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| research:contact_mics [2017/03/19 00:50] – [Impedance converters] mrkva | research:contact_mics [2019/05/15 12:09] (current) – old revision restored (2018/05/27 21:36) mrkva |
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| ==== Resonance ==== | ==== Resonance ==== |
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| Piezos do not detect vibration frequencies in a linear manner. Although they can have an extended frequency range, they have a characteristic resonance that varies with the diameter of the disc. This resonance is usually between 2 and 6 kHz, producing a "metallic" or "honky" sound. The resonance can be reduced by increasing the mass of the disc. However, this can adversely affect the overall sensitivity of the element. A good strategy is to increase the mass of the metallic part, while not excessively loading the central ceramic portion. | Disc piezos do not detect vibration frequencies in a linear manner. Although they can have an extended frequency range, they have a characteristic resonance that varies with the diameter of the disc. This resonance is usually between 2 and 6 kHz, producing a "metallic" or "honky" sound. The resonance can be reduced by increasing the mass of the disc. However, this can adversely affect the overall sensitivity of the element. A good strategy is to increase the mass of the metallic part, while not excessively loading the central ceramic portion. |
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| A comparison of two different bodies is demonstrated in the following video. Notice the "tone" of the piezo without additional mass. | |
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| | A comparison of two different bodies is demonstrated in the following video. Notice the "tone" of the piezo without additional mass: |
| <html><iframe width="100%" height="480" src="https://www.youtube.com/embed/wq8wnFRAPas?ecver=1" frameborder="0" allowfullscreen></iframe></html> | <html><iframe width="100%" height="480" src="https://www.youtube.com/embed/wq8wnFRAPas?ecver=1" frameborder="0" allowfullscreen></iframe></html> |
| ==== Interference ==== | ==== Interference ==== |
| Another way to express this is that the impedance of a contact mic is not "well matched" to typical audio inputs. Piezos transducers need to work at high impedance (over 1 MΩ). A typical line input of an audio mixer has input impedance of about 50 kΩ. This forms a 200 Hz high pass filter (HPF) with typical piezo elements. Worse yet, the microphone input on many mixers and preamps have around a 1.5 kΩ impedance. This results in a 1 kHz HPF, omitting bass frequencies entirely. | Another way to express this is that the impedance of a contact mic is not "well matched" to typical audio inputs. Piezos transducers need to work at high impedance (over 1 MΩ). A typical line input of an audio mixer has input impedance of about 50 kΩ. This forms a 200 Hz high pass filter (HPF) with typical piezo elements. Worse yet, the microphone input on many mixers and preamps have around a 1.5 kΩ impedance. This results in a 1 kHz HPF, omitting bass frequencies entirely. |
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| The solution is to "buffer" the contact mic. The circuit that performs this function is called an impedance converter. Strictly speaking, it does not necessarily "amplify" the signal. But it matches the output impedance of the piezo element to the input impedance of mixers and preamplifiers. This maintains the natural frequency response of the piezo transducer and reduces noise generation in the circuit. | The solution is to "buffer" the contact mic. The circuit that performs this function is called an impedance converter. Strictly speaking, it does not necessarily amplify the signal. But it matches the output impedance of the piezo element to the input impedance of mixers and preamplifiers. This maintains the natural frequency response of the piezo transducer and reduces noise generation in the circuit. |
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| ===== Impedance converters ===== | ===== Impedance converters ===== |
| [[http://hosatech.com/product/mit-129/|Hosa]] makes a typical adapter to convert from 50 kΩ, the typical impedance of the magnetic pickup of an electric guitar, to 200 Ohms, the typical input impedance of an audio mixer. This unit is hence not optimized for piezo disks, but might provide a partial solution. | [[http://hosatech.com/product/mit-129/|Hosa]] makes a typical adapter to convert from 50 kΩ, the typical impedance of the magnetic pickup of an electric guitar, to 200 Ohms, the typical input impedance of an audio mixer. This unit is hence not optimized for piezo disks, but might provide a partial solution. |
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| An **active impedance converter** contains an actual amplifier circuit, based on either Field Effect Transistors (FETs), op-amps, or a vacuum tube. Active impedance converters may be powered by batteries (DC), a standard outlet (AC), or the phantom power provided by the mixer. Active impedance converters are more complicated and are often more expensive than their passive counterparts, but they can be highly optimized to their task. By using a proper active impedance converter with your piezo contact microphone, characteristics of the mixer, particularly how much clean gain is available, become less critical. | An **active impedance converter** contains an actual amplifier circuit, based on either Field Effect Transistors (FETs), op-amps, or a vacuum tube. Active impedance converters may be powered by batteries, a standard outlet, or the phantom power provided by the mixer or the recorder. Active impedance converters are more complex and are often more expensive than their passive counterparts, but they can be highly optimized to their task. By using a proper active impedance converter with your piezo contact microphone, characteristics of the mixer, particularly how much clean gain is available, become less critical. |
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| |<html><iframe width="100%" height="480" src="https://www.youtube.com/embed/LXDjLeU8-kE?ecver=1" frameborder="0" allowfullscreen></iframe></html>| | |
| | Exploring the effects of active impedance converter | | |
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| | Video exploring the effects of active impedance converter: |
| | <html><iframe width="100%" height="480" src="https://www.youtube.com/embed/LXDjLeU8-kE?ecver=1" frameborder="0" allowfullscreen></iframe></html> |
| ===== Useful resources and products ===== | ===== Useful resources and products ===== |
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| * [[http://cafewalter.com/pzp-1/how-to-purchase/|Walter Harley]] sells the PZP-1 Piezo buffer for $49. Uses 9V battery. | * [[http://cafewalter.com/pzp-1/how-to-purchase/|Walter Harley]] sells the PZP-1 Piezo buffer for $49. Uses 9V battery. |
| * [[http://www.yannseznec.com/works/custom-contact-mic-preamps/|Yann Seznec]] builds boutique amps, based on Zach Poff's circuit. | * [[http://www.yannseznec.com/works/custom-contact-mic-preamps/|Yann Seznec]] builds boutique amps, based on Zach Poff's circuit. |
| * [[http://tritonaudio.com/bigamp-pi%C3%ABzo.html|Triton Audio BigAmp Piëzo]] is a phantom powered piezo buffer. | * [[http://tritonaudio.com/bigamp-pi%C3%ABzo.html|Triton Audio BigAmp Piëzo]] is a phantom powered piezo buffer |
| * [[http://pulplogic.com/product/ctact-box/|CTACT Box by Pulp Logic]] is a coin-cell powered impedance converter | * [[http://pulplogic.com/product/ctact-box/|CTACT Box by Pulp Logic]] is a coin-cell powered impedance converter |
| | * [[http://www.radialeng.com/stagebugsb4.php|Radial StageBug™ SB-4 Piezo DI]] is phantom powered "active DI optimized for piezo transducers" |
| | * [[http://www.stompville.co.uk/shop/26-phantom-piezo-preamp.html|Stompville Phantom Piezo Preamp]] is phantom powered piezo preamp module |
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| ==== Other resource ==== | ==== Other resources ==== |
| * [[http://www.musicofsound.co.nz/blog/the-first-rule-of-contact-mic-club|The first rule of CONTACT MIC club]] by Tim Prebble | * [[http://www.musicofsound.co.nz/blog/the-first-rule-of-contact-mic-club|The first rule of CONTACT MIC club]] by Tim Prebble |
| | * [[https://www.zachpoff.com/resources/building-contact-mics/?highlight=contact|Building Contact Mics]] by Zach Poff |
| | * [[https://www.zachpoff.com/site/wp-content/uploads/David-Dunn-Microphones_Hydrophones_Vibration-Transducers__Rolling_Your_Own__Dunn2007.pdf|Microphones, Hydrophones, Vibration Transducers: Rolling Your Own]] by David Dunn |
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| ==== Credits ==== | ==== Credits ==== |
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| Authors, editors and contributors (listed alphabetically): | Authors, editors and contributors (listed alphabetically):\\ |
| Jonas Gruska, Jerry Lee Marcel, Robin Parmar, Terry Setter | Till Bovermann, Jonas Gruska, Jerry Lee Marcel, Robin Parmar, Terry Setter\\ |
| | Videos by Jonas Gruska |