Build a Low Noise Contact Microphone (inducer/transducer)

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I am a sound designer who's studied computer game development, run my own sound design business a...

Intro: Build a Low Noise Contact Microphone (inducer/transducer)

This instructable will show you how I built a great low noise contact microphone for my friend's double bass!

Thanks to the transformer, the microphone has a very good signal to noise ratio. That means that the inherent hum/noise of contact microphones is almost gone! The microphone can also be used in reverse to play back audio.

This instructable is inspired by Nicolas Collins Hack of the Month Club -- Project #6: Drivers & Transducers . Please check out his other videos, they're pretty awesome!

Step 1: Materials

  • Piezo disc
  • Audio output transformer 1200/8
  • Glass jar metal lid
  • Cardboard
  • Pill bottle
  • 1/4 in (6.5mm) female phone jack
  • Shrink tube
  • Wires
  • Guitar cable 1 ft (30cm)

Step 2: Tools

  • Soldering iron
  • Hot glue gun
  • Wire cutter/stripper
  • Double sided foam tape

Step 3: Building

  1. The idea was to use the lid as a cover for the sensitive piezo disc. So the first thing I did was to hot glue cardboard inside the lid to make the piezo disc able to lay flat against any surface I attach it to.
  2. Next I reinforced the solder joints with hot glue and striped the microphone cable and tined the wires.
  3. Use heat shrink tubing to make the connection more rigid.
  4. Glue it in place.
  5. I made a hole in the bottom of the pill bottle and reinforced it with a washer. I fed the cable through the hole and tied a knot so it couldn't slip back out.
  6. The transformer is connected with the high ohm primary winding on the piezo side and the low ohm secondary winding on the amp side. Solder to the outer connections and insulate with heat shrink.
  7. I made another hole in the pill bottle lid and attached the phone jack. Connect the wires from the phone jack to the outer pins on the transformer's secondary winding.
  8. This is the circuit we are building.

Step 4: Info Break! Transformer, Hum and Stuff.

Ok, first off, I'm not an electronics expert. This is the first time i've ever used transformers in my builds. All I know is that when we're using a transformer with a piezo disc like this, we're doing something called impedance matching. The main difference I noticed, compared to contact microphones without transformers, is that the signal to noise ratio is much better. I think it has to do with having a non-galvanic connection between the piezo and the amp. According to the video link in the intro, this build can also be used in reverse to make the piezo behave like a driver. Attach it to different materials to make them vibrate and act like a speaker membrane.

Step 5: Mounting Etc

Almost finished! Just needs a paint job. I use double sided foam tape to attach it to stuff. This also helps a lot with sound quality.

Step 6: Placement

Try placing the mic in different places to find a good sound. The pill bottle sits under the bridge or tailpiece.

Step 7: If You Like This Instructable, Please Comment!

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    28 Discussions

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    blakehx

    2 years ago

    So should the outer wires of the 3 wired side go to the piezo or the amp?

    1 reply
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    simon.w.nordbergblakehx

    Reply 2 years ago

    You want the side with with lesser windings, or Ohms to be on the amp side. You want to go from high impedance to low. I think in your case the 3 wired side should go to the piezo.

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    blakehx

    2 years ago

    My audio transformer has 3 wires on the first side and 2 wires on the other (see pic)! How do I connect it?

    15, 4:34 AM.jpg
    1 reply
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    simon.w.nordbergblakehx

    Reply 2 years ago

    The middle wire is the center tap and is usually half the value of the outer wires. Try different connections, see what sounds best to you :)

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    PerlaV1

    3 years ago

    Would it be possible to make this for a violin?

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    tvil

    3 years ago on Introduction

    check if your audio gear supports balanced mic/line signals then you can use the secondary coil to drive a balanced input line. balanced signal does not get affected by noice coming from powerlines or other electric equipment(tranfered via
    electromagnetism) in same way than an unbalanced signal would get. :)

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    dafy1

    3 years ago on Introduction

    I found a pre-amp piezzo and it sounds perfect !!!

    I love it. Thank u verry much 4 ur tutorial ;)

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    dafy1

    3 years ago on Introduction

    What will hapen if I use a normal piezo and try to connect it directly with the female jack

    2 replies
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    simon.w.nordbergdafy1

    Reply 3 years ago on Introduction

    The piezo and the amplifiers input resistance will form a high pass filter. Leaving the mic sounding tinny.

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    dafy1simon.w.nordberg

    Reply 3 years ago on Introduction

    I tried to use it with my guitar on the amplifier and it wasnt loud i had to turn the volume all the way up to hear what i was playing. Im sory for my poor english

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    simon.w.nordbergdafy1

    Reply 3 years ago on Introduction

    The lid is just to make it look nicer.

    Have you tried using a pre-amp? Mine didn't need one but maybe amps are different in the pre-amp stage. Try using another transformer with different windings?

    I haven't tried this myself yet, but you can use a piezo without a transformer and instead use a piezo pre-amp. A piezo pre-amp can handle the high impedance from the piezo better than a normal pre-amp. Just search youtube for one. The JFET pre-amps seems to work ok. I know Hackaweek posted a video on how to build one.

    Also, f your amp has an overdrive channel, those usually have a higher gain than the clean channel.

    Good luck! :)

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    dafy1simon.w.nordberg

    Reply 3 years ago on Introduction

    i have to add that I didnt use


    • Glass jar metal lid or anything like that i just taped the piezo disk on the guitar
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    popaiscua

    3 years ago on Step 7

    I think that idea is good for classical guitar with nylon wires too!

    1 reply
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    dafy1

    3 years ago on Introduction

    guys i dont understand grounding can u please help me ?

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    Elusiverick

    3 years ago on Introduction

    I Just love this ! I've been waiting for something to use on my acoustic guitar. I would really appreciate more information, prehaps a circuit maybe {?} on your cigar box guitar & the arrangement of multiple pickups you mentioned.

    Very Well Done Sir !