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Author Topic: Bench test DI boxes?  (Read 1690 times)

Chris Horton

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Bench test DI boxes?
« on: September 08, 2014, 02:25:59 pm »

Apologies if this has been covered elsewhere...could not find anything with my search queries.

Essentially looking for advice or help with bench testing DI boxes.  Partially for my own edification, but also because I have been trying to troubleshoot a couple of boxes for my company.  I can't find anything wrong when I open them up, but it seems like every time they got on a gig, they come back labeled as "noisy" or "buzzy".  Of course, there's no way to exactly recreate the conditions on a gig, i.e. was it the guitar, the cable, the amp, the mixer, etc.  Hence, my hope that there is a good method for bench testing under a specific set of circumstances to rule out the box.

One is an old, "homemade", passive DI...the other is a Countryman Type 85 active DI.  Any thoughts or advice would be greatly appreciated, cheers.
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Christopher Horton
Capron Lighting and Sound

Tim McCulloch

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Re: Bench test DI boxes?
« Reply #1 on: September 08, 2014, 04:37:35 pm »

Apologies if this has been covered elsewhere...could not find anything with my search queries.

Essentially looking for advice or help with bench testing DI boxes.  Partially for my own edification, but also because I have been trying to troubleshoot a couple of boxes for my company.  I can't find anything wrong when I open them up, but it seems like every time they got on a gig, they come back labeled as "noisy" or "buzzy".  Of course, there's no way to exactly recreate the conditions on a gig, i.e. was it the guitar, the cable, the amp, the mixer, etc.  Hence, my hope that there is a good method for bench testing under a specific set of circumstances to rule out the box.

One is an old, "homemade", passive DI...the other is a Countryman Type 85 active DI.  Any thoughts or advice would be greatly appreciated, cheers.

Hi Chris-

Yes, there are ways to recreate the gig up to the point of connecting the DI to the actual amp du jour.  You set up your rig in the shop - the same distro and AC cabling, mic cables, DIs... just like you're doing a gig, but with only 1 top box and 1 sub and 1 monitor (all the amps, though).  Hope your DIs make noise so you can find out if they're genuinely bad or are pointing to some other symptom.  If you can rule out your stuff, you'll have some solace knowing that you're putting on a dog and pony show for the benefit of the BE.  Hey, we're in show biz, right?  I've replaced all manner of stage connections (DIs, mics, cables, sub snakes, etc) until someone finally listened to the source and found the buzz...  A Whirlwind Qbox and headphones are your friend. ;)

http://forums.prosoundweb.com/index.php/topic,151222.0.html is from the AC Power & Grounding Forum, about a DI that became an inadvertent part of the mains voltage system.  It's possible that one or both of your problem DIs have taken a hit for the team.  Lots of good stuff in that forum.  Mike Sokol, the mod, is very hip on keeping folks alive and has developed demonstrations with apparatus that can inject voltage into signal grounds (and other unexpected places) in a controlled manner, then he shows how to test for these dangers.  Very informative discussions over there...
« Last Edit: September 08, 2014, 04:40:46 pm by Tim McCulloch »
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Chris Horton

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Re: Bench test DI boxes?
« Reply #2 on: September 09, 2014, 12:15:14 pm »

Hi Chris-

Yes, there are ways to recreate the gig up to the point of connecting the DI to the actual amp du jour.  You set up your rig in the shop - the same distro and AC cabling, mic cables, DIs... just like you're doing a gig, but with only 1 top box and 1 sub and 1 monitor (all the amps, though).  Hope your DIs make noise so you can find out if they're genuinely bad or are pointing to some other symptom.  If you can rule out your stuff, you'll have some solace knowing that you're putting on a dog and pony show for the benefit of the BE.  Hey, we're in show biz, right?  I've replaced all manner of stage connections (DIs, mics, cables, sub snakes, etc) until someone finally listened to the source and found the buzz...  A Whirlwind Qbox and headphones are your friend. ;)

http://forums.prosoundweb.com/index.php/topic,151222.0.html is from the AC Power & Grounding Forum, about a DI that became an inadvertent part of the mains voltage system.  It's possible that one or both of your problem DIs have taken a hit for the team.  Lots of good stuff in that forum.  Mike Sokol, the mod, is very hip on keeping folks alive and has developed demonstrations with apparatus that can inject voltage into signal grounds (and other unexpected places) in a controlled manner, then he shows how to test for these dangers.  Very informative discussions over there...

Thanks for all the help, Tim...and yes, that is my plan to set up what we can and try to recreate as best as possible.  Unfortunately, I don't often get to keep suspect gear (mics, cabling, mixer, etc.) together when it comes back, though I may have to start insisting.

I guess the hope with my post was that someone had an idea about a checklist of steps or methods using test equipment to run signal through a DI, or measuring with a meter across inputs and outputs to find any anomalies.  Not as complicated or in-depth as a schematic, but a little more sophisticated than plugging an instrument into it and seeing what happens.

Anyway, I greatly appreciate everything, and thanks for the link to that forum...I've actually started reading a lot of those posts, as electricity and power is one area in which I'd love to educate myself.  It wouldn't surprise me in the slightest that a long time ago, the DI took some sort of hit and our techs didn't even realize it.
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Christopher Horton
Capron Lighting and Sound
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