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Author Topic: Battery box causing DC offset?  (Read 1827 times)

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Offline Fried Chicken Boy

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Battery box causing DC offset?
« on: May 23, 2014, 05:47:13 PM »
I'm hoping some of the more electronically minded can take a swing at this.  I have a few sets of lavalier mics in stereo pairs that I occasionally run with a battery box.  As I tend to record more open shows nowadays than  >:D they don't get the same amount of use that they used to.  Several months ago, I got a setup out of mothballs, made a pull and when I brought the resultant recording up in my DAW, I could see a little DC offset in the channels.  I chalked it up to the 9v battery in the battery box that was likely less than fresh and moved on.

Fast forward to present day and I bought a pair of Countryman B3s a few weeks ago.  I dust off the 'ol battery box (which I've had for close to 6 years) and take the B3s out for a few spins at two different shows.  Right off the bat while I'm recording I can see that the levels between the L/R are vastly different, at least several dB.  The guy who terminated the mics for me found them to be less than 2 dB apart in his testing.  So I bring the recordings up in my DAW and see very noticeable DC offset, particularly in the left channel. >



What's really odd, and what can be seen in the above screenshot, is that the issue seems to straighten itself out around the 28:30 mark (the red arrow) and it sounds relatively fine for the rest of the recording.  The recorder I've used quite a bit with other setups and had no issues so I can probably rule that out.  I have not extensively tested the different sets of mics, but I'm seeing the same issue with them all and the common denominator seems to be the battery box.  The battery in the box was fresh for these recent recordings and supplying well over 9v.  Checked it just now on a multimeter and it's still reading over 9.5v.

So I guess my big question is: can battery boxes "go bad"?  What would cause it to behave like that?  Until several months ago it appeared to be working fine.  Thanks in advance.

Offline Fried Chicken Boy

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Re: Battery box causing DC offset?
« Reply #1 on: May 24, 2014, 01:17:03 PM »
Thank you very much for the assessment and info, Jon.  How would I go about troubleshooting the issue?  All the electronics in the box appear to be potted in epoxy so I don't think there's access to the individual components.

FWIW, I ran this same B3>battery box setup again last night and, after taking a quick look at it in the DAW, there appears to have been no issues.

Offline Fried Chicken Boy

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Re: Battery box causing DC offset?
« Reply #2 on: May 24, 2014, 02:09:13 PM »
Yes, I do.

Offline Fried Chicken Boy

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Re: Battery box causing DC offset?
« Reply #3 on: May 24, 2014, 04:50:39 PM »
I'm not sure exactly what parts of the battery box you're referring to when you mention "input" and "ground"?  All the electronics are potted in epoxy so, other than the battery terminals, input jack and output plug, I can't access anything else.  For reference, it's this BB without the roll-off circuitry.  Thanks.


Offline Fried Chicken Boy

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Re: Battery box causing DC offset?
« Reply #4 on: May 25, 2014, 12:43:18 PM »
Thank you for the clarification and guidance, Jon.  Got the following:

Input >
9.52 v
1.25 mA
(Same values as the battery)

Output >
No reading for current at all
(Tried switching up the probes on the different parts of the plug between the tip, ring and sleeve and still got nothing)

Resistance to ground >
Minor reading at first and quickly went to 0

Offline Fried Chicken Boy

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Re: Battery box causing DC offset?
« Reply #5 on: May 25, 2014, 03:11:42 PM »
Thanks, Jon, I will.  However, keep in mind that this setup goes from producing these strange results to working perfectly (as it did a few nights ago), and it happened with a different set of mics as well.  Can a leaky capacitor behave like this?  And if it's not the battery box and indeed something with the microphones (which have a 2.2k mod), what in the mics would cause this?  Thanks again.

Offline Fried Chicken Boy

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Re: Battery box causing DC offset?
« Reply #6 on: May 26, 2014, 11:54:55 AM »
After a few preliminary tests with noise (white, pink, brown), a different power source and different mics, it's looking like the battery box is the culprit.  Time for a new one. 

Thank you for all your help and guidance with this, Jon. 

 

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