piezo basics

[Marco Pancaldi]

I have a piezo pickup/preamp system with 2M (pickup out Z) - 10M (preamp in Z) interfacing. I'd wish to attenuate a little the bass response...

1 - I know it is possible to attenuate the bass in a piezo preamp lowering the input impedance. Is this simply a variable first order hi-pass filter (the exact frequency depending from out/in matching of Z) or something more complex/different?
2 - I read from several sources that series capacitors can be used with piezo to "attenuate" the signal...can you explain better? I don't understand if I can use a carefully selected cap inserted before the preamp to obtain another first order filter: in this case I can approximate a 12dB/oct hi-pass using Z-matching and series cap at the same time...

Many thanks
m.p.

[David King]

Marco, the rules of RCL must apply even at high impedances so I'm guessing a second order high pass filter will work for you but that's no guarantee.

[Mike Sulzer]

The piezoelectric sensor looks like a capacitor. The model also has an inductor in series representing its mass, but one would expect that to produce a resonant frequency higher than we care about, so lets ignore that. The cutoff frequency is just as you would expect with an RC network.

As for making another first order filter, or in combination, a second order filter, by just using a C in series with the preamp input, this would probably not work out as you want. Assume that the sensor runs directly to the preamp with no other components. In effect the C you would add would be in series with the C of the sensor and there would be single first order filter at a frequency determined by the series combination.

If there are other components in the circuit, for example a resistor across the sensor separate from the preamp, you would be able to get a second order filter by using a C in series with the preamp input, but in calculating the response, you would have to take all the components into account. So you could make a second order filter, but it is not quite as simple as you might like.

I have a piezo pickup/preamp system with 2M (pickup out Z) - 10M (preamp in Z) interfacing. I'd wish to attenuate a little the bass response...

1 - I know it is possible to attenuate the bass in a piezo preamp lowering the input impedance. Is this simply a variable first order hi-pass filter (the exact frequency depending from out/in matching of Z) or something more complex/different?
2 - I read from several sources that series capacitors can be used with piezo to "attenuate" the signal...can you explain better? I don't understand if I can use a carefully selected cap inserted before the preamp to obtain another first order filter: in this case I can approximate a 12dB/oct hi-pass using Z-matching and series cap at the same time...

Many thanks
m.p.

[David King]

Most piezo preamps that use subsonic filters do so post buffer(s).

[David Schwab]

On piezo tweeters you limit the low frequencies by using a series resistor. Might work on a piezo pickup, but I never tried it.

[Mike Sulzer]

On piezo tweeters you limit the low frequencies by using a series resistor. Might work on a piezo pickup, but I never tried it.

Pulsar Developments Ltd - Piezo Tweeter Application Note

The piezo tweeter looks like a lossy C, and so the series resistor cuts high frequencies (above the audio range, one hopes). Its purpose is stability since the capacitive load alone might be difficult for the amp to handle.

[David Schwab]

Pulsar Developments Ltd - Piezo Tweeter Application Note

The piezo tweeter looks like a lossy C, and so the series resistor cuts high frequencies (above the audio range, one hopes). Its purpose is stability since the capacitive load alone might be difficult for the amp to handle.

You are correct, takes care of the impedance in the higher frequency range. I had read a couple of years ago that it was for the low frequencies. This was after the 10ohm resistor broke off of the piezo tweeter in some cheap PA cab I had.