Undersaddle piezo polarity?

[crazydoc]

Haven't been here for years - glad to see GroupDIY is still around. Sorry that this is a lazy man's question (hope it's in the right place), but I can't find anything by google.

I'm building a resophonic guitar with a magnetic neck pickup (with the usual volume and tone controls), and an undersaddle piezo that is (cheap commercial) preamped. They will both go through op amp buffers to a blend pot and then though an op amp mixer to a master volume pot.

I guess my question is, do undersaddle piezos have a polarity, and if so, is there an easy way to determine it - the reason I'd like to know is if will I need  to invert one of the signals? Of course, after going through the piezo preamp the original polarity could well be changed.

At this point, I'm thinking of putting in a phase reversal switch and going by the sound (which may be interesting anyway), but that's one more piece of hardware sticking out of the guitar.

Any thoughts appreciated.

 

[abbey road d enfer]

I'm building a resophonic guitar with a magnetic neck pickup (with the usual volume and tone controls), and an undersaddle piezo that is (cheap commercial) preamped. They will both go through op amp buffers to a blend pot and then though an op amp mixer to a master volume pot.

I guess my question is, do undersaddle piezos have a polarity,

Indeed they do, but since the stimulus is oriented up/down on the piezo, and lateral for the mag p/u, their outputs have a variable correlation.

and if so, is there an easy way to determine it

Unfortunately no - actually it's also dependant on the way the guitar is plucked, so the polarity relationship may vary.

At this point, I'm thinking of putting in a phase reversal switch and going by the sound (which may be interesting anyway),

That's the best solution; it actually becomes one more tone-control element.

but that's one more piece of hardware sticking out of the guitar.

That's the price to pay for versatility...

 

[mhelin]

Many piezo preamps already come with the phase switch, even these cheap $15 ones like this:

https://www.banggood.com/Cherub-GS-3-Acoustic-4-Band-EQ-Equalizer-Guitar-Preamp-Piezo-Amplifier-with-Tuner-Phase-Function-p-1228351.html?cur_warehouse=CN

 

[mhelin]

There is also this little external device which can be used to adjust the phases of two sources to match with much more precision than simple polarity switch can do:

https://www.radialeng.com/product/phazer

 

[Tubetec]

Taking the fishman style strip as the basic idea , its wired the way it is the way it fits the slot you cant change that  , you do see phase flip on the mic section of some combo piezo/mic pre's for acc guitar . Suppose your taking a mic and a DI off an acoustic guitar , you want to blend the signals of course ,but if ones out of phase , it'll suck all the guts out of your sound , so the abillity to flip the phase of the mic is important , if your talking combo mic/piezo arrangement you need to make sure both outputs add together not subtract .

 

[crazydoc]

Thanks for the replies - very helpful in sorting out my options. I wish I'd known about the preamp with the phase switch - maybe for the next build.

The Phazer looks nice, but I want everything on board, and it's out of my price range anyway for this project.

The piezo I'll be using is the "soft" type - flexible so it can allow access to the tension screw between the spider and the cone, so I suppose any polarity will be random any way.

So it looks like I'll be putting a switch on the magnetic pickup, and play its effect "by ear." 

Thanks again for the info.

 

[abbey road d enfer]

The Phazer looks nice, but I want everything on board, and it's out of my price range anyway for this project.

Actually, the Phazer is a very simple $2 one op-amp circuit. It can be built-in, but it adds one potentiometer.

 

[JohnRoberts]

Actually, the Phazer is a very simple $2 one op-amp circuit. It can be built-in, but it adds one potentiometer.

Yup simple phase shift circuit, but try it before building one into a guitar... YMMV

JR

 

[Tubetec]

I never saw a pickup added to a dobro ,I did once think about attaching a voice coil/magnet assembly  to the back of the cone , maybe using a headphone driver or other mini speaker  as transducer.
For me I just cant see a Resonator guitar sounding good without micing it , Id imagine an undersaddle piezo would have a massive metalic punch at the head of the note , still interested to hear how it turns out though .

 

[crazydoc]

Actually, the Phazer is a very simple $2 one op-amp circuit. It can be built-in, but it adds one potentiometer.

Yup simple phase shift circuit, but try it before building one into a guitar... YMMV

JR

I've looked at allpass circuits, if that's what you're referring to, and they seem quite limited (to me), with phasing centered around a single frequency. Of course, by twiddling the knob there could be some interesting effects. I read a paper (that I can't find now) stating that the perceived difference in audio quality was minimal. The audio spectrum could be broadened by using multiple resistors of different values, possibly with digital or voltage resistance  control (or a ganged pot).
https://pdfserv.maximintegrated.com/en/an/AN184.pdf

Too much for me to put into a guitar at this point. Let me know if you're referring to a different circuit.

 

[crazydoc]

I never saw a pickup added to a dobro ,I did once think about attaching a voice coil/magnet assembly  to the back of the cone , maybe using a headphone driver or other mini speaker  as transducer.
For me I just cant see a Resonator guitar sounding good without micing it , Id imagine an undersaddle piezo would have a massive metalic punch at the head of the note , still interested to hear how it turns out though .

I wouldn't attach anything to the cone, which has to be light weight, and vibrate freely and in balance.

A few years ago I built myself a new square neck spider resophonic, and so retired my cheapo old one and added a preamped piezo undersaddle pickup to it - it sounds pretty good to me, still keeping some of that dobro whine, and I too can turn up if I'm playing in an amped jam.    Of course, I have a tin ear, so it may not sound as good as I think.

 

[abbey road d enfer]

I've looked at allpass circuits, if that's what you're referring to,

Exactly.

and they seem quite limited (to me), with phasing centered around a single frequency.

They are pretty efficient though; that's the typical circuit used in multi-way cross-overs and exactly what's used in the Radial box.

I read a paper (that I can't find now) stating that the perceived difference in audio quality was minimal.

Altering the phase response whilst maintaining linear frequency response is known to be inaudible in most cases. However, when two signals are mixed, the difference is quite noticeable. That's your concern, mixing the signal from the piezo with that of the mag p/u.

The audio spectrum could be broadened by using multiple resistors of different values, possibly with digital or voltage resistance  control (or a ganged pot).

Your original question seemed to be concerned with aligning signals, not effects.

https://pdfserv.maximintegrated.com/en/an/AN184.pdf

This is not digital audio; it's digitally-controlled analog audio. Nothing that can't be achieved with simple circuitry.

Too much for me to put into a guitar at this point. Let me know if you're referring to a different circuit.

The circuit on page 1 is all you need.

 

[abbey road d enfer]

I wouldn't attach anything to the cone, which has to be light weight, and vibrate freely and in balance.

I know a number of players who have fitted a piezo bug to the cone with satisfactory results
https://www.amazon.com/Imelod-Contact-Microphone-Ukulele-Mandolin/dp/B01M5IB4X7/ref=pd_cp_267_3?pd_rd_w=NqSW2&pf_rd_p=ef4dc990-a9ca-4945-ae0b-f8d549198ed6&pf_rd_r=YYJ87YAV7R1NS8E7NWZE&pd_rd_r=db2592a1-7e65-11e9-b54d-15003332d70e&pd_rd_wg=17TGz&pd_rd_i=B01M5IB4X7&psc=1&refRID=YYJ87YAV7R1NS8E7NWZE
Routing the cable is a PITN, though.
If it looks crude to you, there is this
https://kksound.com/products/pureresonator.php

 

[crazydoc]

I'm pretty confused now about pickup polarity and signal phase angle, and what is appropriate to control regarding the merger of magnetic and piezo pickups, so I'll have to let that go for now.

As far as attaching a piezo to a reso, I'm certainly no expert, but am  OK with the K&K attachment to the cone screw, which  is at least balanced, and probably light enough to not dampen the sound even with the wire hanging out. Here's the method I used for the soft undersaddle pickup, bringing the wire out along a spider arm and diving into the sound box through a hole under the cover plate.

 

[crazydoc]

Well, now I'm trying to do the electronics for the guitar - I've started a new thread at  https://groupdiy.com/index.php?topic=72708.0

Thanks for your help and suggestions.