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Author Topic: Impedance Measurement  (Read 9241 times)

Langston Holland

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Impedance Measurement
« on: July 21, 2012, 12:43:20 am »

The following is Tom Young's fault. He started it. :)

This post reads like a review, which is what it originally was, but I've reformatted and updated it since its original posting several years ago. Impedance measurement is so important that I felt that this repost was warranted. Plus a another great guy from the SAC list named Joe Nickell asked about it. :)

Everyone that maintains loudspeakers or does installs would be nuts not to buy the WT3. Once you get used to its two minute warm-up time, you'll find it very consistent and it makes the process so quick and easy you'll find yourself measuring stuff you never would have bothered with before. Woofer tester isn't a good name for it because I've found it accurately tests up to 20kHz and is within 1% of the correct impedance in the audio bandwidth up to about 5kΩ.

Smaart v5.x based impedance measurements whether single-ended (typical) or differential (such as when using Linear X's VI Box) are not good above 300Ω or so. Above this point the measurements are biased downward because of the lowish input impedance of most soundcards. I have found the differential measurement method extremely accurate within Smaart's built-in display limitation (250Ω max) and extremely consistent over time. The VI Box is very cool... More on that later.

WT3 Installation:

I've used WT3 in both XP and Win 7 under Boot Camp on a MacBook Pro. Install the software, then plug in the USB gizmo. I strongly suggest using the same USB port each time you use this thing in Windows - you get a new registry entry every time you change USB ports with the same device and sometimes the operating system gets confused and things stop working.

Use in Smaart v5.4:

This thing is a USB soundcard and single-ended impedance jig in a molded package, thus it will work with Smaart too. Nevertheless, I have found it somewhat inconsistent in Smaart and that may have something to do with the repetitive stimulus generation vs. WT3's single shot sweep. Secondly, the WT3 software corrects for most of the minor errors in the USB gizmo, improving accuracy. Third, you get the very cool autoscaling function in the WT3 software with it's 10kΩ max vs. Smaart's 250Ω max.

Use in ARTA:

The WT3 works better in ARTA's impedance measurement module called LIMP that in any other software, including what comes with the device. I got usable results up to 50kΩ in ARTA with both its quick swept sine stimulus and its heterodyned stepped sine stimulus. The latter is a full, but slow software implementation of Heyser's time delay spectrometry as is the acoustic measurement module called STEPS. TDS noise rejection with impedance measurement - eat your heart out. ARTA is so powerful in so many other ways that I have a hard time believing someone who says they're serious about measurement that doesn't own a copy. It costs $106.50. Put up or shut up. :)

70V Loudspeaker at Various Taps:

I don't have access to such an animal, but I'm convinced it will work nicely for this application based on an observation I make in Charlie Hughes' impedance measurement article. The various taps he measured had a max of 2kΩ, which WT3 is very comfortable with.



Background:

The best articles I know of, the first from Paul Henderson and the second from Charlie Hughes.

http://www.rationalacoustics.com/files/Impedance_Tech-Note.pdf

http://www.excelsior-audio.com/Publications/Impedance_Measurement.pdf

Tools:

A 20 year old and still flawless Fluke 8060A multimeter and one of about 15 flawless GenRad pieces I got from eBay over the years that probably cost 20x what I paid for them. The GenRad 1432N Decade Resistor network can range from 0 - 11,110Ω in .1Ω steps with .01% measured accuracy.



WT3 Measurements:

From dead short to 9kΩ using the decade resistor. Notice the error at high impedances are readings that are higher than actual. A small two way loudspeaker measurement follows as well as a virtually identical differential measurement using the VI Box with Smaart.











WT3 Test Jig:

The alligator clips aren't as useful as the little I/O box I made for it. The ability to measure higher impedances made me want to use WT3 with more than just loudspeakers. The XL3 connector on the WT3 connects the red and black wires to pins 2 and 3 respectively. This will allow testing various balanced I/O impedances with the simple addition of XL3 adapters.

The "ZBox" jig is useful for measuring unbalanced I/O or pin 2 or pin 3 with respect to ground (via a switch). The NL4 jack can switch between 1+/1- or 2+/2-.



VI Box:

I don't pretend to know all the uses of impedance testing with a power amplifier at various volumes, but one thing is for sure - the VI Box tests the loudspeaker as it's actually used. The WT3 alone is fine for maintaining inventory or most install work, while testing at much higher voltages may have its primary use in loudspeaker design. One thing you'll notice from the Smaart measurement above is how the first impedance peak decreases and deforms as the undersized port saturates at higher drive levels. It was basically turning itself into a sealed enclosure due to this design flaw (see picture of loudspeaker below).

Here's the setup. Don't drool when you see the like new condition QSC Series Three dual mono amp that has been in service since about 1986. :)



There is one thing of interest if you're scratching your head like I did trying to figure out how the block diagram of the VI Box correlates with Henderson's and Hughes' differential impedance schematic. The VI Box designer put the guts of the circuit through the negative output of the amp, but that's immaterial. What is material is the diagram included in the literature is wrong as it pertains to the sense output for the current across the shunt.

Original diagram:



Corrected diagram: (by yours truly)



Examples of things I learned:

1. In two hours I used the WT3 to measure (24) KF730's, (8) SB730's and (2) SB150's. It was extremely easy and though I measured each bandpass twice on each cab, the second measurements were effectively identical. I measured through both NL4's on each cab to prove their function as well.

2. One of my employees was standing about a foot from the front of a KF730 when I was measuring the Mid High bandpass and I noticed about a 3Ω drop in the Mid resonance peak. I told him to walk away and the peak resumed its normal level. This incident proves how sensitive these low level single ended impedance measurements are and makes me think that this is the superior way to look for aberrations in my loudspeaker inventory. If I were using higher drive levels with the VI Box, I'm sure this wouldn't have happened.

3. My initial inventory of (16) KF730's measured to factory spec. An additional purchase of (8) that were made in China (EAW has since returned KF mfg to the US) had (2) with issues I wasn't aware of until I made measurements with the WT3.

This one seemed to have a leak in what is supposed to be a sealed cabinet with two 10" woofers. It looked to me like a virtual port has developed with a resonance peak at 15Hz. The effective tuning frequency of the virtual port appears to be around 28Hz. Given the fact that a 4th order LR high pass is applied 81Hz, it's obvious why I wasn't able to hear the problem.



This one seemed to have problems with one or both compression drivers and/or the passive crossover network. Given the location of the error it didn't appear to be crossover related:



EAW's impedance specs:



Well then... The 15Hz resonance bump was in fact due to a cabinet leak. There is an aluminum plate that the two compression drivers bolt to that is in turn bolted on to the cabinet. During mfg, the plate was bent during assembly and this formed a small port to the outside of the cab.

Bent plate:



Replacing the plate corrected the impedance plot (yellow bent, cyan fixed):



On to the compression driver issue. It turned out that one of the compression drivers measured about .5Ω from 1Hz to 5kHz and curved up to about 2Ω at 20kHz. These drivers are connected in series even though four separate wires come from the passive crossover circuit board. Somehow due to the crossover design, the one shorted driver didn't affect the impedance of the good driver as much as I would have expected. Notice how the mid range impedance is affected as well due to the adjacent bandpass interdependence often found in passive crossover designs.

Blue curve with defective driver, corrected green curve after replacement:



Finally, on the amazing sensitivity of the single-ended impedance measurement provided by the WT3, the green curve is with no one near the KF730 and the purple curve is with a road dog standing about 6" away from same:





The great Richard Heyser:

On the subject of testing loudspeaker impedance: http://soundscapesweb.com/files/PSW/Impedance/HeyserZtests.pdf
« Last Edit: July 21, 2012, 03:00:50 pm by Langston Holland »
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Tom Young

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Re: Impedance Measurement
« Reply #1 on: July 21, 2012, 04:48:27 pm »

The following is Tom Young's fault. He started it. :)

.... and proud of it !

This is a great story. Nothing cooler than finding a small but potentially significant flaw, tracking down the cause and then fixing it. And all this done with accurate measurements provided by (very) affordable test systems.

We should write a pilot for a TV "police procedural", as the Brits call them. I'm sure it would be jumped all over ;-)

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Barry Singleton

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Re: Impedance Measurement
« Reply #2 on: July 23, 2012, 01:26:01 am »

Nice "article"!

It brings up a couple of points of interest for me. I have the later model LinearX VI BOX and failed at getting it calibrated to read correctly with TEF SLX, I need to revisit this to see if it is internally wired like yours and try it again. I would love to get this working correctly as its very fast with a bazillion data points unlike LEAP LMS and I can get complex data in one sweep. :)

The little TEF SLX box is similarly sensitive like the WT3 but it's expensive by comparison. Have you by chance horsed around with one?

I'm still in the discovery phase with Smaart (only owned it a month) and hadn't thought to try it there yet.
 
All the best,
Barry.
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Langston Holland

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Re: Impedance Measurement
« Reply #3 on: July 23, 2012, 03:56:36 am »

Hi again Barry. :)

The old and newer versions of the VI Box are wired identically. Chris found a way to make the box less expensively and probably more reliable by replacing the switches with multiple I/O connectors.

I've never even heard of the SLX module for TEF until now, but the great Charlie Hughes has a long history with TEF and has written about the module in the following link. It looks to me like a single ended Z measurement scheme like the WT3 uses as opposed to the balanced two channel scheme that the VI Box employs.

http://www.excelsior-audio.com/Publications/TEF_Impedance_Measurements.pdf

An excellent overview of both methods with applications are covered by Charlie here:

http://www.excelsior-audio.com/Publications/Impedance_Measurement.pdf

Paul Henderson also wrote a great piece on the subject many moons ago:

http://www.rationalacoustics.com/files/Impedance_Tech-Note.pdf

Also read the VI Box manual carefully. Chris is pure scientist and never uses two words when one or less will do, so read it carefully several times.

Smaart v5.x is not required to use the two channel method, it just makes it easy since it has a preset for handling it with a linear display in ohms. This two channel method can be done with any two channel FFT measurement software, though the vertical axis may be goofy with a log response. An old school way of dealing with that was to export the log results in ASCII, open same with Excel, then fix it.

Got enough homework yet? :)
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Barry Singleton

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Re: Impedance Measurement
« Reply #4 on: July 23, 2012, 09:10:44 am »

Thank you Langston.

  I had seen the first paper and years ago did buy the Pearson current monitor that Charlie mentioned. Later when SLX came out I bought that and it became the easiest/fastest Z test method in my arsenal as it will derive TS parameters directly if only at small signal level like you have surmised like the WT3 and does it on the platform I am most familiar with.

   I spoke with Charlie about this exactly but at the time his interest in TEF was waning, he was digging into EASE and he had neither the LinearX VI box nor TEF 25. He's so very helpful and kind but I didn't want to trouble him further so I let it go.

The second and third papers are new to me and I will digest them and see how far I can get. And your absolutely right about Chris, I wish I was smart enoughg to ask him the right questions.

Can one ever have enough home work? :D  This stuff runs in the background in my head no matter what I am doing, non stop. As far as the other homework, I will bounce back to that thread.

Thanks again!!!

 
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Re: Impedance Measurement
« Reply #4 on: July 23, 2012, 09:10:44 am »


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