functioning and the controller volume control works. Repeat this for all controllers.

Then with the wiring still disconnected from the controllers and from the amp, take resistance measurements between all combinations of the four wires. This should show infinity (an open). Then check each wire for a short to AC ground. This should also show infinity. If the pairs daisy chain from one controller to another, you should repeat these measurements for each segment.

If none of this has turned up any problems, then connect the amp to the house wiring and add and test one controller at a time using a low volume setting on the amp. When the fuse blows, you have a controller and its wiring to further investigate. Remove that controller and continue adding others.

• I'm not sure if I learned anything here. Except maybe your last paragraph is the key. So what values should I see with a multimeter? Are the values I saw OK (or irrelevant)? Or, how do I measure the true impedance? I did find one short, and after fixing it, when I hooked up all 7 rooms the measured resistance was 1.5 Ohms, and my Amp fuse still blew again which didn't surprise me.
– TTT
Jul 17, 2017 at 5:28
• I added some troubleshooting steps to help you. Jul 17, 2017 at 11:16