Canton Ergo F - what impedance should show on the DMM?

Talon

Member
2014-08-01 11:54 pm
I bought several Canton Ergo F speakers, used but in excellent condition and produced probably within the past 15 years. Prior to putting them into service, a quick test with my meter showed some very odd impedance numbers for them. Whereas my other Canton speakers show 3.6 or 3.8 ohms nominal, the Ergo boxes show .6Mohm, 1.6Mohm, and the other two show -3.6 and -3.something. I've never seen negative resistance on a speaker. Switching terminals on one speaker got me a positive value, but not on the other.

Since the Ergo F series were designed as HT surrounds, might they have some odd crossover network that might affect impedance values? I haven't bought new speakers since the late 90s so I'm not up to date on this as much as I'd like to be.
 

Talon

Member
2014-08-01 11:54 pm
Your meter can measure impedance? Please show it :)

Once you realise it can't you may also figure out cap input filters are a bit tricky to test with an ohmmeter.

Ok, resistance. Tomato tomatoh. Look in the magazine specs, speaker description shows "nominal impedance", which I check by measuring with a VOM set to "ohms". Then I can talk about the "resistance", which I've called "impedance" my entire life including when I'm talking with audio engineers, but apparently I've gotten that wrong and no one thought to correct me until now.

The point is, I set this meter to ohms, I measure across the speaker terminals. Doing this for decades across many different brands, and no matter whether it was raw drivers or complete 2 and 3 way boxes, I would get a value between 4 and 8 ohms. I have other Canton speakers here just under 4 ohms. These are giving back the oddest numbers I've ever seen. Compare it to some others in my stash:

Case#1: B&W speakers, downstairs media room: 4.6ohm
Case#2: B&W speakers, rec room: 4.7 ohm
Case#3: KEF 100, home office: 3.2 ohm
Case#4: Canton LE102, random location: 3.7 ohm

So I know these are nominal "at rest" numbers that don't take into account the inductance changes over frequency, yada yada yada. I've used an ohmmeter to check a speaker out when I buy it to make sure it isn't blown "OL", shorted "0.0" or something weird in between. And this sure looks like "weird in between".
 
The only number that matters is the sound at your chair. Try the crossovers, no way to damage anything if you aren't blasting away or outside the freq range. Passive crossovers are a crap-shoot at the best of times.

If you want to know the impedance, just put a known-value resistor in series with the driver and apply a known AC signal. Let's say the resistor is 10 Ohms, then if the driver has half the voltage across it (using your DMM AC volts) and the driver has half, then the driver impedance (at that frequency) is 10 Ohms. Do your algebra like that.

B.
 
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AllenB

Moderator
Paid Member
2008-10-18 11:31 am
The negative values can be caused by Voltage present during the test. This may be the result of previous testing (your meter applies voltage when measuring resistance), or due to an amp with an offset.

The large values may be due to working through a capacitance, or from you touching the leads during the test.

Either way, it looks as though the woofers are open. To be sure everything is ok I would want to open the speakers.
 

Talon

Member
2014-08-01 11:54 pm
The negative values can be caused by Voltage present during the test. This may be the result of previous testing (your meter applies voltage when measuring resistance), or due to an amp with an offset.

The large values may be due to working through a capacitance, or from you touching the leads during the test.

Either way, it looks as though the woofers are open. To be sure everything is ok I would want to open the speakers.

Yes I think I'll be taking these apart in a few minutes. The .6 and 1.6 values seem familiar, I think I've seen those on other speakers with blown LF drivers. Thanks for your input.
 

Talon

Member
2014-08-01 11:54 pm
The only number that matters is the sound at your chair. Try the crossovers, no way to damage anything if you aren't blasting away or outside the freq range. Passive crossovers are a crap-shoot at the best of times.

If you want to know the impedance, just put a known-value resistor in series with the driver and apply a known AC signal. Let's say the resistor is 10 Ohms, then if the driver has half the voltage across it (using your DMM AC volts) and the driver has half, then the driver impedance (at that frequency) is 10 Ohms. Do your algebra like that.

B.

You know, that last paragraph did not make sense?

"Lets say the resistor is 10 Ohms, then if the driver has half the voltage across it (...) and the driver has half" is the culprit.
 

Talon

Member
2014-08-01 11:54 pm
I pulled one woofer out of the box, measured it, and the ohms came up 3.6 or something like that. I put the speaker back together and measured, and this time the readings continuously changed, so there was definitely some cap charging going on.

Whatever is going on, the speaker works. I put the pair in service and got decent sound. Its slightly flat and missing the bottom end, which is to be expected with satellites. Actually these are basically effects channel speakers. They'll come alive when I put a couple of subs on them, I'm sure.
 
Ok, resistance. Tomato tomatoh. Look in the magazine specs, speaker description shows "nominal impedance", which I check by measuring with a VOM set to "ohms". Then I can talk about the "resistance", which I've called "impedance" my entire life including when I'm talking with audio engineers, but apparently I've gotten that wrong and no one thought to correct me until now.

The point is, I set this meter to ohms, I measure across the speaker terminals. Doing this for decades across many different brands, and no matter whether it was raw drivers or complete 2 and 3 way boxes, I would get a value between 4 and 8 ohms. I have other Canton speakers here just under 4 ohms. These are giving back the oddest numbers I've ever seen. Compare it to some others in my stash:

Case#1: B&W speakers, downstairs media room: 4.6ohm
Case#2: B&W speakers, rec room: 4.7 ohm
Case#3: KEF 100, home office: 3.2 ohm
Case#4: Canton LE102, random location: 3.7 ohm

So I know these are nominal "at rest" numbers that don't take into account the inductance changes over frequency, yada yada yada. I've used an ohmmeter to check a speaker out when I buy it to make sure it isn't blown "OL", shorted "0.0" or something weird in between. And this sure looks like "weird in between".

Resistance and impedance are NOT the same thing even though they are both measured in ohms. They both measure the relationship between the voltage and current through a circuit, but impedance also requires that an AC frequency be specified. Resistance does not.

Resistance can be measure using either DC or AC voltage, since it is independent of frequency. In essence you can measure a resistor either way.

A reactive component, however, has an impedance that is directly related to frequency. The impedance of a capacitor at low frequencies is very high, At DC (0 Hertz) the capacitor impedance is infinity, i.e., a open circuit. As frequency increases the impedance of a capacitor decreases.

The opposite is true for an inductor(coil). At DC it is essentially just a resistor. But as the frequency increases so does its impedance. At high frequencies it will not pass much current at all.

These are the characteristics that allow high pass and low pass filters, or crossovers if you will, to be constructed.

Even though your DMM or VOM gives you the ability to measure resistance it generally will not allow you to measure impedance. That's due to the fact that measurements on those instrument are usually done with DC only. You can't measure impedance with DC. It has no meaning in that case.

The reason you are getting erratic readings when measuring across speaker enclosure terminals is due to the crossover between the terminals and the drivers. There is no meaningful DC measurement that can be made across those terminals.

On the other hand, if you disconnect a driver from its crossover, and measure directly across the driver's terminals with a VOM you will read the resistance of the coil windings. This is typically given as Re in specification sheets and measured in ohms.

If you look at those same sheets you will see an impedance graph for the driver that varies with frequency. Again, measured in ohms.
 
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Talon

Member
2014-08-01 11:54 pm
It appears these are high passed, as satellites indeed may be. You now have some choices ;)

Yes indeed I do. :D

I won't be able to mess with it any further until next week, but I'm going to be trying these out with my active Velodyne, which is a 120w 12 inch, using the Marantz in <allthechannels mode. After that, dust off one of my two channel amps and try them with the passive Canton sub.

Can you post a photograph of the crossover? That would clear things up.

I wish I could. The crossover is somehow secured to the back of the terminal package, and I'm still trying to figure out how to get at everything. Pulling the two screws on the terminal cup gave me less than half an inch of wiggle room, so I put it back together. When I pulled the mid woofer out I was able to see about half of the crossover but not all. Lots of dacron in there too.

If I feel adventurous when I get my schedule back under control I will see about pulling one of these apart for the curious folks.
 

Talon

Member
2014-08-01 11:54 pm
Resistance and impedance are NOT the same thing even though they are both measured in ohms. They both measure the relationship between the voltage and current through a circuit, but impedance also requires that an AC frequency be specified. Resistance does not.

Resistance can be measure using either DC or AC voltage, since it is independent of frequency. In essence you can measure a resistor either way.

A reactive component, however, has an impedance that is directly related to frequency. The impedance of a capacitor at low frequencies is very high, At DC (0 Hertz) the capacitor impedance is infinity, i.e., a open circuit. As frequency increases the impedance of a capacitor decreases.

The opposite is true for an inductor(coil). At DC it is essentially just a resistor. But as the frequency increases so does its impedance. At high frequencies it will not pass much current at all.

These are the characteristics that allow high pass and low pass filters, or crossovers if you will, to be constructed.

Even though your DMM or VOM gives you the ability to measure resistance it generally will not allow you to measure impedance. That's due to the fact that measurements on those instrument are usually done with DC only. You can't measure impedance with DC. It has no meaning in that case.

The reason you are getting erratic readings when measuring across speaker enclosure terminals is due to the crossover between the terminals and the drivers. There is no meaningful DC measurement that can be made across those terminals.

On the other hand, if you disconnect a driver from its crossover, and measure directly across the driver's terminals with a VOM you will read the resistance of the coil windings. This is typically given as Re in specification sheets and measured in ohms.

If you look at those same sheets you will see an impedance graph for the driver that varies with frequency. Again, measured in ohms.

Excellent explanation. Thank you.
 
Excellent explanation. Thank you.

Actually, I need to correct it a little bit. There is a meaningful DC measurement that can be made in some cases.

A low pass filter connected to a woofer will consist of one or more inductors in series with the driver coil and you can measure the total resistance of those components with a VOM.

That is what you have probably been measuring in the past. But recognize that it is simply the resistance of those components and not the impedance of the speaker. Manufacturers usually specify speaker impedance as the lowest point on the curve and it will be somewhat higher than the DC resistance.

The other erratic measurements that you have gotten are difficult to explain without a schematic, but seems like there are capacitors involved somewhere in the circuit.
 
Excellent explanation. Thank you.

Also keep in mind that if you have a speaker with bi-amp capability you can't measure the tweeter section alone. There is almost certain to be a capacitor somewhere in series with the driver and you can't do a DC resistance measurement.

It could even show an open circuit depending on whether other components are present or not, but that does not mean the driver is blown. It might be perfectly fine. You just can't determine the answer with a VOM.
 

Talon

Member
2014-08-01 11:54 pm
I have just had similar problems with a Fluke 85-III! The problem it transpired was that the terminals on the meter had lost their proper ground contact on the board/PCB...a resolder cured the problems.

The point I am making is that we all need to ensure that our measurement tools are working properly...especially leads!

Very true. Thanks for the info, and it looks like I may take this meter back to the store and get something else. I'll take a look at Amprobe as well as Fluke models. Since I'm not doing mil spec work I'm not going to spend more than $300 on a meter, and this one was already $200.

I'm wondering what the least expensive meter would be that has True RMS and auto-ranging, plus HFE. I have the first two on my meter but not the last.
 
....It could even show an open circuit depending on whether other components are present or not, but that does not mean the driver is blown. It might be perfectly fine. You just can't determine the answer with a VOM.

Post #4 explains how to do the measurement with any AC meter.

A $12 meter from Harbor Freight will be all you need ever* and if you download honourware REW and buy a bunch of alligator test leads, you are well prepared for almost any test.

To test voltage you check a battery. To test resistance, you just touch the probes together. RMS error or some simulation of RMS or even working with rectified peak AC voltage is almost always insignificant.

If in doubt, use good methods like before-and-after tests.

B.
* I have a bunch of them... they are often given free as the monthly freebee.
 
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