Passive compressor for taming a piezo pickup

I do not think passive will be easy.  In the schematic I posted I decide to use 9VDC.  I have not built it it yet.

I used a cap pad like you might find in condenser microphones.

You adjust the cap "pad" value to the piezo pickup used so the JFET SF does not clip.  The stage can be opamp based or tube based etc.  The main idea is to limit the peaks with the cap "pad"
The JFET gate is cap coupled so no DC voltage from the circuit is coupled to the piezo pickup.  The cap value should be adjusted to the rest of the circuit.

The rest of the schematic is a simple passive diode clipper I just added because it is passive.  One could  use an EQ and/or compressor after the non-clipping first stage


 

gyraf said:

It seems that there was (at least some) consensus that the harsh sound you'd most often get from these systems isn't because of the pickups acoustic/mechanic properties themselves, but because of the very-very-large peaks in signal level at the attack of e.g. picking a string, which in turn overloads the preamplifier stage, if this is set to a gain that is anywhere usable for "the rest" of the signal. And this initial overload causes the annoying sonic signature of the piezo systems.

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This seems to be correct but the rest of the assumptions does not seem to be correct.

gyraf said:

So basically a problem in too-large a dynamic range. And compressing later on doesn't help much, as it tends to clip already in impedance-converter stage.

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No, the problem is with large amplitude of the signal from piezo transducer but this does not mean too large dynamic range - it's not the same.
There are two problems with piezo pickups:
1) very high output impedance - this is solved by high input impedance buffers (usually using JFET transistors). Some valve amps with input impedance higher than 3 MOhms will work correctly but it is much cheaper to use a FET buffer and any transistor amp or mixer. I'd say that 5 MOhms input impedance is better and 10 MOhms is very good. Preamps for magnetic pickups will not work because they usually have input impedance below 1 MOhms.
2) high signal level, which leads to distortion in the input stage of the amp or buffer. This can be solved with a resistor divider, or better with capacitance divider as Gus has shown. The part of the schematic with diodes is not needed because the basic rule is not to allow for distortion. Trying to fix the signal with diodes is a bad idea because it is already too late for this.

gyraf said:

Which makes me think: Why not make a full-passive fast compressor to act on the signal already before it leaves the instrument?

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This will not work because compressor cannot be fast. It needs some time to "measure" the signal level. During this , possibly short, time the signal would be distored and applying later a compression will not help. It will sound bad no matter what compression is applied. This problem is solved in many preamps by not allowing to have a distored signal at any moment and in any part of the circuit. And this is done in the way described above.
I'm designing a piezo buffer for my bass guitar. With piezo transducers under each of the string the signal is even higher than 3 V mentioned here. But it still can be handled without distortion with a buffer supplied from a 9 V battery.
Jakob, sorry that I spoiled your plan to conquer the world  .

Mark

Maybe too lo-fi, but what about using a simple zener clamp?
Zeners have relatively 'soft' limiting characteristics, and we're only interested in taming the big peaks so I would think zeners might work nicely, no?

Guys, I just checked some piezo pickups  with squre generator and scope (transducer piezo glued with receiver piezo, both are the same) - these transducers have very high HF peak.  You can read some info here http://en.wikipedia.org/wiki/Piezoelectric_sensor. Usually it is 3-18 kHz peak - dependant on size of element. Maybe it is better to design pickup/preamp system with flat frequency response and then try to work with dynamic characteristics.
2: insead of use Hi-Z FET cascade or follower, you can try Q/U converter. It has very low input Z, maybe it will be good load for piezo element.
3: I didn't tried this, but I think that piezo with some polarisation voltage may have another distortion characteristics. Maybe it will change harmonic distortion balance? Anybody tried?
Thank you for interesting thread!

Ethan said:

Zeners have relatively 'soft' limiting characteristics, and we're only interested in taming the big peaks so I would think zeners might work nicely, no?

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Soft limiting with a Zener diode means changing the signal and this means distortion. Guys who play acoustic guitars with piezo pickups will not agree to this. Basic rule is to not allow for distortion at any moment.

Mark

bezen4uk said:

Guys, I just checked some piezo pickups  with squre generator and scope (transducer piezo glued with receiver piezo, both are the same) - these transducers have very high HF peak.  You can read some info here http://en.wikipedia.org/wiki/Piezoelectric_sensor. Usually it is 3-18 kHz peak - dependant on size of element. Maybe it is better to design pickup/preamp system with flat frequency response and then try to work with dynamic characteristics.

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How did you check the pickups? I checked the piezo bridge I have and I cannot see the HF peak. What circuit did you use?

bezen4uk said:

2: insead of use Hi-Z FET cascade or follower, you can try Q/U converter. It has very low input Z, maybe it will be good load for piezo element.

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Do you have any schematic for such a converter? I thought that they are mainly used in accelerometers but I can check it if you have a schematic.

Mark

Hi,


  any compressor will add distortion, and since it is pretty much normal for the signal to be compressed either in the studio, or live these days, what does it matter where the compression occurs, before the preamp or later in the chain . . . Zeners sound like a good idea to me. Afterall, they are good enough for the *overkillers* . . .



  kindest regards,


      AndyP

Ive been playing around with the idea of a passive limiter for some time and came up with this to go between my mixer and soundcard. ratio adds dirt, but it sounds better than clipping ADC's..threshold values are just a rough estimate and this would be much to low Z for all passive piezo's, so take it for what it is...
"ratio" becomes a voltage divider when the signal reaches the forward voltage drop of the diodes, otherwise it's just a series resistor , adding a little noise, yes. ..but hey, nothing's free...

http://www.aronnelson.com/gallery/main.php/v/Johan_001/DSCF0091.JPG.html?g2_imageViewsIndex=1
J

MarkusBass said:

How did you check the pickups? I checked the piezo bridge I have and I cannot see the HF peak. What circuit did you use?

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Hi Mark,
I used such pickup http://www.lukemiller.org/journal/journal_pics/hydrophone/piezo_disc.jpg. Signal from generator was fed to big electrode and measured by scope at the small electrode. Applying suare wave from generator you can see transient response of pickup. Actually it's not quite fair method and ringing will be less (especially if the pickup is dumped).

MarkusBass said:

Do you have any schematic for such a converter? I thought that they are mainly used in accelerometers but I can check it if you have a schematic.

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Mark you can try this circuit
http://scopeboy.com/elec/chargeamp.gif
or like in attach

bezen4uk said:

I used such pickup http://www.lukemiller.org/journal/journal_pics/hydrophone/piezo_disc.jpg. Signal from generator was fed to big electrode and measured by scope at the small electrode. Applying suare wave from generator you can see transient response of pickup. Actually it's not quite fair method and ringing will be less (especially if the pickup is dumped).

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Thanks for the info but this is not a piezo pickup for an instrument. This is a piezo transducer from a piezo buzzer. You can use is just for tests and fun but this is not a piezo pickup that you can sell to a professional musician. It is highly optimized for one frequency and it has a huge capacitance.
I was rather thinking about a pickup like this one: http://www.thomann.de/gb/abm_3710c_p_single_bass_piezo_cr.htm

bezen4uk said:

Mark you can try this circuit : http://scopeboy.com/elec/chargeamp.gif

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I will check it. It looks simple but I'm affraid what is its frequency response.

Mark

Hi Mark,
Actually, same transducers are used by KK Sound. I don't see any problem using disck pickups for acoustic instruments. Capacitance of transducer is not issue.
Regarding circuit of charge amplifier (Q/U converter): since the piezo has capacitive output impendance, you have flat frequency response at the output. Gain can be matched by feedback capacitor. Another cool feature: you can sum many pickups by paralleling to the amplifier input.
Vladimir.

bezen4uk said:

Actually, same transducers are used by KK Sound. I don't see any problem using disck pickups for acoustic instruments.

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Do you have a link? I'd like to see it. Being frankly I made once a preamp with such a transducer for my friend violinist but the preamp required an equalizer to compensate poor frequency resoponse. And only did this because she had no money for a good transducer.

bezen4uk said:

Regarding circuit of charge amplifier (Q/U converter): since the piezo has capacitive output impendance, you have flat frequency response at the output. Gain can be matched by feedback capacitor. Another cool feature: you can sum many pickups by paralleling to the amplifier input.
Vladimir.

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I thought that charge amplifiers are used only with traffic sensors but a simulation seems to be promising. Maybe I'll build the circuit tomorrow and test it.
Vladimir, don't forget that the thread is about "passive compressor" and we are discussing how to make an instrument with piezo pickup sound good (with an active circuit). I think that the idea of passive compression is not feasible. But I believe that piezo pickup can sound good (even very good) with good electronics - I'm working on it.

Mark

Hi Mark,
Right now I'm making piezo pickup system for a friend's vibraphone, and I'm thinking about how to make the output unclipped. Maybe I'll need limiter or comressor. Will report about results.
The link at KK Sound : www.kksound.com
Vladimir.

bezen4uk said:

Hi Mark,
Right now I'm making piezo pickup system for a friend's vibraphone, and I'm thinking about how to make the output unclipped. Maybe I'll need limiter or comressor. Will report about results.

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It's a very interesting project. How many transducers do you have? I think that you don't need a limiter. Gus on previous page showed how to limit the signal without any limiter  ;D.

bezen4uk said:

The link at KK Sound : www.kksound.com

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I know their web site. But I'd like to see that they used piezo transducers from buzzers as  a guitar pickup.

Mark

Hi Mark,
There are 45 pickups that will be wired in parallel and fed to charge amplifier.