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#160677 03/23/07 05:32 PM
Joined: Mar 2003
Posts: 97
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How would one go about finding out if a microphone is good or bad? The one in question is a "Low Z" Shure.

We got an intercom system that someone else thinks has a bad amp. Before changing out the amp I decided to try the mike on a different amp and it did not work so I suggested we get a new mike first.

The above was somewhat longer to actually do than explain, so if there is a better/faster way to do this, can someone let me know?

Thanks

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#160678 03/23/07 07:15 PM
Joined: Dec 2003
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The only reliable way is to connect it to a known good input on an amp. Probably someone will chime in here and say use an ohmmeter. That's not going to tell you much if anything.

So I would say you "done good". As long as it was connected properly it looks like that's the problem.

-Hal

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Originally Posted by hbiss
Probably someone will chime in here and say use an ohmmeter. That's not going to tell you much if anything.
-Hal

I'd agree with the comment of the ohmmeter not being of any real help, it is after all not a loudspeaker being tested here, where you get the small click in the voice coil when you apply an ohmmeter to it's input.
Microphones always have the problem of bad connections caused by rough handling and the like.
When I was playing in a band, I was forever repairing mike and guitar leads.

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It seems to me that the original concern was someone else thinking that an amplifier was bad. Do we all agree that it is usually very easy to cause a very loud hum at the speakers of a working amp, without a whole lot of effort at a mic input? Just thinking it's easier to show that an amp is working than that a mic is not.
Joe

Joined: Oct 2004
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Very true Joe. All you need to do is tap your fingers on the input terminal to the mic (usually pin #2 if XLR) and the hum you make will tell if the amp is at least working.

Edited to add: Use caution if the amplifier is old or of questionable condition. Be sure you're not grounded in case there is a chassis leakage problem!!

Ohmmeters on any microphone are a bad idea. The "voice coils" on a dynamic mic like the Sure SM series are of very fine wire, and it is possible to burn them open if the meter provides too much current. (Not too likely with a digital meter, but....)

And some audio wizards claim that the magnetizing effect of an ohmmeter run thru the coil can change the sound quality of the mic...

Finally, if the mic works, but sounds "thin" or "tinny" and is run with the balanced XLR input, check for an open in either pin 2 or 3 of the cable.

Last edited by mxslick; 03/26/07 02:48 PM. Reason: To add caution

Stupid should be painful.
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Originally Posted by JoeTestingEngr
It seems to me that the original concern was someone else thinking that an amplifier was bad. Do we all agree that it is usually very easy to cause a very loud hum at the speakers of a working amp, without a whole lot of effort at a mic input? Just thinking it's easier to show that an amp is working than that a mic is not.
Joe

Quite true Joe, how about just making up a simple oscillator (tone source) to prove that the amp is in fact working?. smile

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Thanks Tony! I just had a vision of a grounded artist testing the amp with a hot chassis. I think the yell and hum together is known as the Frampton (Peter) Effect. "Do you feel, feel like I do?"
Joe

Joined: Aug 2001
Posts: 7,520
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Agreed. An ohmmeter will verify a circuit through the coil, but it can't tell you if the mic has had a knock and the coil is jammed etc. Easiest way by far is to plug into a known good amp.

Quote
Use caution if the amplifier is old or of questionable condition. Be sure you're not grounded in case there is a chassis leakage problem!!


Ah.... On the subject of the "finger-injected" test signal that reminds me of this one:

Famous Last Words #249:
"Of course the top cap is the grid."

Joined: Mar 2003
Posts: 97
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Thanks for all your help.

Turned out to be a bad mic and a bad amp.

Thats the short version of the story, after we figured out which amp went with which mic


Joined: Oct 2004
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Originally Posted by pauluk
Famous Last Words #249:"Of course the top cap is the grid."


That reminded me of an old audio amp I had to service for a drive-in theatre..it was made by Quad-Eight, and had integrated circuit (IIRC) input stage, transistor driver stage and TUBES (valves for our non-U.S. readers) for the output!

Rated 350 watts into 4 ohms mono, it would easily drive a 1 ohm load (typical of a field of drive-in speakers) with an output close to 900 watts! (Yes, I did measure it, and the output was fairly clean, with only minor clipping. I also destroyed three sets of dummy loads in the process.)

It weighed around 40 lbs. and had this charming sticker on the top of the chassis:

"WARNING : Lethal voltages inside and on grid caps. To be serviced by qualified persons only."

I think I still have the schematic for the beast in my files (somewhere).


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