Impedance measurement not working - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #11 of 24 Old 07-21-16, 02:36 PM Thread Starter
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Re: Impedance measurement not working

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. Impedance file for HT Shack.mdat
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post #12 of 24 Old 07-21-16, 04:07 PM
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Re: Impedance measurement not working

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?
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post #13 of 24 Old 07-21-16, 04:39 PM Thread Starter
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Re: Impedance measurement not working

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.
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post #14 of 24 Old 07-21-16, 05:12 PM Thread Starter
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Re: Impedance measurement not working

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.

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.
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post #15 of 24 Old 07-21-16, 05:13 PM
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Re: Impedance measurement not working

You can find Java 7 downloads at http://www.oracle.com/technetwork/ja...e7-521261.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.
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post #16 of 24 Old 07-21-16, 05:37 PM Thread Starter
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Re: Impedance measurement not working

Quote:
JohnM wrote: View Post
You can find Java 7 downloads at http://www.oracle.com/technetwork/ja...e7-521261.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.
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?
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post #17 of 24 Old 07-21-16, 06:00 PM
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Re: Impedance measurement not working

Yes, just connect output to input.
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post #18 of 24 Old 07-21-16, 06:02 PM Thread Starter
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Re: Impedance measurement not working

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?
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post #19 of 24 Old 07-21-16, 06:17 PM Thread Starter
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Re: Impedance measurement not working

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
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post #20 of 24 Old 07-22-16, 05:16 AM
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Re: Impedance measurement not working

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.
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