Can I use REW to diagnose system noise electrically?

1198 Views 9 Replies 3 Participants Last post by brucek, Mar 23, 2009

Eric D

Discussion Starter · Mar 19, 2009

First, a little long-winded background…. So, I’m busily working my way through understanding REW at the same time I’m fighting a few problems in my system. One of those is chasing down sources of ground loop humms and noise using the the Jensen Transformer troubleshooting kit. Then, while running around and reading FAQs and older threads, I ran across this post: :

http://www.hometheatershack.com/forums/rew-forum/9872-spectrum-rta-feature.html#post86006

(didn't want to include it, as it's quite lengthy, but just look at the first picture. If someone can tell me how to include it directly - all I get is the Attachment # - I'll edit this to paste it in - thanks!)

It left me wondering: can use REW to directly measure the electrical noise going INTO the speakers. I know I can hear hiss and/or humm on the speaker, but measuring it electrically would seem to remove differences in mic setup or ambient sound levels in the room and make it easier to quantitatively measure what effect different fixes bring. I didn't find this procedure already described, but I may not have looked under the right search terms. Is there something I missed?

I figure that I need a simple circuit to convert speaker-level voltages back to line-level (or just to read them off). Would probably like to have it with an RCA line-out so I can just plug it into my sound card line-in and complete a loop just like we do in calibration. It seems like it would be an easy circuit to build. Or is it something that’s already available - I didn't find anything on search terms I could come up with?

Hope I'm not :coocoo:.

I hope I'm :coocoo: like a fox!

Thanks

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brucek

· #2 · Mar 19, 2009

I don't think you need to go to much trouble to do a spectrum reading in REW of what is being sent to your speakers. Simply place the REW microphone near field and set up REW to look at the spectrum. I can certainly tell you the controls you need to set in REW to use the spectrum tool.

brucek

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Eric D

Discussion Starter · #3 · Mar 19, 2009

That may very well be what I need to do. I just received my calibrated mic (got the phantom power licked - just need to get a cable and either get a stand or jigger a way to connect it to my camera tripod). I'm thinking I can trust the results better than my ole SPL meter setup for this kind of use.

But I'm still very interested in pursuing the electrical-only connections. I'm measuring the family room which is completely open to the kitchen and can't be sealed off from the rest of the house. It can be a bit hard to tell the wife and cats not to make noises at just the wrong time. It would also allow me to play music through some other source while I'm fiddling with the system. Finally, removing the speaker and room from the measurement seems like a bit of elegance to me and helps narrow the phenomena which have to be explained in measurements.

take care,

P.S. Are the Spec Analyzer settings what you discussed in the thread I referenced above? In any case, thanks so very much for all the time you devote to this area!

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brucek

· #4 · Mar 19, 2009

I'm still very interested in pursuing the electrical-only connections.

So you want to take a measure of your preamp? What is your goal? Without the room, the electronics is usually fairly perfect. The room is the culprit.

re the Spec Analyzer settings what you discussed in the thread I referenced above?

Yes.

brucek

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DrWho

· #5 · Mar 21, 2009

You don't want to use REW for tracking down ground loops....either your ground loop will be very audible or you won't have any ground loops. It is extremely rare that you get a ground loops that is very low in volume.

Can you post details about your setup - specifically, what gear you have hooked up when the ground loop is present? Any 3 prong devices or cable TV in the system? What about your computer?

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Eric D

Discussion Starter · #6 · Mar 21, 2009

My bad - I agree that ground loops are more than evident, and I don't need REW to hear them. I'm thinking more of lower-level sources of noise. The graph that Bruce had posted showing spikes in the noise and labeling the sources had me thinking that taking the background sound out of the graph might make for a clearer image.

So I thought that looking at the electrical signal going to the speaker, instead of the speaker conversion/transmission/conversion back to electrical by the mic - especially especially when you add the ambient noise in the room, might be a good tool for troubleshooting the system itself as well as electrical noise from things like transformers, computers, etc. And for me, being able to look at it graphically/numerically was better to see what changes were really more effective, especially before throwing money at the solutions.

Thanks for any inputs,

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Eric D

Discussion Starter · #7 · Mar 21, 2009

brucek said:
So you want to take a measure of your preamp? What is your goal? Without the room, the electronics is usually fairly perfect. The room is the culprit....

Please don't take this as being argumentative - I'm sure I wasn't making myself clear. When I can hear hiss coming out of a speaker, the room is not the culprit. I agree completely that it would be informative to just measure the room and to work on those issues. But I was hoping to get the room out of the equation when trying to troubleshoot noise coming out of the speakers.

I think there's also utility in measuring them both together and independently to see what is the larger source of problems and where it would be more worth spending money.

take care,

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brucek

· #8 · Mar 22, 2009

I can hear hiss coming out of a speaker

Ah, OK. It would be hard to measure the power amp section though, but you could measure the preamp portion if it had outputs, using the Spectrum feature.

brucek

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DrWho

· #9 · Mar 23, 2009

Yes, REW can be used to measure the S/N ratio of your system....however, it will be limited by the noise floor of your soundcard (not something that is trivial in this situation).

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brucek

· #10 · Mar 23, 2009

it will be limited by the noise floor of your soundcard

Yeah, I would do a quick loopback cable spectrum check of your soundcard first and examine the THD+N and all the harmonics, then connect the device under test. This will reveal the differences you're looking for.

brucek

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