Shure MX412 and MX418 buzzing...

[Solstace]

Hi all

 

So today's mystery. We've got a pair of Shure Mics (as above), each fitted with R185 Cardioid capsules, that each randomly decide to go crazy when in use. When the mic is in "crazy" mode, it's hard to describe exactly what happens... Here goes:

 

• The source sounds like it's being clipped somehow - but at a much lower level than would be apparent with clipping. It's almost like the sound source is triggering a white-noise generator that sits gently behind the signal.
  
• There's also other random repeated clicking - varying in frequency (less than 10 clicks per second).
  
• The mic will become very resonant and therefore prone to feedback.
  
• Thinking about it, it almost sounds like radio interference - but I can't pin down any logical source.
  

 

The problem is more acute when we use them with our Intelix 1U mini-mixer, but it's happened also in the same venue when using them to feed our Soundcraft Spirit 8 desk. Tried different feeds from the stage and each gets the same result. Plugging the mic directly into the back of the mixer kills the problem. Swapping the mic to any other phantom-powered condenser mic from our current collection (mostly AKG 451, SE300/CK91, C1000) using the same feed kills the problem and has proved 100% stable so far.

 

Interestingly, the AKG GN15/CK31 combos these Shure mics have replaced were also prone to this problem - but far less so than their replacements.

 

 

Well I'm stumped - thoughts anyone?

 

C :)

[fogg]

If plugging the mic directly into the mixer kills the problem it sounds like a problem with the multi... Could it be that the XLRs have been wired wrongly ie unbalanced? Definatley worth checking the connections at the desk end and the stage box end to check the wiring hasn't been switched? just a wild guess but worth a look if bypassing the multi sorts the problem.

[Solstace]

...but worth a look if bypassing the multi sorts the problem.

 

Would love to - and I think the only way I'm going to do it is to try rigging a long cable and trying it for a day while I'm doing other things. It's not always instantly reproducible even in its present state...

 

Any other ideas? Anyone?

[nothingatall666]

I would also say its the multicores to blame,

 

I may not be an expert but if all your dynamics don't work on the channel, but do if you just use different lead in to the same desk channel then its obviously a problem with the multi.

and I think the only way I'm going to do it is to try rigging a long cable and trying it for a day while I'm doing other things. It's not always instantly reproducible even in its present state...

you don't have to rig it, how about a 15m XLR (or a few joined together) just near desk plugged in to both the mic and desk and seeing if you have a problem then?

I'm not sure on this but as the multi works with phantom power then maybe its a distance problem?

 

 

just my idea's

[Solstace]

Fair-fair.

 

Bear in mind that the MX412/418 are themselves condenser mics... and that we're running SM57's and SM58's (very definitely dynamic mics), AND all our other mics over that same multi, without issues. It's only the gooseneck-mounted mics that have ever given us problems, either the old AKG's that are no longer in use, or the Shures that have replaced them.

 

So right now it's only the Shures that are playing up. Since both were purchased a month or more apart then I find it difficult to believe that exactly the same fault will show in both of them unless there's something common to their design that makes them less stable than they ought to be... Could it be that for some reason gooseneck-mounted mics are more prone to RF interference? Or am I just seeing a coincidence here?

 

I might well try the long XLR approach - but I'm not sure that helps us much since everything else works 100% BUT the Shure's...

 

Or am I barking up the wrong tree?

[iCorpRoadie]

Do you have a blackberry/nextel or iphone? The data portion of those devices can cuase interference with those mics along with the wireless mics. tell the presenter to turn them off.

 

That is if the XLR/multi problem didn't fix it.

[Solstace]

Yes, I and some others have RIM Blackberry or Apple iPhones. Tests with my own Blackberry have shown the mic to be immune to calls or data coming to my phone when placed anywhere near it.

 

I've swapped the R185 capsule in our main mic for the one from our backup, and the noise issues have been reduced, We now only tend to experience a "grainy" sound on low-level sources - rather than the full-blown crazy distortion and random pops/squeeks.

 

Any one got any more ideas? I know some have mentioned cabling and the like - checks have shown that it is consistently in good condition and wired correctly. The worst fault I've found is a swap between pins 2 and 3.

[Solstace]

Thread necromancy alert...

 

Okay - so today's shown a little revelation on what's going on. It seems that at certain output levels of our induction loop amplifier, the microphone gets noisy. What puzzles me is that it's not a proportional thing - if I turn up the volume of the mic's feed to the induction loop to drive a higher level out of it, the problem goes away.

 

Now, I could play with the input gain and compression settings - but the problem then is that we won't be getting the desired output level from the loop. And it doesn't solve the problem of why a balanced microphone placed and cabled well outside the working range of the loop is having such issues.

 

Thoughts anyone?