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Microphone Amps

Discussion in 'Amps [BG]' started by Journey55, Sep 9, 2013.


  1. Journey55

    Journey55 Supporting Member

    Joined:
    Jun 10, 2011
    Location:
    Melbourne, Florida
    Hello, I was wondering what some inexpensive microphone amps would be (like with an XLR input) I was gonna use an extra guitar amp I have lying around but it doesn't have an XLR input so I don't think it'll work. By inexpensive I mean, I'm in college and would prefer to spend as little as possible, it doesn't have to be really loud, just enough to be heard (my little 38w amp is doing fine keeping up with everything bass-wise so around that loudness). Thanks!
     
  2. TomB

    TomB Supporting Member

    Joined:
    Aug 24, 2007
    Location:
    Burlington, Vt.
    Buy an XLR TO 1/4" adaptor, $7 or $8. Plug in your mic. Don't expect great results without matching impedance, but it should work, anyway.
     
  3. Journey55

    Journey55 Supporting Member

    Joined:
    Jun 10, 2011
    Location:
    Melbourne, Florida
    Thanks, that's a lot cheaper! I feel a little dumb I didn't think of it, but thanks
     
  4. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    Joined:
    May 6, 2000
    Location:
    Groom Lake, NV
    Disclosures:
    Independent Contractor to Bass San Diego
    Several companies make adapters that convert low-impedance XLR inputs to high impedance quarter-inch. I believe Shure makes one.
     
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  6. deathsdj

    deathsdj

    Joined:
    Sep 18, 2010
    Location:
    Wichita, KS
    Agreed on the adapter. I use one when I'm DJing sometimes depending on what board is running. Often after a couple of years I'll wear out the balanced line for the mic so I use an adapter and move to the mic / line section of the board.

    Cheers!

    MJW
     
  7. T-Bird

    T-Bird

    Joined:
    Apr 29, 2007
    Location:
    Finland (Northern Europe)
    Hi.

    Depending on the amp and the mic, not necessarily so dumb at all.
    The impedance mismatch may cause severe frequency response anomalies that are annoying at least.

    OTOH, on some amps the difference is inaudible.

    Obviously, no harm will come to either of the equipment because of that mismatch, and in Your intended application, the reduced performance is of no concern.

    BTW, on cheaper end of the mic price spectrum, XLR connector is not a guarantee of a low impedance mic.
    Also, quite a few vintage mics are switchable between HiZ and LoZ operation.


    Regards
    Sam
     
  8. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    Joined:
    May 6, 2000
    Location:
    Groom Lake, NV
    Disclosures:
    Independent Contractor to Bass San Diego
    You're going to have a tough time finding a modern high-impedance microphone. They're so scarce it's nothing to worry about. The first clue that it's a high-impedance mic is the quarter-inch plug on one end of the cable that comes with it.
     
  9. Projectile

    Projectile

    Joined:
    Feb 5, 2009
    This is not made to match impedance. What it is designed to do is give the signal an extra 24db of gain while (potentially) sacrificing high frequency response. That's why they call it a "line matching transformer". It matches the signal levels, not the impedance. It provides extra gain at the expense of making the impedance match worse!

    Almost any guitar/bass amp in existence will have a high enough input impedance to handle most microphones just fine. Guitar pickups typically have much higher output impedance than most mics, so guitar/bass amps are designed to not load down the source and work more than fine with mics (though they are not ideal mic amps for other reasons that have nothing to do with impedance). There are always exceptions to the rule, but they are few and far between.

    Just use a regular XLR to 1/4" adapter, keep your cable runs short, and you will be fine. If you find your mic is not driving your guitar preamp hard enough to get a strong signal level, then consider buying one of those line matching transformers mentioned above to increase the signal level from the mic. But you do not need a transformer to match the impedance. That's absolute nonsense.

    No impedance matching is required in this situation. A guitar amp is a better impedance "match" to a microphone than most microphone preamps! There is so much ridiculous misinformation about impedance matching on the internet that it makes my head spin.
     
  10. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    Joined:
    May 6, 2000
    Location:
    Groom Lake, NV
    Disclosures:
    Independent Contractor to Bass San Diego
    Notwithstanding my link to the "Shure A85F - Low to High Impedance Microphone Matching Transformer - In-Line XLR Female to 1/4" Male (Barrel)"
     
  11. Projectile

    Projectile

    Joined:
    Feb 5, 2009
    Yeah, the description of that product is really terrible. If you look carefully, though, nowhere does it actually say it matches the impedance. The device is designed to work with a very low impedance output going to a very high impedance input, as it says. This is a requirement. Otherwise, you wouldn't be able to get a passive gain boost without loading down the input. It trades a superior impedance match for a slightly worse match (but still perfectly adequate) in order to boost gain.

    For some reason, no sound tech I've ever encountered seems to understand this. They all think its an impedance issue, when it's a gain issue. The whole term "impedance matching" in this context is a misnomer. The only time you want to literally match the impedance of an output stage to an input stage is when you are trying to achieve maximum power transfer, such as when driving a speaker or an antenna. The connection between a microphone and a guitar amp, like the connection between most audio devices, is a bridged connection. In this case, we say we have an impedance "mismatch" when the output impedance of one stage loads down the input stage of another device. So a good "impedance match" means that the input impedance of one device is at least 10x greater than the output impedance of the source device, the higher the better. An ideal bridge connection has an input impedance of infinity and a source impedance of zero.
     
  12. seanm

    seanm I'd kill for a Nobel Peace Prize!

    Joined:
    Feb 19, 2004
    Location:
    Ottawa, Canada
    Another solutions is the Art Tube MP. Only $50 NEW and works as a mic preamp or DI or instrument preamp.
     
  13. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    Joined:
    May 6, 2000
    Location:
    Groom Lake, NV
    Disclosures:
    Independent Contractor to Bass San Diego
    I guess Shure needs to hire some qualified engineers.

    From their site:

     
  14. Projectile

    Projectile

    Joined:
    Feb 5, 2009
    I don't see what you are getting at. The engineers don't write the ad copy. There is nothing factually wrong with it anyway. It's just misleading.

    What I'm explaining here is very basic electrical engineering stuff. You can look it up in any amplifier design textbook.
     
  15. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    Joined:
    May 6, 2000
    Location:
    Groom Lake, NV
    Disclosures:
    Independent Contractor to Bass San Diego
    Don't confuse me with facts.
     

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