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8 ohm speaker metering 3.2ohms??? Help please

post #1 of 24
3/8/13 at 3:28pm
Thread Starter 
I recently acquired a pair of Paradigm monitor 7v.3 tower speakers. They sound pretty great, and are definately an upgrade from my old Athenas that I was previously fairly satisfied with.

I noticed my AVR (Denon 790) going into protect mode a couple times at somewhat high listening levels. It has happened 3 times over about 4 weeks, which is since I got the Paradigms. I was able to push my volume up higher when I had my athenas, and I never once had my AVR go into protect mode. Only now, with the addition of the Paradigms. I checked all my wires to make sure a positive and negative weren't touching together somehow, but that was not the cause. From here, I started worry that my fairly recent purchase of the Denon head unit could be showing signs of weakness. :/

I decided to put my multimeter on the speakers to see what they read for resistance, since the issue seemed to start with the speaker swap. They are reading 3.2 ohms! Right by the speaker terminal, the speakers very clearly read 8 ohms! There are two sets of terminals, and I currently have the plates in between them for a single speaker wire connection. I took the plates off, and checked the terminals separately. The top, which is the tweeter, reads 1 ohm. The bottom terminals ready 3.2 ohms. Together at the plates, the 3.2 reading...

What the heck? Am I doing something wrong here? I can't see wiring the two sets of terminals up parallel as being anything the manufacturer intended just to reach the 8 ohm load.. And I don't think that would even work because the tweeter doesn't carry the resistance... (please correct me if I'm wrong here)

Any help or even explanation as to what might be going wrong here is apprecaited!

Thanks

James
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post #2 of 24
3/8/13 at 6:04pm
All you're measuring is the DC resistance of the voice coils and crossover, which is not the same as the impedance of the speaker as a system while playing. As for why your AVR has been shutting down, it is possible that the impedance of these speakers gets too low at certain frequencies, possibly with a challenging phase angle at the same frequencies, causing the amps to overheat. My point is that you can't always tell from measuring the DC resistance.
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post #3 of 24
3/8/13 at 6:09pm
OK, I went ahead and looked up some measurements, and sure enough, this speaker does dip quite low and with somewhat of a phase angle:
http://www.soundandvisionmag.com/article/test-report-paradigm-monitor-series-7-speaker-system?page=0,2

This could definitely shut down a non-4-ohm-capable AVR if you turn the volume high enough, and evidently in your case it does. frown.gif The writer seems to think that this is an easy load, but while 4 ohms at some frequencies and a low phase angle can generally be driven by any AVR, 3.3 ohms can make a big difference. That said, if others can successfully use these speakers with their Denon AVRs at a similar volume, then it is also possible that your AVR or one of your speakers is faulty. The most likely scenario is that you are playing these speakers too loudly for your AVR--or most any low-end or mid-range AVR--to handle for long.
Edited by Robert Cook - 3/8/13 at 8:38pm
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post #4 of 24
3/8/13 at 6:33pm
Thread Starter 
Quote:
Originally Posted by Robert Cook View Post

All you're measuring is the DC resistance of the voice coils and crossover, which is not the same as the impedance of the speaker as a system while playing.

Is there a better way to get the accurate reading with my multimeter?
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post #5 of 24
3/8/13 at 7:36pm
Quote:
Originally Posted by dj_james View Post

Is there a better way to get the accurate reading with my multimeter?
No, it cannot read impedance. But that doesn't matter. The DCR that you can read is also the minimum impedance of the system, the only thing that you can't know is at what frequency it reaches that minimum. In any event the absolute minimum DCR you should read with an 6 ohm speaker is 4 ohms. If you're reading 3.2 ohms that means either it's a 4 ohm speaker or it's a 6 ohm speaker with a design flaw. Not that it matters, either way your alternatives are a receiver that has higher current capacity, a different speaker, or don't play it that loud.
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post #6 of 24
3/8/13 at 7:42pm
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Quote:
Originally Posted by Bill Fitzmaurice View Post

No, it cannot read impedance. But that doesn't matter. The DCR that you can read is also the minimum impedance of the system, the only thing that you can't know is at what frequency it reaches that minimum. In any event the absolute minimum DCR you should read with an 6 ohm speaker is 4 ohms. If you're reading 3.2 ohms that means either it's a 4 ohm speaker or it's a 6 ohm speaker with a design flaw. Not that it matters, either way your alternatives are a receiver that has higher current capacity, a different speaker, or don't play it that loud.

Thank you Bill, while I resent your response because I don't like what it means... I do appreciate the feedback. I went ahead and metered my Athena speakers and they came up 4.2... That's really not that big a difference but hey, it's not the only factor obviously.

Do you think that if I simply changed my AVR/Audyssey settings to "small" speaker config that I would probably have less issues keeping my current setup?
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post #7 of 24
3/8/13 at 8:37pm
Quote:
Originally Posted by Bill Fitzmaurice View Post

The DCR that you can read is also the minimum impedance of the system, the only thing that you can't know is at what frequency it reaches that minimum.

I knew it was a lower bound, but does the impedance of all speakers necessarily get this low at some point(s) while reproducing sound?
Quote:
Originally Posted by dj_james View Post

I went ahead and metered my Athena speakers and they came up 4.2... That's really not that big a difference but hey, it's not the only factor obviously.

It may not look like much, but all else being equal (rarely is, but just to illustrate) it means about 35% higher current and at a phase angle that generates even more heat on top of that. You were already on the edge of what your AVR could handle with the Athenas, and the Paradigms pushed you over it.
Quote:
Originally Posted by dj_james View Post

Do you think that if I simply changed my AVR/Audyssey settings to "small" speaker config that I would probably have less issues keeping my current setup?

The article that I linked to above says that the low point on the impedance curve was at 10 kHz, so crossing the speakers over with the sub is not going to address the known issue, and without a full impedance measurement, we don't know how many other issues there may be. That said, reducing the overall burden on your AVR's amps certainly can't hurt and may well help (stands to reason). Why don't you just try it?! wink.gif
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post #8 of 24
3/9/13 at 5:13am
With the demand for more current comes more heat. Do you have adequate ventilation and cooling for your AVR?
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post #9 of 24
3/9/13 at 5:22am
Thread Starter 
Quote:
Originally Posted by flyng_fool View Post

With the demand for more current comes more heat. Do you have adequate ventilation and cooling for your AVR?

I'm forced to stack my equipment, and so I'm using large TV mount spacers to seperate the Direct HD box that sits on top of my AVR. When it trips to protect, it doesn't really feel that hot. I will try giving it more room to vent, I'm sure it can't hurt. I'm also going to take some of the bass out of the fronts and see if that helps.

This thread has shined some light on a few areas that I needed lit up, thanks everyone. Feel free to keep it coming...
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post #10 of 24
3/9/13 at 5:34am
Quote:
Originally Posted by Bill Fitzmaurice View Post

The DCR that you can read is also the minimum impedance of the system, the only thing that you can't know is at what frequency it reaches that minimum. In any event the absolute minimum DCR you should read with an 6 ohm speaker is 4 ohms. If you're reading 3.2 ohms that means either it's a 4 ohm speaker or it's a 6 ohm speaker with a design flaw. Not that it matters, either way your alternatives are a receiver that has higher current capacity, a different speaker, or don't play it that loud.

Exactly. Always look at the swale in the impedance magnitude just above the one or two impedance peaks in the bass. If it's under four ohms, it's a nominal four ohm speaker and shouldn't be rated at eight ohms. If it's over about five and a half ohms, the device is probably rated at eight ohms.

What the OP is experiencing is one of two issues misleading impedance ratings incur. The other is that sensitivity - which is not efficiency - in a true four ohm speaker rated at eight and driven at 2.83v reflects two watts input and not one. If you're pulling double the current because the load is miss-rated, then in addition to stressing some amplifiers, it stands to reason that the free lunch you're getting at two watts - that higher initial loudness hit, should you want to value that artificially high sensitivity specification - expends at the loud end of their range. There you find that assuming the amplifier can even drive four ohms at full power, it naturally cannot continue to deliver a full 3dB more loudness.
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post #11 of 24
3/9/13 at 7:12am
Quote:
Originally Posted by dj_james View Post

I'm forced to stack my equipment, and so I'm using large TV mount spacers to seperate the Direct HD box that sits on top of my AVR. When it trips to protect, it doesn't really feel that hot. I will try giving it more room to vent, I'm sure it can't hurt. I'm also going to take some of the bass out of the fronts and see if that helps.

This thread has shined some light on a few areas that I needed lit up, thanks everyone. Feel free to keep it coming...
Lots of guys have fans actively cooling their gear as well.

http://www.coolerguys.com/
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post #12 of 24
3/9/13 at 7:36am
Quote:
Originally Posted by Robert Cook View Post

I knew it was a lower bound, but does the impedance of all speakers necessarily get this low at some point(s) while reproducing sound?
On occasion minimum impedance will be even lower than DCR. That can happen when the crossover capacitive/inductive reactance is even less than the DCR. This would occur in the vicinity of the crossover frequency, where current demand is far less than where DCR tends to be at a minimum ( the middle of the woofer passband), so it's usually not a problem. With a typical 8 ohm woofer DCR will be about 5.6 ohms, and the minimum impedance might dip as low as 4 ohms in the crossover region with no ill effect. But if the crossover components aren't chosen correctly you could get a dip down to even 1 ohm, and that would be a major problem. Knowledgeable crossover designers know enough to avoid that trap, but not all crossover designers are as knowledgeable as they should be.
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post #13 of 24
3/9/13 at 8:16am
Quote:
Originally Posted by dj_james View Post

Quote:
Originally Posted by Robert Cook View Post

All you're measuring is the DC resistance of the voice coils and crossover, which is not the same as the impedance of the speaker as a system while playing.

Is there a better way to get the accurate reading with my multimeter?

If you want to make reasonble measurements of speaker impedance, you want something like this:

http://www.parts-express.com/pe/showdetl.cfm?partnumber=390-804



I found the impedance curve of a similar speaker from Paradigm:



I would not be surprised if its DC resistance is somewhere between 3 and 4 ohms, too.
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post #14 of 24
3/9/13 at 8:19am
Quote:
Originally Posted by Bill Fitzmaurice View Post

On occasion minimum impedance will be even lower than DCR. That can happen when the crossover capacitive/inductive reactance is even less than the DCR. This would occur in the vicinity of the crossover frequency, where current demand is far less than where DCR tends to be at a minimum ( the middle of the woofer passband), so it's usually not a problem. With a typical 8 ohm woofer DCR will be about 5.6 ohms, and the minimum impedance might dip as low as 4 ohms in the crossover region with no ill effect. But if the crossover components aren't chosen correctly you could get a dip down to even 1 ohm, and that would be a major problem. Knowledgeable crossover designers know enough to avoid that trap, but not all crossover designers are as knowledgeable as they should be.

That's interesting--thanks! But is the opposite ever true, that some speakers' system impedances would never get as low as the DCR? I'm just trying to determine what and how much a speaker's DCR really tells us. I had thought that it sometimes told us something useful, but we couldn't be sure (which implies that it doesn't tell us anything much regarding amplifier compatibility).
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post #15 of 24
3/9/13 at 8:29am
Quote:
Originally Posted by Robert Cook View Post


That's interesting--thanks! But is the opposite ever true, that some speakers' system impedances would never get as low as the DCR? I'm just trying to determine what and how much a speaker's DCR really tells us. I had thought that it sometimes told us something useful, but we couldn't be sure (which implies that it doesn't tell us anything much regarding amplifier compatibility).

Unless you can come up with negative resistance, the impedance of any real world speaker is always greater or the same as its DC resistance. Negative resistance can occur in the real world but there are rules about how it manifests itself and they go strongly against it ever showing up in a loudspeaker.

I'll turn the amplifier compatibly problem around by pointing out that due to the nature of music, it is reasonable to consider the impedance of a speaker playing music to be several times what test bench measurements say.

I read about problems with an AVR tripping out while driving these speakers, whch I find to be unexpected.

I question the thermal situation. I recommend measuring the temperature of some metal part of the chassis of teh AVR with a IR thermometer.

http://www.ebay.com/itm/Mini-Pen-Type-LCD-Non-Contact-IR-Infrared-Pen-Type-Thermometer-Meter-Red-/380595555147



I would measure the impedance of other identical speakers by whatever means for the purpose of making a comparison. A rubbing voice coil can short out a power amp, but the short can disappear when the speaker is not being driven hard.
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post #16 of 24
3/9/13 at 8:36am
Just to clarify, my read is that Bill was speaking of the crossover region where multiple drivers are seen in parallel, reducing the net impedance, not an actual negative resistance.

I bet a speaker with a tunnel diode someplace in it could be used as a marketing ploy... smile.gif
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post #17 of 24
3/9/13 at 9:45am
Quote:
Originally Posted by Robert Cook View Post

That's interesting--thanks! But is the opposite ever true, that some speakers' system impedances would never get as low as the DCR?
Yes, that's possible, but rare. The usual instance would be with a horn loaded driver, where the horn adds acoustic impedance to the electrical impedance.
Quote:
Just to clarify, my read is that Bill was speaking of the crossover region where multiple drivers are seen in parallel, reducing the net impedance, not an actual negative resistance.
It's not just the multiple drivers that's at issue, it's also the crossover filters. Keep in mind that part and parcel of a 2nd order or higher filter is a shunt to ground, capacitive in the case of a low pass, inductive in the case of a high pass. If the component values are incorrect the capacitive or inductive shunts can present a very low impedance load. This explains:
http://www.bcae1.com/xoorder.htm
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post #18 of 24
3/9/13 at 11:03am
dj-james'

I have witnessed 2 occasions in which the bare wires from the back of the speaker cone to to it's terminal board were intermittently shorting.( Both were Paradigms) Might want to take a peek. Just my 2 cents.
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post #19 of 24
3/9/13 at 11:33am
Quote:
Originally Posted by Bill Fitzmaurice View Post

Quote:
Just to clarify, my read is that Bill was speaking of the crossover region where multiple drivers are seen in parallel, reducing the net impedance, not an actual negative resistance.
It's not just the multiple drivers that's at issue, it's also the crossover filters. Keep in mind that part and parcel of a 2nd order or higher filter is a shunt to ground, capacitive in the case of a low pass, inductive in the case of a high pass. If the component values are incorrect the capacitive or inductive shunts can present a very low impedance load. This explains:
http://www.bcae1.com/xoorder.htm

Agreed. And of course DCR may not be terribly relevant for some designs, like ESLs...
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post #20 of 24
3/10/13 at 11:05am
Thread Starter 
Quote:
Originally Posted by Skytrooper View Post

dj-james'

I have witnessed 2 occasions in which the bare wires from the back of the speaker cone to to it's terminal board were intermittently shorting.( Both were Paradigms) Might want to take a peek. Just my 2 cents.

Wow, once is one thing, but twice from the same manufacturer? That's unlikely odds!

Since I have two of the towers, metered the other and got the same reading. The speakers also both sound as expected, good of course. I guess I thought that I could get an accurate measurement with my meter, but it sounds to be MUCH more complicated than that, lol.

Am I correct to think that I can get the ohm reading from each diver individually, directly from it's terminals? (before the crossover, box terminal, ect)
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post #21 of 24
3/10/13 at 12:08pm
Quote:
Originally Posted by dj_james View Post

Am I correct to think that I can get the ohm reading from each diver individually, directly from it's terminals? (before the crossover, box terminal, ect)
That doesn't tell you what effect the crossover has on the final result. If you want to get an accurate impedance sweep you need testing gear.
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post #22 of 24
3/10/13 at 12:34pm
It really sounds like your receiver is under-powered from the volumes you are trying to achieve in your room size.
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post #23 of 24
3/10/13 at 2:42pm
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Quote:
Originally Posted by Audiojan View Post

It really sounds like your receiver is under-powered from the volumes you are trying to achieve in your room size.

Bummer, I guess I have another upgrade to save up some money for.. In the meantime, I crossing the fronts and center over at 80hz, they are full right now. I also will try to get more than what I currently have for air movement..

Thanks for the education everyone
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post #24 of 24
3/10/13 at 6:42pm
Still can't see your 8 ohm speakers shutting down your receiver without something intermittently wrong.
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