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Old 4th October 2011, 12:51 AM   #1
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Default Reduced Output Level from a Dynamic Mic

I have recently acquired an old SM58 that sounds clean/undistorted, however putting out much lower signal than a regular mic would.

I suspected that there may have been a disconnection in wires leading towards the capsule, but upon further inspection, everything from XLR to transformer, transformer to capsule wires measures well on the DMM.

The mic almost seems to "reject" sound coming from a distance, and output level suddenly goes back to normal if within a inch to the grille, however at this distance a singer can easily clip the mic/preamp.

I am really confused about what might be wrong with this mic, I was more or less under the impression these things are virtually indestructable. I am pretty certain this is a genuine mic too.(has the correct colored wires, actual transformer with isolated primary/secondary, right XLR connectors...etc)

If any one could chime in and let me know if this could somehow be fixed without sending it back to Shure, any advice is greatly appreciated.
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Old 4th October 2011, 01:01 AM   #2
Enzo is offline Enzo  United States
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I guess I'd want to know what you consider a "regular" mic. Inexpensive crystal mics put out a very strong signal, compared to something like the SM58.

Take your mic somewhere where you can side by side compare it to similar mics. If it sounds about the same as they do, it is OK. If not, it isn;t.

I don;t know how experienced you are with microphones. The SM58 is a cardioid mic. It is directional. The more around the sides and to the rear you go, the less it picks you up. It is designed that way.

Ther are various failure modes for the thing, but if the internal transformer is OK, pretty much replacing the element is the repair. If you had a cracked diaphragm for example, there is no fixing it. Just replace the works.

I haven;t bought one of those replacements in ages, so I have no idea what they cost. I am sure Shure Brothers would tell you.

And just in case, you can disassemble the mic, take the wires coming up from the transformer in the handle off the element, and tack on a cable or otherwise connect the element alone to a hi-Z amp input. Does that perform any different?

These are very rugged and reliable good sounding mics. Doesn't mean they can't fail or be damaged. Even the Rolls Royce dealer has a service department.
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Old 4th October 2011, 02:25 AM   #3
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The only other "real" mic I have in comparison is a fet condenser. The output is quite hot and I use about 40dB of gain on my preamp.

While I do not have another dynamic mic for comparison, I thought that needing 60dB of gain for recording vocal 3 inches away on axis seems somewhat wrong. At this sort of gain a pair of dynamic transducer earbuds plugged into the preamp can do louder than the mic in question, so I am guessing maybe the cartridge was damaged.

The only strange thing I can't wrap my head around is the sudden increase in sensitivity when within 1 inch to the grille, it seems almost as if I tripped a +10dB pad once I get close enough, and the sound suddenly cuts back when I move my mouth away. My experience with proximity effect is that it affects the low frequency mostly, and not the sensitivity, and this is the first time I've encountered this in a microphone.

I may try bypassing the transformer just to be sure, but my guess is that would result in lower sensitivity.
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Old 4th October 2011, 02:41 AM   #4
imix500 is offline imix500  United States
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We are dealers for Shure at work and when we've have a 57/58 go down it's rarely been worth repairing. I can't remember exactly what the replacement capsules go for, but it's never been worth it for us. Considering a new 58 is maybe $99 Street. Less for dealer but markup is comparatively very small for these mics.

Last edited by imix500; 4th October 2011 at 02:44 AM.
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Old 4th October 2011, 09:34 AM   #5
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Originally Posted by imix500 View Post
We are dealers for Shure at work and when we've have a 57/58 go down it's rarely been worth repairing. I can't remember exactly what the replacement capsules go for, but it's never been worth it for us. Considering a new 58 is maybe $99 Street. Less for dealer but markup is comparatively very small for these mics.
The Shure price sheet is practically logarithmic when it comes to quantity discounts. Most small dealers are taking a loss on these things, just so they can sell other stuff. Only the chain stores can buy enough at once to make a profit at $99.


nereis, Enzo's right - you really need to compare your '58 to a known good one. These things are all about proximity - it's why they're still the standard vocal mic after all these years. (That, and they can survive an enormous amount of abuse.) In a live situation, an SM58 won't feed back when many other "better" mics will.
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Old 4th October 2011, 10:55 PM   #6
ChrisA is offline ChrisA  United States
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Originally Posted by nereis View Post
..
The mic almost seems to "reject" sound coming from a distance, and output level suddenly goes back to normal if within a inch to the grille, however at this distance a singer can easily clip the mic/preamp....

That is the whole point of an SM58. You can have an stack of guitar amps as big as a house right nearby but the mic only picks up the vocals that are an inch away. And yes the SM58 needs a ton of gain for most normal singing. But on the other hand you can use an SM58 to mic a guitar speaker at 12 to 18 inches. I'd say if the mac has any output at all it is 100% good. When it fails it's dead.

If you have a singer with a soft voice in a studio and want the mic a foot away with a pop filter then get a condenser mic. The SM58 is not going to work in that setup.

Look on You Tube at anyone who is using an SM58 on stage and notice how they hold the mic. It is as you describe, about an inch away.
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Old 4th October 2011, 11:39 PM   #7
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Thanks for all the input, so it might not be a bad mic afterall!

Might be worthwhile to actually just record some things with it to get a feeling as to how these things work.
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Old 5th October 2011, 03:04 AM   #8
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Originally Posted by nereis View Post
The only other "real" mic I have in comparison is a fet condenser. The output is quite hot and I use about 40dB of gain on my preamp.

While I do not have another dynamic mic for comparison, I thought that needing 60dB of gain for recording vocal 3 inches away on axis seems somewhat wrong. At this sort of gain a pair of dynamic transducer earbuds plugged into the preamp can do louder than the mic in question, so I am guessing maybe the cartridge was damaged.

The only strange thing I can't wrap my head around is the sudden increase in sensitivity when within 1 inch to the grille, it seems almost as if I tripped a +10dB pad once I get close enough, and the sound suddenly cuts back when I move my mouth away. My experience with proximity effect is that it affects the low frequency mostly, and not the sensitivity, and this is the first time I've encountered this in a microphone.

I may try bypassing the transformer just to be sure, but my guess is that would result in lower sensitivity.
Let's look at the sensitivity specs for the SM58 and an example FET condenser mic, say a U87i in cardioid mode:

SM58 => 1.85 mV/Pa
U87i => 28 mV/Pa

which is about a 24 dB difference in level for the given measurement conditions. so it doesn't surprise me and is consistent with my own experience to need about 20 dB more gain for an SM58 vs a condenser mic.

Cheers,

Michael
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Old 5th October 2011, 10:09 AM   #9
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Originally Posted by nereis View Post
Thanks for all the input, so it might not be a bad mic afterall!
I don't know about your one - but in my experience it's a common failure mode on Shure mikes, come to use them after a few years and their output is almost non-existent, presumably it's failure of the capsule in some way? (perhaps the magnets lose their power?).
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