Piezo pickup: high-to-low impedance passive converter-resistor ???

[Schertler]

Hi there,

All my guitars are equipped with internal condenser and dynamic
microphones and amplified through the 2-channel 10 kOhm low-impedance
input preamplifier (mic preamp), 1st channel does also power the condenser
mic by 4.5DCV through the cable.....
I do also have ukulele equipped with ECM mic and passive K&K piezo-disks,
ukulele does not work with my basic preamp, so it has a battery box
built-in for powering the ECM ..... I do plug ukulele directly in to the combo.

Well, I just want to have a single universal preamp system for guitars and ukulele, so I would like to get the piezo element's impedance down passively and then plugging it in to my basic preamp.....

So the question: have you ever tried converting piezo's high-impedance
into low-impedance passively with a single 1+ mOhm resistor in-series
?? in other words, wiring some 1-10 mOhm resistor in-series within the
signal-chain ?? this resistor can be soldered in the jack's metal barrel right on the input, so it would be totally shielded and located just in 3-4 cm distance from the preamp's electronics PCB.......

Thank you!

Sincerely,
Misha

[Cuki79]

It can not work.

You think impedance as a resistivity. But a piezo is a capacitive charge "source".

[Rudy4]

Impedance and resistance are not the same thing, but if they were you would be going the wrong direction and adding to the number, not reducing it.

If that's all there was to it then every piezo-based transducer would simply be made with the resistor already added.

[Schertler]

I am aware of the nature of technology...... However, the case is not about the rule, but the solution .......
This method does work but the trouble is a noise which may occur......
Interesting link: https://www.richardmudhar.com/blog/2...ower-supplies/

The resistor's value should be something between 56-560 kOhm !!
I am going to test this way, so will share the result after !!

[varmonter]

Let us know how it works for you...

[Cuki79]

Originally Posted by Schertler

I am aware of the nature of technology...... However, the case is not about the rule, but the solution .......
This method does work but the trouble is a noise which may occur......
Interesting link: https://www.richardmudhar.com/blog/2...ower-supplies/

The resistor's value should be something between 56-560 kOhm !!
I am going to test this way, so will share the result after !!

I am not sure the guy does have the same problem as you do.

He hits directly on huge piezo discs making 60V spikes.

He uses a voltage divider to reduce those spikes
https://en.wikipedia.org/wiki/Voltage_divider

The Olympus LS14 is a digital recorder. It means everything works in 5V inside. So the maximum voltage his Line input can handle is 5V. This line input has 10kOhm impedance (see manual page 95).

Adding a 330kohm inline will divide the voltage by 10/(330+10) making the 60V spikes becoming 1.76V spikes.

You need to measure the output of both mic and pickup and be sure you level down the K&K to mic level while maintaining a 1Mohm load. So with a 1 Mohm inline that makes -40dB attenuation that should be compensated by the mic gain. If you put 500k it will make -34dB.

I still have hard time believing you won't have a very poor signal to noise ratio. It does not sound right to attenuate -34dB a signal to amplify it later on... On the webpage you just gave, the guy is getting 60V spikes, and he is recording hits, not music...

I'd rather make myself a 2-channel preamp or buy a small mixer with hi-Z input and mic input.

[Schertler]

Thanks for sharing your thoughts!
That passive SBT pickup in my ukulele is the last piezo-system left in one of my instruments, basically I do have condenser/dynamic mics built-in and thus my basic preamp is a standard 2-channel mic preamp with 12 kOhm input impedance, so I should have it made with another high-Z input section for such pickups but I was totally ignoring everything related to piezos at the time of constructing the preamp......
Considering such a primitive way for making passive piezos sound good is everything I can wish to do, if it does not work I will try to build another dynamic pickup for ukulele rather than constructing any new preamp for piezo..... however, finding a good 200-300 Ohm dynamic capsule is not a easy task, that's where the idea of a resistor/converter comes from.
Actually, there is a little condenser mic in the ukulele, it might be absolutely fine and enough to amplify it well, different type pickups are always better than single sources. If it were steel-string guitar, I would have all the piezos off but they sound OK in ukulele.

[Villamarzia]

Have you considered a Lehle pedal in the chain? Something like Little Lehle or maybe the P-split might solve your issue as it accepts different kind of incoming signals (balanced, unbalanced, hi-z, low-z, stereo, mono etc) and it will match your preamp input (the channel you normally use for the dynamic mics, the other will be directly dedicated to condenser mics as it provides power too).

[Schertler]

Originally Posted by Villamarzia

Have you considered a Lehle pedal in the chain? Something like Little Lehle or maybe the P-split might solve your issue as it accepts different kind of incoming signals (balanced, unbalanced, hi-z, low-z, stereo, mono etc) and it will match your preamp input (the channel you normally use for the dynamic mics, the other will be directly dedicated to condenser mics as it provides power too).

Thank you for the notes! Very handy device, sorry I did not know about that....
However, I would try to build something similar from an audio matching transformer, it should not be a very hard task.... starting a search for more details. Thanks!

[sam.spoons]

A simple fet preamp is the answer, a few years ago I would have DIYed but they are so cheap these days there's not much point.

https://www.zachpoff.com/resources/alex-rice-piezo-preamplifier/

https://www.ebay.co.uk/itm/Preamp-Piezo-Pickup-Kit-Active-Endpin-Jack-Piezo-Transducer-for-Guitar-Parts/153766321690?hash=item23cd2fde1a:g:dkMAAOSwmENd-eW5

[Schertler]

Well, I have got tested/simulated the following source follower circuit as my high-Z input-pre, I am going to add these few components on the actual PCB with a mini selection-switch on the panel, so this way I will be able to select my 2nd channel between the two inputs - 10kOhm low-Z and 2mOhm high-Z.
The circuit truly has a decent specs (LTSpice).
R2 and R3 combination allows the output sensitivity to be increased/decreased.