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Old 3rd January 2003, 06:17 PM   #21
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Iíve tried 1/32Ē piezo material between the bridge and the body and Iíve tried it in the bridge. Our guitar player is also a violin maker and has made all of our violins. He explained to me the reason I couldnít get the right sound, and the reason the violin player didnít like the sound, acoustically, was because the pickup changes the violin. The pickups that use the bridge cause problems. It ruins the instrument from being played acoustically. Just the pick-up sitting under the bridge changes the sound of the violin too much. The bridge of the violin is a mechanical preamp/EQ for the instrument's body which amplifies the sound. By placing anything in or under the bridge changes the violin's sound too much for the musician. It ruins it from being played unamplified. We found that one pick-up under each foot of the bridge and one pick-up in either side of the bridge, gave us a harsh, brittle sound. It sounded like the violin was being played in a long pipe. By completely removing the side pick-up (only using the 2 under the feet) got rid of a lot of the harshness. 2 pick-ups worked better than the 3 but still not a good sound even with EQ'ing. By removing the 2 pick-ups under the feet of the bridge and placing them in the side slots of the bridge, we diminished some of the pipe sound, but the brittle, harsh, sound was really bad. The problem with the acoustic properties of the violin does NOT return by moving the pick-ups from under the bridge to into the sides of the bridge. It apparently is not a problem of isolation between the violin body and the bridge but more of a problem of change the vibrating characteristics of the bridge.
I have not used PVDF foil and maybe should try some before giving up on bridge pickups.
As for input, a current input should be good. This was used for measurement inputs like for vibration transducers. Protect it very well by clamping with fast diodes to supply rails.
Will you please explain this more? I do not understand what you are telling me. I am just a beginner at electronics.
Thank you,
steve
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Old 3rd January 2003, 07:59 PM   #22
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A current input is the input of an I-V convertor like it was/is used on DA convertors. It is nothing more than when you take an inverting opamp, and remove the input resistor, connecting the piezo direct to the inverting terminal of the opamp.

A piezo works by shifting charge. So it represents a current source. That's why it is so strongly influenced by cable and input impedance, it has a very large output impedance.

Protection is needed because if you tap a piezo hard (even the 12mm ones) they can easily output 10 to 30 volt. Now most opamps do not like this at their inputs, certainly not if you use the I-V stage, where the input is directly connected to the inverting terminal. So you put some very fast, low capacitance diodes from the input to the supply rails of the opamp. In normal operation, the diodes are reverse biased, put if the input voltage rises above/below the supply, they clamp the input to the supply rail, preventing damage. Something like a 1N914B, or BAV99 or BAT54 should do.
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Old 3rd January 2003, 09:50 PM   #23
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I once experimented with such a cheapo transducer on a Kora, where I placed it under the bridge. Since this instrument has a soft thingie between the bridge and the "body" the transducer's resonances are damped quite well.

I think a current input would damp resonances even better. I don't think that it is necessary to connect the protection diodes between the input and the supply lines. The inverting input of an opamp in this circuit represents a "virtual ground" so that the protection diodes can be connected antiparallel between the inverting input and signal ground. And don't even bother to question if this works at all, I already used it successfully in another situation !!!!!

Regarding the sound quality of commercial piezo pickups: Just have a listen to Victor Wootens "A show of hands". The tune "overjoyed" for instance is recorded using piezo pickups. The sound is very tranparent, the only disadvantage are the subsonics generated by the plucking, that make you fear for your woofers if you crank it up a little......

The transducer which Bob pointed at reminds me of a thingie called "C-Ducer" a colleague once used on an upright piano. It must have been resonant like hell on some frequencies because some keys sonded sharp as hell and other ones nice. It behaved very well on an acoustic guitar however.

Regards

Charles
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Old 4th January 2003, 07:05 PM   #24
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A virtual ground is just that: virtual. In fact this is only near ground as long as the feedback loop can follow.
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Old 4th January 2003, 08:02 PM   #25
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Quote:
In fact this is only near ground as long as the feedback loop can follow.
Which it will always be under normal conditions, i.e. if the opamp insn't driven into clipping or if it's max slew-rate isn't reached.

If these two conditions happen, then the influence of the two diodes doesn't matter anyway !!

As I already mentioned before: I used this principle with success. I used it to protect the input of an inverting OP-AMP, that is preceeded by a tube stage, from turn on- and off- transients within a hybrid bass preamp.

Regards

Charles
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Old 4th January 2003, 08:03 PM   #26
Variac is offline Variac  United States
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havoc:
I searched on PVDF foil and didn't get a supplier.
Do you have any hints? I assume that this is being used in some tweeters such as that old Pioneer cylinder and maybe some Gallo ones? Might be fun to play with. Sorry to go back to a DIY audio topic
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Old 5th January 2003, 10:55 AM   #27
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Variac,
I donít know if it will work as a tweeter but Trout Cove Lutherie sells PVDF Piezo Film Sensors.
http://tclutherie.hypermart.net/page4.html
This site has a lot of information on using the stuff which you can get to by clicking the links on the left side.
steve
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Old 6th January 2003, 12:37 PM   #28
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Don't have one to be honest. We had some of that foil in the lab when I was at university. But that is some time.

Murata has some nice looking piezo tweeters.
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Old 20th January 2003, 09:18 PM   #29
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Thumbs up condenser mic for guitar

I have build mic using this circuits
http://www.linkwitzlab.com/sys_test.htm#Mic

http://sound.westhost.com/project93.htm

I used the wm55 cardiode mic for guitar omni make feedback to easy.

If and body have good tips about condenser mic let me know

best regards jc dk.
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Old 21st January 2003, 02:11 PM   #30
csd is offline csd
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Default PVDF transducers

Hey guys,

I work as a development engineer for Measurement Specialties, the company that manufactures piezo PVDF film. Here is a link to the website:

http://www.msiusa.com/piezo/index.htm

We build numerous custom film products that are used in instrument pickups (guitar, violin, drum, etc.) and speakers. Please don't ask for details, as these designs are typically proprietary to the company who sells the end product.

Piezo film (or disks) can make wonderful instrument pickups, but as they say, the devil is in the details. The film itself is a transducer, that is, it converts mechanical strain into electrical charge. The frequency response is linear to far below and far above the audio range. It is possible to generate over a thousand volts by stretching the film. To make a quality instrument pickup requires careful consideration of the transducer shape, interface materials (including adhesives), and mechanical clamping conditions. All of these will affect the final frequency response!

By all means experiment - if the pickup doesn't sound right, try gluing or clamping it someplace else. Also experiment with adding different materials to dampen it. Some of the best designs have come from a lot of trial and error.

Any pickup will require some form of buffering, and this will also affect the frequency response. Remember, the transducer is basically a capacitor, and any induced strain will produce a charge across the capacitor. We can either use a charge-to-voltage converter (charge amp), or a high-impedance voltage amp (since V=Q/C). Input protection may be necessary, since it is not difficult to generate high voltages.

A voltage amp can be just a simple JFET source-follower or a non-inverting JFET-input opamp circuit. The input impedance and the capacitance of the film set the low-frequency response. A voltage amp is usually placed very close to the transducer to minimize stray capacitance which would lower the signal level.

A charge amp is useful if you want to place the amplifier far from the transducer, since additional capacitance won't attenuate the signal.

Take a look at this document for more technical info:
http://www.msiusa.com/techman.pdf

Craig
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