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Mic Impedance

Discussion in 'Recording In Progress' started by Martin R, May 16, 2018.

  1. Martin R

    Martin R Friend of Leo's Silver Supporter

    Jun 26, 2008
    Albuquerque
    I know changing impedance on the preamp changes the gain and sound of a mic. So what's going on when I turn the dial?
     

  2. Obelisk

    Obelisk Tele-Holic

    812
    Apr 1, 2013
    NW USA
    Someone far more knowledgeable than me will hopefully chime in, but you are changing the load impedance of the input when changing this switch. The basic rule of thumb is the optimal load at input should be 4-5 times higher than the output load coming from the mic(some sources will claim 10:1 ratio being optimal). I believe most mics with a transformer are commonly 150-200 ohms hence why much equipment comes with a 1000 ohm input. Some of the transformerless mics get down to 25-30 ohm level and they tend to have way more level that sometimes requires a pad. Usually you don't need to play with this switch much though. In my experience the impedance is most important when using ribbon mics. If the impedance isn't in the right relationship, you will have to boost the gain control beyond reason and the frequency response is compromised(especially the lower frequencies). Changing the value of the load impedance shouldn't hurt the mic or preamp. If you are using a pre with variable impedance, switch the setting until you find the best sound.
     
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  3. paratus

    paratus Friend of Leo's Ad Free Member

    Dec 2, 2010
    Michigan
    Increasing the gain just means you are amplifying the signal more. In some preamps, that increased output will start to include distortion byproducts, which can add warmth, or even a bit of hair. If you have an o'scope, you can look at your waveform as you increase the gain. If you take too much off of the top of the waves you will hear that.

    Some preamps are not real graceful as they distort, adding intermodulation distortion that is not a harmonic of the signal. That sounds bad.

    Also consider that as you increase gain, you are hitting the next stage in your signal chain harder. In most cases that will be your record interface. Smacking the input of your interface hard is not the same as hitting the input on a nice analog console, so make sure you aren't over driving it. If you have meters, use them.
     
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  4. codamedia

    codamedia Friend of Leo's

    Apr 4, 2009
    Western Canada
    As @paratus says... the dial (assuming you mean the "gain" or "trim" control) does not change the independence, it simply changes the input level. The input impedance is determined by the two input options... low (XLR or TRS 1/4") or high (typically TS 1/4") impedance.

    The common approach with "turning the dial" is to use your pfl to accomplish a nice strong signal that doesn't clip.
     

  5. Martin R

    Martin R Friend of Leo's Silver Supporter

    Jun 26, 2008
    Albuquerque
    It sounds like you're talking about volume...gain and trim. My ISA One has four impedance selections plus the usual gain and trim.
    Page 14 of the manual describes the effect each position has on various mics.
     
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