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Limp 5th February 2013 01:18 AM

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.

http://www.megalithia.com/sounds/tec...o/piezoopa.pdf

Tesla88 5th February 2013 08:32 PM

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...

JMFahey 5th February 2013 09:31 PM

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.

Limp 5th February 2013 10:41 PM

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?

KMossman 5th February 2013 11:46 PM

Quote:

Originally Posted by Limp (Post 3357597)
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I am out-of-date on the latest [audio] op-amps but I am sure you can find on other threads.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?

Limp 6th February 2013 01:01 AM

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?

JMFahey 6th February 2013 01:42 AM

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.

Limp 6th February 2013 02:06 AM

Quote:

Originally Posted by JMFahey (Post 3357763)
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?

Limp 6th February 2013 04:51 AM

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?

JMFahey 6th February 2013 01:18 PM

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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