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DIY contact mic advice?

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  • DIY contact mic advice?

    I want to experiment with electro-acoustic sounds with a piezo contact mic, but one issue remains unresolved after thorough googling- do I need to build a pre-preamp for it? My audio interface has inputs rated at 220K ohm and provide phantom, but will this be enough to get low frequencies in? I've understood that low input impedance results in a (much too high) high-pass filter together with the piezo, but this is usually mentioned with line levels. Still some sources mention 1M ohm as appropriate.

    The preamp itself doesn't look very complicated to build, there's several design available and components are cheap, I just wonder if I should order them with the piezos and cabling to avoid disappointments.
    https://soundcloud.com/noiseconstruct/

  • #2
    i've only ever used a piezo straight into a mic input with, as you say, limited low end response. Sounded fine otherwise, depends on what you're recording I guess whether the low end is important. I plan on building a preamp sometime as I think that's the way to go (especially when you consider how cheap the components are). I think a design on Leafcutter Johns site was the one I plumped for. He sells pro quality mics too.
    Latest release: never to be repeated

    Hearthis | Soundcloud

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    • #3
      Thanks Gary!

      Was the mic input you used a normal preamp input, or a Hi-Z instrument input combo? Typical mic inputs have much (some up to hundreds times) lower input impedances than those meant to also for guitar DI. Anyway, mine is still 5x lower than most seem to recommend for piezos, so I'll build a preamp. I also want to try using two piezos in series.

      Leafcutter John used the the design by Alex Rice, which is also described in detail here: http://www.zachpoff.com/diy-resource...-preamplifier/ looks like a good option, but today I also found this:

      http://www.stompville.co.uk/shop/en/...-soldered.html which has matched transistors, something I'm not equipped to do myself.
      https://soundcloud.com/noiseconstruct/

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      • #4
        To be honest, it was a few years ago when I had a small Roland interface, I believe it was straight into the normal mic input (no phantom or anything).

        It's the low impedance that kills the bottom end; I *think* (not being a trained electronic engineer or anything...) that the piezos have a low capacitance that, coupled with a low input impedance effectively creates a simple high pass filter. From that logic I guess a hi imp. guitar input would work a lot better but never tried it. Obviously using a proper preamp will buffer the signal nicely for a full sound. That Stompville one looks a good deal if DIY isn't an option/your thing etc.
        Latest release: never to be repeated

        Hearthis | Soundcloud

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        • #5
          I thought I had posted this here but maybe it was somewhere else

          http://www.musicofsound.co.nz/blog/t...ntact-mic-club

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