SM-81 Problem

T

tennbidder

New member
I have a problem with a Shure SM-81 microphone.

This mic was purchased used but is in mint condition.

There is a static hum when the mic is powered up. It sounds similar to what you would on a portable radio if placed to close to something like a flourescent light. The volume of the hum is loud enough to be detectable in headphones when preamp gain is set for recording.

I have isolated the hum to the mic by changing cords, changing preamps, and turning off any potential interference sources. Two cheaper mics hooked with exactly the same conditions have no hum.

Any ideas that would help eliminate the hum ? Do i have a defective mic?
 
Sounds like a bum mic. It's probably a wire that has come loose, though I wouldn't know how to fix that. You might try sticking it in a ziplock back with a few dessicant packets for a couple of days, as sometimes mics that have been in a humid environment will collect some moisture on the capsule that may lead to excess noise (usually clicks and pops). Can you still return it to the sender?
 
All the other comments make sense, especially the one about drying out the mic if it's been in a humid environment.

One more thing to check is to see if one of the wires soldered onto the inside of the XLR in the mic is broken or has a cold solder joint. A bad hum makes me suspect that one of the conductors (especially the shield) is faulty. How do the pins on the mic's XLR look? Are any of them really corroded or covered with some type of goo or film? That could cause a poor connection, which could cause a hum.

I have a pair of SM-81's they're both dead-quiet.
 
Big Kenny said:
bad phantom?
Click to expand...

Yeah, could be. The SM81 only has a few wires inside, and they aren't subjecte d to any stress. Most of the mic is a long surface mount PC board, there is a transformer at the XLR end with a few leads. If one of those isn't obviously bad, then it's off to Shure with it. Maybe not a bad idea in the first place.
 
Thanks for the notes.

At this point, I am unable to return it.

I have tried another preamp and still get the hum.

If I put my hand on it, the hum is not as loud.

Does that bring up any other ideas ?
 
The ground is shorted

Something's not grounded right.

Look for loose connections between the capsule and body, etc.

Look for loose wires.

Use a Volt-Ohm-Meter to test continuity between leads.

I had a grounding problem on a mic once, turned out the body had come disconnected from the ground wire. That was it.

Good luck.
 
definitely

sounds like a poor ground to me too. Not that I am an expert but have a few years of experience to draw from.
If you've ruled out all other possibilities in your signal chain then the mic has a problem. The sm81 is a pretty good mic. I don't know how much you have invested but it might be worth the cost of letting Shure fix it for you.
good luck

(just my .02)
 
tennbidder said:
Thanks for the notes.

At this point, I am unable to return it.

I have tried another preamp and still get the hum.

If I put my hand on it, the hum is not as loud.

Does that bring up any other ideas ?
Click to expand...


If you can get the noise to disappear by simultaneously touching the grill and the XLR release tap (the unpainted part), then that will indicate a grounding issue between the body parts.
Continuity can be established by scraping a bit of the plating/paint off where the body pieces connect.
Also, insure that the mic's XLR jack is screwed in tight.
I have seen this problem with Shure VP88's.
Another possible thing to check is whether or not your XLR cable is "case grounded" on both ends. This may or may not be an issue.

Brent Casey
Studio Projects Microphones
877-563-6335
 
Thanks guys! This is a great forum. I have certainly learned enough to get my home studio up and running.

I took the unit apart and saw some very light corrosion which I remedied.

Looks like the problem has been fixed.

Once again, THANKS !!
 
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