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Discussion Starter · #1 ·
Hi Guys,

I guess this question should be placed here: I recently got a Behringer UCA 202 USB soundcard and a Galaxy CM-140. Since I don't own a stereo jack to RCA just yet, I tried a mono jack to RCA as well as a composite jack to RCA, which immediately both showed me much higher dB levels on the SPL meter LCD screen (40dB goes to 65 dB) in the same quiet room. Measuring my subwoofer and checking RTA in a quiet room showed me this;



Nothing is connected to AC, I'm simply using the laptopbattery. Also the soundcard and the SPL meter are both calibrated. Disconnecting the SPL meter shows me this, which also lowers the SPL readings on it's display;



Why would connecting the *output* of the SPL show me different readings on it's display, and could the 'not-stereo' jack possibly be the cause?

I'm actually pretty worried if I might own a faulty soundcard or SPL meter. It was quite a bit of money for my standards :sad:

Can anyone take away my worries?
 

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Discussion Starter · #5 · (Edited)
Thanks for the help so far!

The problem seems to be with the Behringer UCA 202. It is axaggerating the low frequencies. The weird thing is that the calibration of the soundcard seems to be going fine. See the results in the graph below (between -10 dB to 0 dB range). Right after the calibration I did a measurement without anything connected to the soundcard, and it gave me the results as shown below in blue;

snap spl 3.jpg

I don't understand how this is possible. How can it find a result with an 'empty' measurement, when the calibration I just did should tell 'what zero is', if I understand the calibration process correctly?

These are the results without marking the 'C weighted SPL meter' and without the downloaded SPL calibration file;

snap spl 4.jpg

[edit:] of course the calibration won't tell what zero is, since it loops back a signal @ -12 dB 0_o.

Is there a way to zero this problem out? Should I return the soundcard, or is this a probable PEBCAK? I get the same results on another laptop following this step-by-step. If I attach the line-out to my receiver, will I be able to hear this signal, when I flick the 'monitor' switch to 'ON'?

[edit2:] I just checked via remote desktop, and Windows doesn't show a loud noise in the line-in. Also, I just compared the 'sweep results' (with no connected inputs to measure anything) with RTA (also nothing connected). This shows a big difference, so somehow the unconnected output sweep is actually generating an input, even though monitor is off. :blink:;

snap spl 5.jpg

In this thread someone was having a comperable issue, but 'only' at a slope of 82db to 32db from 2 Hz to 200 Hz, with nothing connected. Mine has 112db to 76 dB, without applied C-curve/calibration.
 

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The UCA202 doesn't seem to like high impedance sources, it may have high leakage current from its inputs - that might be the cause of the SPL meter reading changing when connected to the soundcard due to being backfed by the soundcard's input. The soundcard's output is a low impedance source so the problem doesn't show up for loopback measurements. The answer might be to ditch the UCA202 :(
 

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Discussion Starter · #7 · (Edited)
Thanks John. That would explain a lot of what's happening here.

But do you believe this is normal behaviour for the soundcard? I mean the signal from USB sent to line-out leaking to line-in like that? I want to know if I should return the soundcard as being faulty, or just buy a new one.
This guy had much better results with the sweep on the unconnected soundcard. I just did a sweep through my onboard soundcard which gave me these results on the unconnected line in of the UCA202 (green), compared to previous measurement with the sweep on the line out of the usb soundcard (gray);



What soundcard is recommended for usage with the CM-140? Or maybe a DI box in between would be a cheaper solution? Or will that ruin the actual signal?

[edit:]I read the following about the SPL;
AC Output: 1 Vrms at FS (full scale).
Output impedance: Approx. 100 Ohms


And the following of the UCA202;
Input impedance: Approx. 27kΩ
Max. input level 2 dBV

Output impedance: Approx. 400Ω
Max. output level 2 dBV


Seems the output impedance of the SPL is actually lower than the output impedance of the UCA202 itself...

[edit2:] I'll try and compare my results with these, and see if this unit is an oddity or not...
 

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Also worth making sure there is no software monitoring going on, i.e. 'Listen to this device' not ticked if using Win8 or the UCA202 input volume muted in the playback mixer if using XP.
 

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Discussion Starter · #9 · (Edited)
Thanks John. Yes, I followed this step-by-step. I've changed the output to the optical output of my internal soundcard to my receiver, and have done the loopback calibration from receiver to the UCA202. That way there was no feedback anymore, so I'm now only left with the 50Hz noise peak problem.

I opened the UCE202 and noticed some heavy hand soldering near the monitor switch at a component called Q1;



Touching that soldering changes the 50Hz noise from this;


to this;


Does anyone have any knowledge of what that component does, and if that could be the culprit?
 

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Discussion Starter · #11 ·
Pictures of a good working copy of the thing on NwAvGuy show the same solderblob, though;



But I'm done with workarounds, and will now focus on returning the thing as being faulty.

Thanks for all the help, John. Really appreciate it...

Tom.
 

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Discussion Starter · #12 ·
The UCA202 doesn't seem to like high impedance sources, it may have high leakage current from its inputs - that might be the cause of the SPL meter reading changing when connected to the soundcard due to being backfed by the soundcard's input. The soundcard's output is a low impedance source so the problem doesn't show up for loopback measurements. The answer might be to ditch the UCA202 :(
I've sent back the UCA202, but recently noticed that hooking up the CM-140 SPL to my mic-in port on my laptop (yes, I know:nono:), the reading on the SPL also shifted almost 30 dB. Since I haven't read anyone else complaining about this issue, I'm now wondering if this might be a problem with the SPL?

Is there anyone else that owns the Galaxy XM-140 that gets different readings on it's display, as soon as you hook up the SPL to a line-in?
 

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Mic inputs usually carry power (5V) for PC condenser mics, that can affect some sources - the RS meter, for example.
 

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I suggest that your new SLM ( CM-140 ) is faulty ( or possibly your cabling is ) .

The display ( & apparently the output voltage ) of an SLM shouldn't simply jump up ( 25 to 30 extra db ) just because you loaded it's output ( ie; which occurred when you plugged the SLM into those 2 soundcards ).

I would return the CM-140 ( if possible ) & buy a miniDSP UMIK-1 ( the recommended test mic for newbs ) .

:sn:
 
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