trying to understand "impedance" and "line level"

Discussion in 'Microphones (live or studio)' started by Unregistered, Aug 12, 2011.

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  1. Unregistered

    Unregistered Guest

    I'm trying to understand inputs. I have an electric bass guitar that I'm running into a UCA202. The bass is passive and I don't have a pre-amp. Going from the UCA202 to my DAW (Logic Pro), the bass sounds okay.

    I also have an acoustic violin. It has a piezo contact pickup that is clamped to the bridge. I'm running that into the same UCA202 and DAW. It sounds weak and thin. I was told that I have an impedance mismatch and should run through a mixing board or pre-amp.

    I will do that but I don't understand the problem that I'm addressing. I've googled on the term and read a few pages but they're not making sense to me. I'd appreciate pointers to any really basic sources that I can read and learn from.
  2. Kapt.Krunch

    Kapt.Krunch Well-Known Member

    Nov 21, 2005
    I don't know what the input impedance is on that thing is since, like everything Behringer, the link on their website to download a manual stopped working.

    Read the following (at least down through "Impedance Considerations of Electric Guitars"):

    Understanding Impedance

    The impedance of the interface input needs to be high enough, and the output impedance of the bass and fiddle pickups need to be low enough. It seems counterintuitive that you need a lower output impedance to drive a higher input impedance to get a good signal, but that's the way it is. You are amplifying voltage, instead of power.

    One of the reasons a bass with active pickup electronics can sound louder and more clear is that the circuitry actually lowers the bass' output impedance (in addition to other factors). A passive pickup is more high impedance than a passive. See the following, which is the first thing I stumbled across that explained it a bit:

    Active VS. Passive

    When one device has a higher ouput impedance than another, the higher output impedance one is closer to the input impedance of the device they connect to. Conventional wisdom states that for a good signal, the input impedance of the receiving device should be at least 10 times the output impedance of the device feeding it. So, if the fiddle pickup has an output impedance of, say, 100 ohms, and the input impedance of the interface is only...say..600 ohms, you don't have your minimum 10x multiple to properly drive the interface, and it will sound weak. Even if it has an input level knob, all you'll do is boost noise surrounding too low of a signal.

    Maybe the bass pickup is fine, and the fiddle pickup isn't? Maybe it's basically "loading down" the fiddle pickup too much?

    So, to start, do some homework. Dig out your manuals, and/or find the models of your interface, your bass pickups and your fiddle pickup. See if you can find specs for the input impedance on the Behringer, and the output impedances on the all the pickups. If there is too much mismatch, you'll need to amplify with a preamp, or possibly, a Direct Box, (though I'm not sure a Direct Box would do any good on that Behringer. I don't know what the inputs are).

    I THINK I got that right. If not, please correct, anyone.

    Good luck,

  3. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    Jan 23, 2010
    Boulder, Colorado
    Home Page:
    For a piezo pickup you'll need something with an impedance on the order of a megohm. Mixing board line inputs may not be that high. An active direct box or some kind of active preamp would be best.
  4. Boswell

    Boswell Moderator Distinguished Member

    Apr 19, 2006
    Home Page:
    What you were told about an impedance mismatch for your violin pickup is correct. The UCA202 has a line-level input with gain but does not have an "instrument" input, i.e. a high-impedance input suitable for piezo pickups. You will need to put a pre-amp between the pickup and the UCA202, or use a pre-amp that has a computer interface to by-pass the UCA202. You should look for one that has specifically an instrument input, preferably with an impedance of 1MOhm or more. I use an Audient Mico, which is a bit of an overkill for a single channel of piezo input, but it gives you a second channel you can use for a mic and it connects directly into the optical input of a Mac computer. I'm guessing you are using a Mac to run Logic Pro.

    That said, piezo pickups on fiddles do not in general produce good quality sound, although the bridge type are better than the strap-on ones. The best close sound is got through a specialised clip-on microphone, and the best of those is the DPA 4099V.

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