Why won't this work?

scootron · #1 11-09-2011, 06:28 PM

Okay, non-engineer with little aptitude for electronics, but I learned that I could use my multimeter to determine the Ohms of a speaker by hooking it up to the + and - connectors of the speakers.

So, it seemed to me that if I cut off one end of a speaker cable, I could hook my multimeter to the red and black wires of the speaker cable, plug the jack into a cab, and see the Ohm rating for the cabinet. I was confident this would work, so I put such a rig together and...no dice.

Why doesn't this work? What am I missing?

What is the accepted way to determine the Ohm rating of a cabinet if you don't know the ohms of the speakers and don't know how they are wired inside the cabinet? There has to be some easy way to do it, without tearing the cab apart.

Smart guys, a little help, please?

1958Bassman · #2 11-09-2011, 06:45 PM

Quote:

Originally Posted by scootron

Okay, non-engineer with little aptitude for electronics, but I learned that I could use my multimeter to determine the Ohms of a speaker by hooking it up to the + and - connectors of the speakers.

So, it seemed to me that if I cut off one end of a speaker cable, I could hook my multimeter to the red and black wires of the speaker cable, plug the jack into a cab, and see the Ohm rating for the cabinet. I was confident this would work, so I put such a rig together and...no dice.

Why doesn't this work? What am I missing?

What is the accepted way to determine the Ohm rating of a cabinet if you don't know the ohms of the speakers and don't know how they are wired inside the cabinet? There has to be some easy way to do it, without tearing the cab apart.

Smart guys, a little help, please?

You really want to know how they're wired. It matters.

Did you test the leads on the meter before starting? Your battery may be dying if you got an 'open' reading. Check the speaker cable out of circuit, too.

If you have a good cable with a well soldered plug, you should definitely be able to test the speaker's DC resistance. You can't use a multi-meter to test the actual impedance because impedance is resistance to alternating current and a meter uses DC for testing the resistance.

scootron · #3 11-09-2011, 06:56 PM

Quote:

Originally Posted by 1958Bassman

You really want to know how they're wired. It matters.

Did you test the leads on the meter before starting? Your battery may be dying if you got an 'open' reading. Check the speaker cable out of circuit, too.

If you have a good cable with a well soldered plug, you should definitely be able to test the speaker's DC resistance. You can't use a multi-meter to test the actual impedance because impedance is resistance to alternating current and a meter uses DC for testing the resistance.

Okay, I'll check all connections, make sure the battery is fresh, and give it another try. I'm assuming that measurement of DC resistance will yield information from which we can infer or deduce impedance. Yes?

Thanks for the response.

Chainsaw Willie · #4 11-09-2011, 07:04 PM

In what way is it "not working"? Does the DMM read wide open? Dead short? Some crazy value you are not expecting?

As the last post says, the speaker voice coil is not a resistor, it is an inductor. So those ohms are not ohms of resistance, they are ohms of inductance. You need a different tool to measure inductance.

However a regular old DMM should read "close" when measuring a single speaker. If you had a single speaker on the workbench and put an ohm meter on it you would measure 7 ohms or so. Close enough - you would know that is really a 8ohm speaker.

A 4x12 cabinet with 8ohm speakers may often have two pairs of speakers wired in series ( doubling the impedance for 16 ohms) and then the two bundles are wired in parallel ( which halves the impedance back down to 8 ohms). So you may get screwy numbers measuring dc resistance on a quad of inductors wired series/parallel.

scootron · #5 11-09-2011, 07:11 PM

By not working, I meant that I was just getting zeroes, nothing that I could use.

When using the meter on single speakers, I would in fact get something close to what the speaker impedance should be, so I inferred (obviously incorrectly) that I was reading ohms.

I generally understand that wiring speakers in series and speakers in parallel will determine the impedance of the cabinet. Should the readings I get from a cabinet be close to its actual impedance?

will33 · #6 11-09-2011, 07:21 PM

Capacitors block DC. If the cab has any filter components in it like for a tweeter or if it's anything other than speakers wired straight to the jack, this method won't work. You'd have to pull the speakers out, test them all individually and then trace the wiring to find out.

will33 · #7 11-09-2011, 07:22 PM

Quote:

Originally Posted by scootron

By not working, I meant that I was just getting zeroes, nothing that I could use.

When using the meter on single speakers, I would in fact get something close to what the speaker impedance should be, so I inferred (obviously incorrectly) that I was reading ohms.

I generally understand that wiring speakers in series and speakers in parallel will determine the impedance of the cabinet. Should the readings I get from a cabinet be close to its actual impedance?

4ohm speakers will read 3 point something. 8ohms read between 5&6.

DukeLeJeune · #8 11-09-2011, 07:30 PM

Something's not right. You should be getting resistance readings of ballpark 65-90% of the cab's rated impedance. Make sure the meter's set right, that there is good continuity in your speaker cables, and that you're getting good contact between probes and conductors in the speaker cables. You could double-check the speaker cable with a single speaker, might have to "turn the cable around" so you that have the bare wires going to the speakers while your multimeter probes are touching the + and - areas of the plug.

If all connections are good, another (remote) possibility is that your speaker cab has a capacitor in series with the woofers for some reason (in series with the tweeter wouldn't matter, because that would in turn be paralleled with the woofer).

will33 · #9 11-09-2011, 07:39 PM

Sure the meter is set to ohms and not voltage or something?

brendanbassist · #10 11-09-2011, 07:46 PM

Quote:

Originally Posted by will33

Capacitors block DC. If the cab has any filter components in it like for a tweeter or if it's anything other than speakers wired straight to the jack, this method won't work. You'd have to pull the speakers out, test them all individually and then trace the wiring to find out.

Not likely that there is a HPF for the woofers; so he would still be able to measure the DC resistance of the woofers. He wouldn't be able to infer anything about the impedance of the HF driver / tweeter, but that's not really relevant to the overall impedance of the cabinet.

will33 · #11 11-09-2011, 07:50 PM

Quote:

Originally Posted by brendanbassist

Not likely that there is a HPF for the woofers; so he would still be able to measure the DC resistance of the woofers. He wouldn't be able to infer anything about the impedance of the HF driver / tweeter, but that's not really relevant to the overall impedance of the cabinet.

On a regular bass cab probably so. The coils in a low pass would add to the reading in a real crossover.

scootron · #12 11-09-2011, 08:13 PM

Checked everything. The cab has two 10" speakers, I don't know the ohm rating. Don't recall whether they are series or parallel, but I think it is the one that makes the pair have an ohm rating lower than the individual speakers. No tweeter, nothing other than speakers connected to the jack.

Can't get it to work, tried every setting on the meter.

I thought it was a great idea, guess it was a stupid idea.

It still seems that it would be nice to have a simple way to read a cab's impedance. For example, I have a small tube amp with three jacks, 4 ohms, 8 ohms, and 16 ohms. If I were going to use it with a cabinet that wasn't mine, I wouldn't know which jack to use to get the most efficiency out of my tube amp.

Thanks, everyone, for the answers, even the ones I didn't understand.

Chainsaw Willie · #13 11-09-2011, 08:13 PM

Checkout your test adaptor itself. Your problem may not be the speakers or the meter.

Measure tip to sleeve and look for infinite (or out-of-range) ohms.
Measure tip to red wire and look for the same near-zero value you get when you short your test leads together.
Measure the sleeve to the black wire for the same near-zero ohms value.

will33 · #14 11-09-2011, 08:18 PM

Does the cab play? May have a wire off in there. When you touch the meter leads together do you get a small .1 or .2 reading? That would be the resistance in the lead wires.

brendanbassist · #15 11-09-2011, 08:20 PM

Quote:

Originally Posted by will33

...The coils in a low pass would add to the reading in a real crossover.

Yes, but probably not enough to interfere with determining 4/8/16 ohm cabs apart. Air Core Coils | 14 Gauge XQ | ERSE

basscooker · #16 11-09-2011, 08:21 PM

could you have your meter set to "x .01" or something like that? just stabbing in the dark here. short the leads on the meter to make sure it reads 0 and wiggle the wires to make sure its not a test equipment issue.

will33 · #17 11-09-2011, 08:26 PM

Quote:

Originally Posted by brendanbassist

Yes, but probably not enough to interfere with determining 4/8/16 ohm cabs apart. Air Core Coils | 14 Gauge XQ | ERSE

True. Anything that's highpassed won't show. You will get funky readings. I later learned this is your meters battery trying to charge the cap. After it's fully charged, it shouldn't read anything if it's good. It stops the dc and won't let it complete the circuit.

If this guy is having problems and it's not his meter, he should probably take the cab apart anyway just to check everything out. Can test the raw drivers then.

skychief · #18 11-09-2011, 08:35 PM

He either:

A: has a defective meter.

B: is not using it correctly.

Try another meter. If this fails, take the cab to your local electronics supply store -- if they're nice they can throw a meter on your cab and tell you the impedance.

georgestrings · #19 11-09-2011, 08:42 PM

Quote:

Originally Posted by skychief

He either:

A: has a defective meter.

B: is not using it correctly.

Try another meter. If this fails, take the cab to your local electronics supply store -- if they're nice they can throw a meter on your cab and tell you the impedance.

I agree - and you don't have to have a special cable, any speaker cable will do... just plug it in, and test across the tip and sleeve on the other end... An 8 ohm cab will typically read around 6-7 ohms resistance, a 4 ohm cab will read around 3 ohms...

- georgestrings

DukeLeJeune · #20 11-09-2011, 08:45 PM

Quote:

Originally Posted by scootron

I thought it was a great idea, guess it was a stupid idea.

Your approach is basically how I check my cabs before I ship them, so it's not a stupid idea (or else we're both stupid, a possibility I'm not yet ready to accept). I run an impedance curve instead of just measuring the DC resistance, but the basic concept is the same: You can get a good electrical analysis of a cab through a speaker cable.

Something isn't right somewhere, and you'll have to check each link of the chain to find out where the problem is. Make sure the meter is giving you readings that make sense, then make sure the meter + speaker cable are giving you readings that make sense, and so on until it stops making sense. Then investigate the link that introduced the problem.