Need a Piezo buffer with Balanced, mic level output - diyAudio
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Old 31st March 2011, 10:33 AM   #1
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Default Need a Piezo buffer with Balanced, mic level output

I require a phantom powered piezo surface mount mic buffer with balanced mic level output.

I have been searching the net for a few weeks now, but can only find unbalanced, instrument level buffers. It seems silly to convert the inherently balanced signal from a piezo to unbalanced with a buffer, then back to balanced with a DI box.

Having the whole process sorted with one box would be very convenient.

Would this system create more problems than it solves?

Cheers,
LEVY

Levy Sound Design
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Old 31st March 2011, 12:26 PM   #2
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Most active DI boxes will accept piezos with no problem; of course, you'll have to apply the equalisation after.

I've never seen a circuit that actually took in a piezo (although not inherently having any preference for grounding) as anything but a ground and signal, ie, grounding one end. It would seem that doing a multi-meg input impedance on a balanced circuit is not worth the effort.

There used to be some contact mics – Barcus Berry, if I remember correctly – which could be bought with balanced preamps (= buffers, sometimes, with mix possibilities for several sources), but I'm not even sure these were phantom powered. And I do know the pickups were run in unbalanced.
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Old 29th April 2012, 10:19 PM   #3
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Default True balanced output from a piezo : Use two

Hey

I found this question, and wanted to post a follow up, in case someone comes by with the same question at some point.

Sandwiching two piezo elements will give you true balanced output.

I have just discovered this for myself, and I have written a little guide to the wiring here:

Noise free, balanced piezo microphone - Christian Liljedahl

Kind regards
Christian Liljedahl
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Old 30th April 2012, 03:22 AM   #4
Simon B is offline Simon B  United Kingdom
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Quote:
Originally Posted by bigredlevy View Post
I require a phantom powered piezo surface mount mic buffer with balanced mic level output.
......
Having the whole process sorted with one box would be very convenient.

That last sentence is already on my list of future sigs - full credit will of course be given

The essence of the interference rejection capability of balanced systems is the balance of impedances in the two legs. Piezo elements have very high frequency-dependent impedance, and no centre-tap.

As you're using them as contact mics, a scheme involving mounting two, back to back, along the lines of Christian's hydrophones might not be appropriate, for this application.

I suggest that you run the connection from the element in twin and shield cable, connecting the shield to ground only at the preamp. With regard to the circuit, the left hand part of this might be the thing:

http://www.jensen-transformers.com/as/as004.pdf

Possibly try increasing R1 to several megohms, whether any benefit shows in your frequency response depends on the type of piezo element.

As for the "in one box" part, that's up to you.
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