# Impedance measurement not working



## satx (Nov 29, 2012)

Hi, I've been using Rew for a couple of years, so I do know the basics. I have an old laptop running XP that I use to take room and frequency measurements. I've never been able to make it take an impedance measurement though, so I've always used my desktop for this. I use a homemade jig and it's always worked fine. The thing is is that I've just upgraded to a new desktop computer, so instead of doing all the calibrations again, I would like to get the laptop doing everything because it's already setup for REW and it's just plain easier to take it to a large, heavy speaker than the other way around. 

When I try to run an impedance sweep on the laptop I can see something in the small tab graph on the left, but it's obviously wrong and I see nothing on the main graph. I can see a constantly rotating phase response though. And it isn't the graph scaling or the check tab for the measurement. Even if I just try to measure a resistor or inductor, nothing shows up. Does anyone have any ideas to get me moving again? It'd be much appreciated!

Evan


----------



## JohnM (Apr 11, 2006)

Difficult to say much without anything to look at, can you attach an example mdat file from a measurement? One of the most common issues is to have the L and R input connections the wrong way around, can check that by just changing the input channel selection in REW's soundcard preferences. What sense resistor value are you using?


----------



## satx (Nov 29, 2012)

Hi John, thanks for getting back to me. 

I'm using a 100 ohm resistor. 

It could be that the inputs are switched. It always worked normally on my old desktop, so I never thought to consider that they could be switched. But, since I hadn't heard anything and wanted to measure a new driver last night, I set up REW on my new computer and after doing the impedance cal. My 10 ohm resistor was measuring 110 ohm, After switching the input settings everything is fine. So.... I probably had switched it on the old machine too, because I'm using the same jig. 

I'll try changing the settings on the laptop and if it still doesn't work then I attach a sample .mdat file

Evan


----------



## satx (Nov 29, 2012)

Hi, I hate to bump an old thread, but once again I am having problems with the impedance measurements. Frequency response with a mic works fine. I basically get nothing when I make impedance measurements, regardless of settings, L/R input, etc. This is on two different computers. I've checked the jig and everything looks fine. This is the same jig that I've always used without issues. When I run the calibration (with shorted resistor) I do get the straight line at about 100 ohms. The input seems to clip really easily and I only avoid clipping with a level of -20db or so. Here are a couple of screen shots if they are of any help. Thanks!

Edit: I had to attach them. They are screenshots that I emailed myself from an old laptop I use for measurements.


----------



## JohnM (Apr 11, 2006)

Since it was working and now isn't then a problem with the jig or wiring is the most likely cause. 

On the REW setup front it is better to select the soundcard device, input and output directly in the soundcard preferences than to leave things set as default, since Windows may change the defaults depending on what you plug into the PC.


----------



## satx (Nov 29, 2012)

I'll set up my old computer and try it on that. If it doesn't work then we'll know it must be the jig or wiring. 

I tried every combination of devices including selecting the specific devices the computer says it's using. But, I'll set it to those and leave it from now on. 

Thanks,
Evan


----------



## satx (Nov 29, 2012)

Hi, I'm sorry to keep resurrecting this thread, but I still cannot get this to work on either on my current windows computers. 

I have checked the jig and everything is good. This worked on my old desktop, but will not work on my old measurement laptop or new desktop. Frequency response measurements work fine on the laptop. One question about the jig- should all grounds be connected together, speaker, input and output? I know that I only use the Right output, but the grounds are common, is that an issue. 

I am using the computer's soundcard for impedance measurements and an external amp for frequency response. I could try an amp for impedance, but I'm afraid of damaging the input. 

Please see the graphs posted previously as this is still all that I get. A blank graph with a downward sloping graph in the thumbnail. All settings in preferences, ie. input device, output device and left/right input give the same results. 

One oddity is that the output needs to be very low to not clip the input. This wasn't the case on my old computer, but maybe the laptop just has a more sensitive input or stronger output. I tried to go back to my old computer to test the jig, but the power supply appears to be toast and it would not power up, hence, why I replaced it when I did. 

Any help would be greatly appreciated. I am getting pretty frustrated with this as it worked fine before. If all else fails, I'll just bite the bullet and buy a tester. 

Thanks,
Evan


----------



## jtalden (Mar 12, 2009)

I just noticed that the version of REW is 'V5' per above screen shot. I believe that version may have had some issue with impedance measurements? That may not be correct. One of the earlier versions had an issue but I don't remember which one. It would is possible to check the change log, but instead I would suggest that you just update to V5.17 or the latest beta version  and try that out. It may not be the cause of the problem, but the latest version has more features and less bugs than V5 anyway so it is a more stable release in several ways.

No Joy? If you post a .mdat we can see if there are any other obvious problems with the file.


----------



## satx (Nov 29, 2012)

jtalden said:


> I just noticed that the version of REW is 'V5' per above screen shot. I believe that version may have had some issue with impedance measurements? That may not be correct. One of the earlier versions had an issue but I don't remember which one. It would is possible to check the change log, but instead I would suggest that you just update to V5.17 or the latest beta version  and try that out. It may not be the cause of the problem, but the latest version has more features and less bugs than V5 anyway so it is a more stable release in several ways.
> 
> No Joy? If you post a .mdat we can see if there are any other obvious problems with the file.


Thanks, I'll try an update and see. I know the reason that I'm still using V5 is because I had some kind of issue when I updated and had to revert back. This was a year ago or so so maybe it'll work now. 

I'll post an .mdat file shortly, don't know why I didn't think of that before:dontknow:


----------



## satx (Nov 29, 2012)

Ok, now I remember why I'm using an old version- when I try to install the newest version the install fails because it says no JRE can be found. But, When I try to download the latest JRE, that doesn't work either. I am using a seriously old laptop, like circa 2005 that I only use for two programs. It is running XP. The Rew downloads with JRE aren't supported on XP. 

My current laptop is a 2014 Macbook Pro, but does Rew work on a mac? I would prefer to keep this software on the old computer if at all possible though.


----------



## satx (Nov 29, 2012)

I was able to reinstall Java and update to the latest version of Rew. Still no dice though. One weird thing that now happens is when I check levels before making an impedance measurement, nothing shows up on the input meters to the right and I get the warning that input is very low. But, if I go ahead and take the measurement anyways, the meters just up into the red and I get clipping. I have to set the output to about -20db to avoid clipping. 

Here is a sample .mdat file.
View attachment Impedance file for HT Shack.mdat


----------



## jtalden (Mar 12, 2009)

I can't determine the real issue, but I can point out things to look at. Maybe that will help you find the problem.

> REW has an install for Mac OS X (10.7 - 10.11) on the same install page. Try that.
> The info panel still doesn't indicate that the headphone output was specifically selected in the soundcard setup. Also the line inputs are not specifically selected. It's best to specify both the device and the inputs/outputs.
> The graph panel does show a portion of the measurement (>10 kHz) when the scales are fully zoomed out. The cursor reports ~36.4 kohm at ~30Hz and ~5 kohm at ~20 kHz. The shape does not follow the normal shape so this is probably not just an issue with a mistaken value of the sense resistor.
> Have you been following the REW 'Help/Impedance' info from the REW Menu or are you referring to 3rd party procedures? The info in the REW Help is very good. If any of that info is not clear as you step through it then that may help to pin down the problem area.
> The laptop headphone output is good to use. Be sure the line input jack correctly connecting to L and R inputs. [Some jacks have 3 and others 4 connectors maybe. I don't really know for laptops.]
> The sweep started too high at 20 Hz if this is a woofer being measured. 0 Hz is normally selected for LF drivers.

Again these are only overall observations/thoughts that I had in looking at the file. I found no likely cause, only these observations. Maybe there is something here that helps?


----------



## satx (Nov 29, 2012)

Thanks for taking a look. 

- The measurement file I attached was just one of many I tried. I selected every possible combination of inputs and outputs. None of them provide different results. 

- I used the REW impedance help section to initially get set up with impedance measurements. Like I said in the first post, I have been using this jig with Rew to take measurements for years without issue on my old desktop. It was only when I bought a new desktop that it no longer works. And of course it doesn't work on the laptop either. 

-The sense resistor is the same 100 ohms that I've always used. Like you say, even if the value was off, I should still see the correct shape. BTW, it tested R with a volt meter and it's not bad. 

-The sweep was just for a woofer that I had laying around. I start many tests at 20Hz without issue as I often measure full range speakers, much more than subwoofers. I also, tried to measure a resistor and get the exact same results. 

-Not sure what you mean by 3 or 4 inputs in the jack. The plugs i'm using for the jig has a R/L and a common ground. I did verify that if was wired correctly and it is. I used the impedance help file schematic to build the jig, btw. 

Thanks again.


----------



## satx (Nov 29, 2012)

Here is a file I made using my desktop computer. It's a bit different, but still wrong of course. Maybe you can see something here. Thanks. 

View attachment Impedance file for HT Shack desktop.mdat


BTW, when checking levels I get the warning "Ref input level low". If I continue anyways with the check, I get clipping until the level is adjusted unreasonably low.


----------



## JohnM (Apr 11, 2006)

You can find Java 7 downloads at http://www.oracle.com/technetwork/java/javase/downloads/java-archive-downloads-javase7-521261.html

REW does run on OS X.

The problem looks more likely to be with your jig or the connections to it though. The images you posted show the impedances measured are very high, > 6k, which is why they are off the top of your graphs. 

If gain needs to be set very low as you mentioned that might be due to mixer settings causing a feedback loop (see notes on volume controls) or trying to feed a line level signal into a mic-level-only input. You should check that you can make a valid loopback measurement with the input and output you are using first.


----------



## satx (Nov 29, 2012)

JohnM said:


> You can find Java 7 downloads at http://www.oracle.com/technetwork/java/javase/downloads/java-archive-downloads-javase7-521261.html
> 
> REW does run on OS X.
> 
> ...


Thanks a lot John, 

I got Java reinstalled on the old laptop and also the newest version of REW. Not sure why it didn't work before. 

I suppose I can just build another Jig. I went through it with a meter set to continuity though and also rechecked it against the schematic. It appears to be fine. 

How do I make a loop back measurent? Is this the same as when calibrating the soundcard?


----------



## JohnM (Apr 11, 2006)

Yes, just connect output to input.


----------



## satx (Nov 29, 2012)

Ok, I've been playing with the desktop setup. I may have had the jig plugged into the wrong mic location. It was plugged in the the 3.5mm jack that said mic, but I found in the sound manager that the default device is another jack. (I am choosing specific input output devices) So I tried that new jack and now I can get level readings when that device is chosen in preferences. However, I still don't get anything when I actually measure. And the input clips until the output level is very low. I looked, but can find no mixer that is set to on or create a loop back. 

I was able to make a loop back measurement under the spl tab, but not the impedance tab. Is this correct?


----------



## satx (Nov 29, 2012)

Here are attached files for the loopback connection and a new impedance attempt using the new Jack on the back of the computer (I know that sounds stupid, but this computer is fairly new for me) The driver under test is 8 ohm nom. and has an fs of somewhere around 65Hz, so the 25 ohm flat reading is obviously not correct. Thanks


----------



## JohnM (Apr 11, 2006)

The loopback shows that there is some form of EQ or audio effect active. You'll need to track that down and turn it off. 

It is a little odd that the sweep level needs to be so low (-42 dB) when the input volume is 1.0 on a line input, but it may be that the line input applies gain above some volume setting. What was the input level when you did the loopback? Could set the input gain to 0.25 to even things up a little. 

If you need a low level for the loopback then you would need a similarly low level for the impedance measurement - one of the connections is a direct loopback from output to input.


----------



## satx (Nov 29, 2012)

I thought that the loop back looked odd. I didn't see any mixer or equalizer, but I'll hunt around some more. 

When I set the input to .25, I get the same results as when I don't control the input level. With a driver attached right now I get -16db for an OK level reading.

Any thoughts on the impedance sweep?

Evan


----------



## satx (Nov 29, 2012)

Ok, I think I have finally got it working!!!

I can't seem to find the EQ so far, but when I change to the communications headphones sound driver, I get a normal looking loop back response. 

Please see the two files below and let me know if they're ok.

Also, I was able to make a driver impedance sweep that looks normal! The first one showed the result too high by the value of R so I switched the input and it works correctly. The only issue I had is that once I switched the input and checked level, no beep sounds and the warning appears saying the Ref input is very low. If I continue anyways, I get the pink noise and an OK reading at about -16db. 

Anymore feedback would be greatly appreciated, but thank you both for all of your help on this!


----------



## jtalden (Mar 12, 2009)

Joy! :sn:


----------



## satx (Nov 29, 2012)

jtalden said:


> Joy! :sn:



Yes! hallelujah! Now I'm able to measure at about -6db too. Don't why it keeps getting lower, but I'll take it at this point.


----------

