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Old 5th February 2013, 01:18 AM   #1
Limp is offline Limp  Norway
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Default Piezo stereo preamp

I'm currently putting together a contact mic system to record various instrument and environmental sounds using 20mm piezo discs. Now I've come to the preamp part and need a little help. I'm very new to this.

This design has been chosen: megalithia.com/sounds/tech/piezo/opamp.html, simply because it seems to be the only one with decent documentation.
I know how to follow a simple circuit, what I need help with is how to turn it into a stereo amplifier. I suspect this is stupidly simple, but as I said, I'm very new to this.
Sincerely sorry for all the fuss.

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Old 5th February 2013, 08:32 PM   #2
Tesla88 is offline Tesla88  Italy
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To make it stereo , just duplicate the circuit , ground must be common to both circuits , you can use a dual op-amp ...
What i doubt is linearity , piezo are not that linear IMHO so i would use a circuit with a frequency response inverse to the piezo response , this would compensate the lack of piezo giving a quasi-flat curve.
I bet better conditioning circuit for piezo exists...
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Old 5th February 2013, 09:31 PM   #3
JMFahey is offline JMFahey  Argentina
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How many mics will you use?
How will you attach piezos to instruments?
Can you give an example of an instrument or "thing" whose sound you want to catch?
Thanks.
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Old 5th February 2013, 10:41 PM   #4
Limp is offline Limp  Norway
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Thanks for the replies.
I have two discs per channel, connected in series, attached to various resonant surfaces. A guitar, a tin flute and a steel plate.
Currently they're just attached with some mild adhesive directly on surface, but I have some disc neodymium magnets in the mail.

This circuit seemed like a good compromise between simplicity and performance. I intend to keep this at a modest hobby level.
Given the noise performance documented I believe moderate frequency non-linearities can be ironed out post ADC.
Any recommendations for a suitable dual op-amp?

Last edited by Limp; 5th February 2013 at 10:44 PM.
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Old 5th February 2013, 11:46 PM   #5
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Quote:
Originally Posted by Limp View Post
Thanks for the replies.

Any recommendations for a suitable dual op-amp?

I am out-of-date on the latest [audio] op-amps but I am sure you can find on other threads. I would think low noise would be critical. Do you want single-ended or dual? Will they be battery powered?
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Old 6th February 2013, 01:01 AM   #6
Limp is offline Limp  Norway
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Dual, battery powered.
Problem is that all the piezo preamp designs I've found have been mono, and I don't really know enough about circuit design to discern desirable op-amp properties.
I could of course just double up as proposed by Tesla. It would be the simplest solution, I suppose. If I change the op-amp I'd have to adjust the rest of the circuit too.
What about power, do I need two batteries?
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Old 6th February 2013, 01:42 AM   #7
JMFahey is offline JMFahey  Argentina
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TL072 is the workhorse if the Industry.
Very good, available anywhere and inexpensive precisely because it's so much used.
And it's a dual Op Amp, so it lends itself to your stereo setups.
One detail: your life will be easier if you connect Piezos in parallel, rather than in series.
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Old 6th February 2013, 02:06 AM   #8
Limp is offline Limp  Norway
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Quote:
Originally Posted by JMFahey View Post
TL072 is the workhorse if the Industry.
Very good, available anywhere and inexpensive precisely because it's so much used.
And it's a dual Op Amp, so it lends itself to your stereo setups.
One detail: your life will be easier if you connect Piezos in parallel, rather than in series.
But won't that only increase the already problematic capacitance?
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Old 6th February 2013, 04:51 AM   #9
Limp is offline Limp  Norway
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Alright, reading has been good. I feel like I'm about ready to start ordering parts, but first.
What adjustments do I need to do to make the above circuit more adapted for the TL072.
I'm in particular uncertain about R1, 2 and 3 at the input (determining input Z as I understand it), and the feedback topology.
And what's the role of C2 in this scheme?
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Old 6th February 2013, 01:18 PM   #10
JMFahey is offline JMFahey  Argentina
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Quote:
What adjustments do I need to do to make the above circuit more adapted for the TL072.
None.
By definition "all Op Amps are the same" , which holds true 90% of the time.
In *some* borderline applications (very high or low supply voltage/much lower than usual noise/ultra low power consumption/etc.) you may need to use a special one, but the concept besides Op Amps is "Universal gain blocks".
In this case, requirements are easily met by most.
Only adaptation you'll need is pinout: shown is for a single Op Amp, while TL072 has 2 inside, with different pin numbers.
Quote:
I'm in particular uncertain about R1, 2 and 3 at the input (determining input Z as I understand it), and the feedback topology.
Just build it as is.
Input Z is 1M (fine) and gain is 10X, which is probably right.
Piezos glued straight to the vibrating surfaces produce strong and clean signals.
Quote:
And what's the role of C2 in this scheme?
You are using an Op Amp meant for dual supplies (say, +/-9V if you run from batteries) with a single one.
Then you need to "split" those 9V into something equivalent to 2 4.5V supplies in series, which is achieved using R2/R3.
You need to couple that center point to ground, using C2.
I suggest you read something about Op Amps or you'll find a problem every 5 minutes
Maybe somebody can suggest a tutorial .
As I understand it, you'll build 2 preamps, and feed those outputs into some kind of stereo mixer, is that so?

Quote:
But won't that only increase the already problematic capacitance?
In this case, increasing is good, because it lowers source impedance .
What was problematic was wiring them in series, because it lowers capacitance, makes source impedance higher, and complicates a lot proper shielding of the "floating" one.
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