Deviation in DC Resistance/ Ohm, tweeter. How much is acceptable?

[DanielT]

What do you think, or do you have to say about this?

I have measured, and measured again with my (cheap) multimeter. One driver shows 5.6 Ohm and the other driver 6 Ohm. Measurements taken directly on the contacts of the compression drivers. Okay, there's some solder left, but here's what it looks like and sorry for the blurry picture, not easy to measure and take a picture at the same time. :

This is what the manufacturer states:

Edit:
Changed title of the thread, see attached picture.

 

[fpitas]

I'd be concerned, unless they were very inexpensive. Can you mount them on a waveguide and measure them?

 

[solderdude]

DC resistance is not the same as impedance.
8ohm rating usually means it is somewhere in that region.
Could also be a short between windings when it has been overloaded that can cause a lower resistance. 5.6 ohm is a bit too far below 8.0

 

[DanielT]

I'd be concerned, unless they were very inexpensive. Can you mount them on a waveguide and measure them?

Shit. Inexpensive or cheap, well I bought them used. Gave $50 for them.

These are:

Eighteen Sound - Professional loudspeakers

www.eighteensound.it

How then do you mean, mount them on a waveguide and measure them?...Should I remove the horn, the waveguide, it is fixed with two screws. But what does it matter? The horns are made of plastic:

 

[fpitas]

Oh ok, thought they were raw compression drivers. In that case measure them and see if you get a reasonable sonic match with the same sine sweep amplitude etc.

 

[DanielT]

Ok, I can try removing all the solder on the contacts and then measuring, but it shouldn't make any difference, or? Otherwise,if that's the case, it would be hopeless to make sense with soldering, I mean.

Thanks for the tips so far! I'll be back a little later!

 

[fpitas]

Ok, I can try removing all the solder on the contacts and then measuring, but it shouldn't make any difference, or? Otherwise,if that's the case, it would be hopeless to make sense with soldering, I mean.

Thanks for the tips so far! I'll be back a little later!

I too doubt it's the solder.

 

[DanielT]

DC resistance is not the same as impedance.
8ohm rating usually means it is somewhere in that region.
Could also be a short between windings when it has been overloaded that can cause a lower resistance. 5.6 ohm is a bit too far below 8.0

Phew..fu**. I figured something out. I probably messed up. Tested them with ONLY one capacitor. I thought it would work considering that my subwoofer has sub out with 12 dB cut HP filter at 80 Hz. That plus a capacitor I thought should be enough, even if I was only playing at low volume. I used an RCA splitter from the sub out and ran a pair of bass/mids with one amp and the XD125 with another amp.

But the manufacturer says:

 

[fpitas]

Even if you blew one coil, replacement diaphragms are usually available.

 

[DonH56]

Phew..fu**. I figured something out. I probably messed up. Tested them with ONLY one capacitor. I thought it would work considering that my subwoofer has sub out with 12 dB cut HP filter at 80 Hz. That plus a capacitor I thought should be enough, even if I was only playing at low volume. I used an RCA splitter from the sub out and ran a pair of bass/mids with one amp and the XD125 with another amp.

But the manufacturer says:
View attachment 243703

The manufacturer says DC resistance is 5.7 ohms, so your measurement of 5.6 ohms is very close.

 

[LTig]

The manufacturer says DC resistance is 5.7 ohms, so your measurement of 5.6 ohms is very close.

Yep, and if the other one measures 6.0 Ohm it's certainly not totally blown. Maybe a bad contact?

 

[DanielT]

The manufacturer says DC resistance is 5.7 ohms, so your measurement of 5.6 ohms is very close.

Yep, and if the other one measures 6.0 Ohm it's certainly not totally blown. Maybe a bad contact?

Aha, missed that. It says in the data sheet, DC 5.7 ohms!

How the hell can one measure 6 Ohms? I will really clean the contacts, after that I will measure.

In any case, thanks for the help.

 

[DonH56]

A standard DMM should read 6 ohms without too much trouble. You'll have to look up the meter's accuracy, but I expect within an ohm or so is "close enough". Usually you just want to see if it is shorted, open, or very far off spec (say factor of two).

Measuring <1 ohm and >100 M ohms gets interesting... For very low resistance, like trying to measure speaker cables, a four-point (Kelvin) connection is commonly used, but an inexpensive DMM likely does not support that. Recently to measure <<1 ohm supply impedance over a fairly wide bandwidth (~0.01 MHz to 1+ GHz) I used a VNA in a "Kelvin" configuration but that is rarely needed for audio.

 

[DVDdoug]

That seem strange bit if they are both working OK and putting-out equal volume I wouldn't worry about it.

Low resistance is difficult to measure accurately (but I do believe there is a difference). It's only about 10% difference. But I assume the windings are counted (automatically) during manufacture so I wouldn't expect a 10% difference.

That plus a capacitor I thought should be enough,

That would depend on the capacitor value. A capacitor has a 6dB per octave slope so you should probably calculate a higher crossover frequency (maybe 5kHz) if you are running near the rated power. (Tweeter power ratings depend on most of the power going to the lower-frequency drivers.)

If you your capacitor cuts-off at 80Hz that's no good. ... And it's not a full-range driver so a sub and this compression/tweeter is not a complete speaker system.

 

[mhardy6647]

I wouldn't worry about it. Their impedance behavior is far more important (if a bit more tedious to measure).

 

[DanielT]

That seem strange bit if they are both working OK and putting-out equal volume I wouldn't worry about it.

Low resistance is difficult to measure accurately (but I do believe there is a difference). It's only about 10% difference. But I assume the windings are counted (automatically) during manufacture so I wouldn't expect a 10% difference.


That would depend on the capacitor value. A capacitor has a 6dB per octave slope so you should probably calculate a higher crossover frequency (maybe 5kHz) if you are running near the rated power. (Tweeter power ratings depend on most of the power going to the lower-frequency drivers.)

If you your capacitor cuts-off at 80Hz that's no good. ... And it's not a full-range driver so a sub and this compression/tweeter is not a complete speaker system.

And it's not a full-range driver so a sub and this compression/tweeter is not a complete speaker system. See #8, I have bass/mid too.

Well, it was an experiment. This is what the FR looks like, then I connected a capacitor I had at home, but it is not quite the right value 3.3 uF.

I think 2uF- 2.2uF would have been better. Why, then the cut off occurs around 10Hz-9 kHz and should flatten the FR. There is apparently an old "trick" that can be done with compression drivers that have that kind of sloping FR, to even out the FR that way, that is.

From sub, HP filter so everything above 80 Hz goes to the speakers:

Outputs
There are two sets of output connectors on the STA-400D. One set (labeled Line Out) is unfiltered and carries
the same signal as found on the input connectors. The set labeled High Pass Out has a high pass filter applied
which has a corner frequency of 80Hz and a slope of -12 dB per octave

Power Led - SV Sound SB12-NSD User Manual [Page 11]

SV Sound SB12-NSD Manual Online: Power Led. The power LED will be green when the subwoofer is on and ready to receive (or is receiving) an audio signal. When the Power Mode switch is On, the LED will constantly be green. When the Power Mode switch is on Auto, the LED will be off when...

www.manualslib.com

So that sub out + cap 3.3 uF on the XD125.

A standard DMM should read 6 ohms without too much trouble. You'll have to look up the meter's accuracy, but I expect within an ohm or so is "close enough". Usually you just want to see if it is shorted, open, or very far off spec (say factor of two).

Measuring <1 ohm and >100 M ohms gets interesting... For very low resistance, like trying to measure speaker cables, a four-point (Kelvin) connection is commonly used, but an inexpensive DMM likely does not support that. Recently to measure <<1 ohm supply impedance over a fairly wide bandwidth (~0.01 MHz to 1+ GHz) I used a VNA in a "Kelvin" configuration but that is rarely needed for audio.

Aha, aha! Very interesting. I learned something new there.

I wouldn't worry about it. Their impedance behavior is far more important (if a bit more tedious to measure).

 

[hex168]

If you are still worried, you could do this and compare impedance curves:

Impedance Measurement

 

[MAB]

Hi DanielT.
I have six T25CF002 Seas tweeters, 6 Ohm. Two voice coils are ~20 years old, have a ton of hours, and were used with a really low crossover point to the point the ferrofluid dried up due to heat. You can see the visible discoloration due to age and heat. The other four tweeters are relatively new.

Here are the DC Resistances:

Tweeter	DCR (Ohms)
Old 1	4.575
Old 2	4.602
New 1	4.792
New 2	4.751
New 3	4.735
New 4	4.766

If I go back and remeasure, I get the same readings to better than 0.01 Ohm. I am guessing your meter is giving you repeatable measurements.
Here is a potentially good way to look at the data:

Old and New do differ: Old is 0.18 Ohms lower that the New group. Each group is tightly distributed, New has a range of ~0.06 Ohms across the four measurements.

New vs. Old:
The Old tweeter I consider worn out due to age and heat. Old are also slightly different design and manufacturing than the New tweeters if you look closely. Not knowing if the delta is due to age, use, or manufacturing differences... The delta is 0.18 Ohms.
Within New:
Voice coils are matched to 0.06 Ohms.

Based on this, your tweeters are quite a bit mismatched. I am more concerned about the matching to each other, not sure what to make of the matching to the spec since I don't know what "8 Ohm" actually means and find that spec inconsistently published (manufacturers seem to round up in 2 Ohm increments...)

Even comparing New to Old I only see 0.18 Ohm difference, some may be slight differences in the pieces, but the Old tweeters were actively crossed over at 1.2kHz crossover point with a 300 Watt tweeter amp. Even then the two voice coils are only 0.027 Ohms different.

Given the amount of difference in your tweeters, I anticipate if you measure with REW you will see significant response differences.

Hope this helps.

 

[KSTR]

One driver shows 5.6 Ohm and the other driver 6 Ohm

Few driver manufacturers do actually spec a tolerance on DCR but one that does, PHL Audio, usually have a +-10% spec, and from my own measurements on batches of drivers I would state you can't expect less than +-5% DCR variation from standard processes used during driver manufacture, so a +-10% spec seems to be a reasonable choice.

In your case, I thus would not loose sleep over it. At any rate, in case of doubt a comparative impedance measurement should be done to check if everything is okay.

 

[DanielT]

If you are still worried, you could do this and compare impedance curves:

Impedance Measurement

Hi DanielT.
I have six T25CF002 Seas tweeters, 6 Ohm. Two voice coils are ~20 years old, have a ton of hours, and were used with a really low crossover point to the point the ferrofluid dried up due to heat. You can see the visible discoloration due to age and heat. The other four tweeters are relatively new.
View attachment 243711

Here are the DC Resistances:

Tweeter	DCR (Ohms)
Old 1	4.575
Old 2	4.602
New 1	4.792
New 2	4.751
New 3	4.735
New 4	4.766

If I go back and remeasure, I get the same readings to better than 0.01 Ohm. I am guessing your meter is giving you repeatable measurements.
Here is a potentially good way to look at the data:
View attachment 243715

Old and New do differ: Old is 0.18 Ohms lower that the New group. Each group is tightly distributed, New has a range of ~0.06 Ohms across the four measurements.

New vs. Old:
The Old tweeter I consider worn out due to age and heat. Old are also slightly different design and manufacturing than the New tweeters if you look closely. Not knowing if the delta is due to age, use, or manufacturing differences... The delta is 0.18 Ohms.
Within New:
Voice coils are matched to 0.06 Ohms.

Based on this, your tweeters are quite a bit mismatched. I am more concerned about the matching to each other, not sure what to make of the matching to the spec since I don't know what "8 Ohm" actually means and find that spec inconsistently published (manufacturers seem to round up in 2 Ohm increments...)

Even comparing New to Old I only see 0.18 Ohm difference, some may be slight differences in the pieces, but the Old tweeters were actively crossed over at 1.2kHz crossover point with a 300 Watt tweeter amp. Even then the two voice coils are only 0.027 Ohms different.

Given the amount of difference in your tweeters, I anticipate if you measure with REW you will see significant response differences.

Hope this helps.

REW you say.. I thought I would look into in the future, hopefully soon as I will connect my UMIK. But I might have to look over my laptop and possibly upgrade some programs. I'm lousy with data so I've been dragging my feet on getting things up and running.

Hope this helps. Yes.

Few driver manufacturers do actually spec a tolerance on DCR but one that does, PHL Audio, usually have a +-10% spec, and from my own measurements on batches of drivers I would state you can't expect less than +-5% DCR variation from standard processes used during driver manufacture, so a +-10% spec seems to be a reasonable choice.

In your case, I thus would not loose sleep over it. At any rate, in case of doubt a comparative impedance measurement should be done to check if everything is okay.

Sounds reassuring. Nop, I won't lose sleep over this, I was mostly curious about how it works, with deviations between different speaker elements. What to expect and accept.

_____

In any case, I thank you all for your answers! This is interesting. Good tips and advice.