Piezo Phantom Preamp

Matt CookElectronics7 Comments

This device is for amplifying simple contact microphones made from piezo discs. Piezoelectric transducers, meant as low quality buzzers can also be used as simple contact microphones, allowing the budding phonographer to listen to a hidden world of internal vibrations.

The problem is that they have grounding issues and pick up a lot of interference and are therefore noisy. Also, when connected directly to a jack plug they present an impedance mismatch and lose a lot of low end frequencies. This circuit aims to fix these problems.

The circuit design is based on the the original design by Alex Rice, documented here. It incorporates improvements from Stompville allowing the circuit to offer significantly better low frequency response, who knew such low end sounds could be captured with a piezo!

Like similar designs it fits into an alloy box to be used as a modular piece of kit for field-recording or performances. It is phantom powered, so requires no battery to be used with the Tascam DR-100, and incorporates two preamps for use with two piezos at a time or a stereo pair.

The circuit design is based on the the original design by Alex Rice, documented here. It incorporates improvements from Stompville allowing the circuit to offer significantly better low frequency response, who knew such low end sounds could be captured with a piezo!

Like similar designs it fits into an alloy box to be used as a modular piece of kit for field-recording or performances. It is phantom powered, so requires no battery to be used with the Tascam DR-100, and incorporates two preamps for use with two piezos at a time or a stereo pair.

Ingredients:

R1, R2, R3, R4 – 3M3
R5, R6, R7, R8 – 220
R9 – 150
R10 – 1k
C1 – 680p polypropylene
C2, C3 – 22n polyester
Q1, Q2, Q3 – 2N3819 N-channel JFET

2 x stereo minijack socket
2 x stere jack socket
Alloy box

The test microphone is based on recommendations from Alex Rice. The piezo element is wrapped in electrical tape, then in copper tape which is grounded to the shield in the cable. Effectively the entire signal path is shielded, including the piezo, wires carrying audio signal and the circuit, all the way to the recorder – It’s quiet! It’s a balanced signal any interference it does pick up is rejected.

The mic plugs into the box via stereo minijack, so different ones can be used, probably some instruments will get made by gluing piezo discs onto various objects.

The test microphone is based on recommendations from Alex Rice. The piezo element is wrapped in electrical tape, then in copper tape which is grounded to the shield in the cable. Effectively the entire signal path is shielded, including the piezo, wires carrying audio signal and the circuit, all the way to the recorder – It’s quiet! It’s a balanced signal any interference it does pick up is rejected.

The mic plugs into the box via stereo minijack, so different ones can be used, probably some instruments will get made by gluing piezo discs onto various objects.

7 Comments on “Piezo Phantom Preamp”

  1. Another hint is that the wire itself can be a source of noise. Try hitting a section of cable you ll find that it is microphonic. I try and keep the distance between the piezo and it s pre-amp nice and short which is easy with a phantom powered preamp.

  2. Love, love your site. Been researching piezo preamps for months and this is the exact configuration I’ve been looking for. Would it be possible to add a gain knob? Would there be any potential pitfalls of doing that? I know nothing about electronics and am starting from scratch, but will be studying your schematic closely. Again, great site and really cool projects.

    1. Thank you for such kind words.

      The best place to adjust the gain would be at the mixer or recording device that is providing the phantom power. Because this is a balanced circuit there is no obvious place to add a gain control, changing R7 and R8 will have an affect on gain so you could experiment by replacing these with a dual-gang trimpot but this would make the circuit unbalanced and therefore increase the noise.

      A design more suited to adding a volume control is the Unbalanced Piezo Preamp or Altoids Peizo Preamp at Stompville.

      1. Been a while, but I have one more question if you don’t mind.

        I noticed you used stereo input jacks for plugging in the piezo. Are you using mono cable/TS plug as opposed to stereo cable/TRS plug? Not a lot of info out there about mixing the two, but seems to have worked for you.

        The Zeppelin Design Cortado mic seems to use stereo mic cable without soldering the ground, but their’s doesn’t use an input jack, it’s hardwired to the PCB.

        http://www.zeppelindesignlabs.com/wp-content/uploads/docs/ZDL%20CORTADO%20MKII%20ASSEMBLY%20and%20OWNERS%20MANUAL.pdf

        Grateful for your thoughts!

  3. I have seen those designs and have considered them. Your design has a dual mic input which is the first I’ve seen and is what I’ve been wanting to build.

    Good to know about the balanced circuitry and lower noise. With this info I think the gain control won’t be necessary as I am running it into an audio interface with relatively nice preamps. Thanks for the info!

  4. Hi, nice design! Thank you very much for all that info.

    I want to build this device, and I have one question: What’s the board you used to replace the PCB? Is that a kind of prototyping board where you can solder components?

    Cheers.

    1. Hi Clara,
      I don’t have the technology to print PCBs so I use stripboard which is basic prototype board you can solder to.
      The reason it is orange is because I painted a lot of it for a project a while back.

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