A question about my cabs resistance

Shinbone · #1 11-08-2011, 09:52 AM

I have an SWR 1x15 cab. It's supposed to be an 8 ohm speaker (at least that's what the specs and the label on the back says) but when I read the jack on the back of the cab with a DMM I get a reading of a little under 4 ohms.

I'm reading from tip to shaft on the speaker cable plug connected to the jack. Is this correct? Shouldn't I be seeing close to 8 ohms here?

will33 · #2 11-08-2011, 10:10 AM

Does the cab have a tweeter in it? If so, the filter components will upset your meter reading. Check the 15 by itself. A reading in the 3's is a 4ohm speaker, in the 5's is an 8ohm speaker.

If it's just a 15 wired to the jack and nothing else, your reading is good and it was likely replaced with a 4ohm at some point in history.

BassmanPaul · #3 11-08-2011, 10:11 AM

No you shouldn't. You cannot measure impedance with a DMM. What you are reading is the DC resistance of the voice coil which will read in the region of 60% of the impedance.

SurferJoe46 · #4 11-08-2011, 10:28 AM

Yeah - but it'll be somewhat close to the same value to one who knows what they are seeing on a DVM.

There is however, a corollary to the actual value. I'm not sure I'd say it's a 60% differential factor though.

billfitzmaurice · #5 11-08-2011, 11:04 AM

Quote:

Originally Posted by Shinbone

I'm reading from tip to shaft on the speaker cable plug connected to the jack. Is this correct? Shouldn't I be seeing close to 8 ohms here?

You should get a reading between 5 and 6 ohms resistance to correspond to 8 ohms impedance.

Munjibunga · #6 11-08-2011, 11:07 AM

Is your multimeter properly calibrated?

Shinbone · #7 11-08-2011, 12:48 PM

I'll have to check the meter against a known resistance.
The cab does have a tweeter but I have it turned off. Don't know if that matters.

will33 · #8 11-08-2011, 12:55 PM

Quote:

Originally Posted by Shinbone

I'll have to check the meter against a known resistance.
The cab does have a tweeter but I have it turned off. Don't know if that matters.

There still may be a capacitor in the way. Check the raw 15" driver by itself to know for sure. Meaning remove the driver from the cab or could possibly do it by removing the jackplate. Just make sure the voicecoil in the 15 is the only thing in between your leads.

gumtownbassman · #9 11-08-2011, 01:00 PM

Typical DMM's are not very accurate at the lower end of the scale, unless there is a special low ohms scale and/or the meter has a current calibration certificate (my work ones are tested/calibrated each year).
Resistance and Impedance are not the same thing, even though they both use "ohms" as a reading.
But as mentioned above, 'typically' for a speaker the D.C resistance is about 60% less than the rated impedance (only as a rough rule of thumb guide), passive crossovers and tweeters will also complicate the issue too.

will33 · #10 11-08-2011, 01:58 PM

Ya, the figure I see everywhere and I assume is pretty standard is .7, or 70%.

OP, technically, capacitors don't pass DC but you'll still get funky readings. I later found out (again thanks to BFM) this is your meters battery trying to charge the capacitor. You have to have the filter completely out of the works to get an accurate reading.

Also, if it's a cheap little meter like most of us have, you have to hold the leads together and subtract that from your original reading to know the true resistance of what's between the leads. You can do this on a close tolerance resistor to know how close your meter is. Pro grade meters like an electrician or other tech would use can be zeroed and calibrated so you don't have to do that stuff.