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piezo pot value?

SarasotaSlim

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1,569
Just got my new Republic Hwy 61 and want to mount a pickup on the cover plate and also a piezo on the biscuit. I want a knob for the pickup and a separate knob for the piezo. What pot value should I try to use for the piezo? Not interested in the pre-made thing that National makes (it doesn't include a piezo and it's really pricey plus I wanna do it myself)
 
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SarasotaSlim

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You want to get piezos to an amplifier ASAP (transistor, tube, opamp). They don't "like" passive circuits.

For DIY try this circuit. If you don't already know how to DIY baby simple electronics... oh yeah I just remembered DIYStompboxes (Aron's Stombox Forum) has a beginner project that is practically the same thing and shows you exactly how to incorporate a volume control. Plus there are people there who will hold your hand all the way through the project. That's your best bet by far.
Thanks - interesting stuff there - not the info I'm looking for but interesting. I have a store bought preamp - SansAmp acoustic DI. There are reso instruments that use piezos with pots that don't have onboard preamps and they blend the piezo signal into the electric guitar pickup signal via a blend pot from it's own pot. The Dean reso uses this system - no battery - no onboard preamp. I have a mid 70's chrome Dobro that has multiple piezos and a 1 meg* pot wired to a separate output jack. (* I think it's 1 meg - it was done many years ago) However - I tried the Dean and it seems to work OK and I like the idea of adding the piezo sound to the pickup sound into my guitar amp. My intention is to have 2 knobs - 1 for the pickup and 1 for the piezo - no switches and a single output jack.
 

walterw

Platinum Supporting Member
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39,359
Passive piezos don't blend with magnetic pickups; electronically, they're "oil and water".

Also, without a real piezo preamp (which the sansamp is not) the piezo will be hideously thin and weak.

The fishman powerchip does a really good job of both "fixing" the piezo signal and blending/splitting it with magnetic pickups at the output.
 

SarasotaSlim

Member
Messages
1,569
Passive piezos don't blend with magnetic pickups; electronically, they're "oil and water".

Also, without a real piezo preamp (which the sansamp is not) the piezo will be hideously thin and weak.

The fishman powerchip does a really good job of both "fixing" the piezo signal and blending/splitting it with magnetic pickups at the output.
Yes I'm aware that they are totally different animals. However I have played and heard the result on that Dean and it is acceptable. It has no onboard preamp or battery and it works OK. I will experiment and see what happens.
 
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Thanks - interesting stuff there - not the info I'm looking for but interesting. I have a store bought preamp - SansAmp acoustic DI. There are reso instruments that use piezos with pots that don't have onboard preamps and they blend the piezo signal into the electric guitar pickup signal via a blend pot from it's own pot. The Dean reso uses this system - no battery - no onboard preamp. I have a mid 70's chrome Dobro that has multiple piezos and a 1 meg* pot wired to a separate output jack. (* I think it's 1 meg - it was done many years ago) However - I tried the Dean and it seems to work OK and I like the idea of adding the piezo sound to the pickup sound into my guitar amp. My intention is to have 2 knobs - 1 for the pickup and 1 for the piezo - no switches and a single output jack.
If they have solved that age-old puzzle, I would love read about how they did it. This is the guitar version of Fermat's Last Theorem: many have tinkered with it but it has proven elusive. Of course FLT was eventually solved, almost 400 years later.

The problem with piezos is ultra high impedance, which makes it unruly. Generally they don't play well with others. If you know something about how to do it, please share. Can you trace the guitar you're talking about?

The time-tested way of doing it is with buffers. You convert divergent signals into easy-to-use, low-impedance signals and mix them any way you like.

Here's another idea: I was recently grappling with a similar issue. I have an archtop guitar that had homemade PAF-type humbuckers and I wanted a more traditional jazz sound. I set about making some Charlie Christian pickups but I noticed that all the recordings I really liked had some of the sound of the acoustic guitar present. So I wanted to mix an acoustic sound with a pickup. I thought really hard about it and tried a few things, finally deciding the best compromise was to run out separate signals for a Schatten Dualie (soundboard transducer, works okay over short cable runs) and the regular guitar circuit. This was the literally the simplest way I could think to do it without an onboard pre.

Here is a sample of that project to give you an idea. BTW for the Dualie signal I use an outboard pre running into the "CD input" on a solid state amp for this setup; the regular guitar circuit goes into the amp input like normal. The guitar works either with a special TRS cable I made, or with a regular guitar cable. With a regular guitar cable I can have either the CC pickups or the Dualie (but not both - can't be mixed). BTW the Freddie Green comping and bassline are the Dualie by itself.

 

SarasotaSlim

Member
Messages
1,569
If they have solved that age-old puzzle, I would love read about how they did it. This is the guitar version of Fermat's Last Theorem: many have tinkered with it but it has proven elusive. Of course FLT was eventually solved, almost 400 years later.

The problem with piezos is ultra high impedance, which makes it unruly. Generally they don't play well with others. If you know something about how to do it, please share. Can you trace the guitar you're talking about?

The time-tested way of doing it is with buffers. You convert divergent signals into easy-to-use, low-impedance signals and mix them any way you like.

Here's another idea: I was recently grappling with a similar issue. I have an archtop guitar that had homemade PAF-type humbuckers and I wanted a more traditional jazz sound. I set about making some Charlie Christian pickups but I noticed that all the recordings I really liked had some of the sound of the acoustic guitar present. So I wanted to mix an acoustic sound with a pickup. I thought really hard about it and tried a few things, finally deciding the best compromise was to run out separate signals for a Schatten Dualie (soundboard transducer, works okay over short cable runs) and the regular guitar circuit. This was the literally the simplest way I could think to do it without an onboard pre.

Here is a sample of that project to give you an idea. BTW for the Dualie signal I use an outboard pre running into the "CD input" on a solid state amp for this setup; the regular guitar circuit goes into the amp input like normal. The guitar works either with a special TRS cable I made, or with a regular guitar cable. With a regular guitar cable I can have either the CC pickups or the Dualie (but not both - can't be mixed). BTW the Freddie Green comping and bassline are the Dualie by itself.

Very nice tone on your project - I've always wanted to try some of those Charlie Christian pickups but have never had a chance to hear any. I do like the idea of using a TRS jack just in case you want to separate the signals but wiring it so that they join if used with a standard guitar cord into a guitar amp. Here's the Dean I tried that has the passive system with a pickup and a piezo.

http://www.deanguitars.com/resonatorchromeG.php



Notice the 3 knobs - they are a pickup volume - piezo volume - blend. I just wanna go with 2 knobs on mine. A friend suggested wiring them like a jazz bass and they will be independent of each other.
 

walterw

Platinum Supporting Member
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39,359
I couldn't find anything about the tech specs of those, either. A lot of the deans that are straight dobros have internal preamps, but nothing about these one way or the other.

Anyway, any pot below say 10MΩ (including the blend pot) will likely wreck the output of the passive piezo.
 

SarasotaSlim

Member
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1,569
Here's a pic of my new reso toy - Republic HWY 61 - now with an old Kay pickup and piezo transducer under the cone.
 

SarasotaSlim

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1,569
This is gonna be a very sloooow project. Turns out that both of the old Kay pickups I've been saving for this kind of thing are about useless. Now that I've already cut the cover plate I'm committed to this pickup. It's one of the few that are chrome and shallow and I really like how it looks so now I'm looking into rewrapping them to see if that works. Worst case scenario is I might have to break down and get a new cover plate and cut new holes for a different style of pickup.

The Piezo seems to like the 1MEG pot best.
 

Keyser Soze

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1,472
Sorry, late to the party.

The ultra high impedance of a piezo means a high value volume pot, you might get a better response from something even larger than 1 meg. The only downside is that the higher the value the more the pot behaves like a tone control - robbing highs as you turn down. A small value treble bypass, anywhere from 100-500 pf can help with this.
 
Last edited:

SarasotaSlim

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1,569
Sorry, late to the party.

The ultra high impedance of a piezo means a high value volume pot, you might get a better response from something even larger than 1 meg. The only downside is that the higher the value the more the pot behaves like a tone control - robbing highs as you turn down. A small value treble bypass, anywhere from 100-500 pf can help with this.
AHA! Thank you so much for the info - better late than not at all. So does this mean you have actually done this before and gotten some kind of predictable results - or is this just a suggestion that you think will work??

So far we have used little clippy things to temporarily wire different values to the piezo and listened briefly to a 5k - 10k - 100k - 500k and 1meg. The lower values seem to decrease the output volume as well as reduce desirable tone if there is such a thing from a piezo.

My wiring plans may have to change from the original concept of having the JassBass style with the magnetic Kay pickup on 1 knob and piezo on the 2nd knob. If it squeals too easily with a mono output when the piezo dials in then I may use a stereo jack and split the signal with the piezo going to and external preamp and then to the PA.
 

Keyser Soze

Member
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1,472
No, I've really done it. Nothing fancy, mainly uke/mando type bar piezos in 4 string cigar box guitars. Volume control, using high value pots, is fairly straightforward. But, as you note, the tone is a tad unique.

Not only being high impedance piezos are also peaky as all hell. They can generate voltage swings substantially larger than any overwound humbucker, so will often overload the input stage of a sensitive amp. Sometimes placing a fixed resistor, in line with the piezo out and prior to the volume pot, can help with this. The value can vary anywhere from 100k-470k. But again, treble loss will be an issue.

Adding in an actual tone control gets much more tricky, and I've never had good results, but this method looks promising (and includes a sound clip.)

http://www.cigarboxnation.com/forum/topics/passive-tone-control-for-piezo-solved?xg_source=activity

Another thing CBGers do is wire multiple piezos in parallel, which reduces the overall impedance down towards what a typical guitar amp is voiced to handle. Mostly this is done with disc type piezos mounted in the body cavity. I prefer a rod under the bridge saddle, so have never tried it, but I suppose it is technically feasible to use multiple rods, so long as you are getting even string pressure on them.

If you are going for a lo-fi mojo kind of tone to add to your repetoire I'd say continue pursuing the passive approach. If you are looking more to capture the rich acoustic type tone of a reso then I'd strongly advise adding an on board powered pre-amp. Either that, or run the piezo volume pot to a second (or stereo) output jack, one that can be sent to an outboard graphic EQ pedal.
 

SarasotaSlim

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1,569
No, I've really done it. Nothing fancy, mainly uke/mando type bar piezos in 4 string cigar box guitars. Volume control, using high value pots, is fairly straightforward. But, as you note, the tone is a tad unique.

Not only being high impedance piezos are also peaky as all hell. They can generate voltage swings substantially larger than any overwound humbucker, so will often overload the input stage of a sensitive amp. Sometimes placing a fixed resistor, in line with the piezo out and prior to the volume pot, can help with this. The value can vary anywhere from 100k-470k. But again, treble loss will be an issue.

Adding in an actual tone control gets much more tricky, and I've never had good results, but this method looks promising (and includes a sound clip.)

http://www.cigarboxnation.com/forum/topics/passive-tone-control-for-piezo-solved?xg_source=activity

Another thing CBGers do is wire multiple piezos in parallel, which reduces the overall impedance down towards what a typical guitar amp is voiced to handle. Mostly this is done with disc type piezos mounted in the body cavity. I prefer a rod under the bridge saddle, so have never tried it, but I suppose it is technically feasible to use multiple rods, so long as you are getting even string pressure on them.

If you are going for a lo-fi mojo kind of tone to add to your repetoire I'd say continue pursuing the passive approach. If you are looking more to capture the rich acoustic type tone of a reso then I'd strongly advise adding an on board powered pre-amp. Either that, or run the piezo volume pot to a second (or stereo) output jack, one that can be sent to an outboard graphic EQ pedal.
This is interesting news regarding the multiple piezos in parallel. I have a Dobro with pickups and multiple piezos and it did seem to help a little when I added the 2 to the biscuit to help the one under the cone. I rarely use the piezo out as it requires a second separate guitar cord and it's just too cumbersome to deal with live as a band. If I did a single then it might be worth the trouble. When I do use it I plug it in this thing - http://www.tech21nyc.com/archive/acousticdi.html



On this new project I'm hoping to get it to work all off the one mono style jack if it doesn't feedback too bad when the piezo is dialed in with the pickup. I'll experiment with small resistors like you said too and finally I may add another piezo to the top of the biscuit in parallel with the one under the cone.

Here's another twist that might be fun but more complicated. I also have this old XDS Plus Digital Wireless thingy that has 2 channels sort of. It has a stereo jack on the transmitter and a stereo out as well. They describe it as tip being a slightly colored sound like a guitar cord and the ring as the complete spectrum uncolored. I could use that ring to send to my Tech21 acoustic DI and on to the PA and the tip would go to my normal pedalboard and then my amp.
 

Keyser Soze

Member
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1,472
I've never tried blending piezo and magnetic pickups passively, primarily because everyone I've talked with, and everything I've read, says the two systems just do not play nice together.

But if anything was going to succeed it would makes (some) sense that the multiple piezos in parallel could help.

IN CBGs people will often cover the disc types with various substances/objects (spray foam insulation, blocks of wood, etc.) to limit their response and reduce feedback. You also want to avoid mounting them in places that will be most exposed/responsive to outside noise sources. Stage volume is tough and in those situations a loud monitor is not your friend.
 

SarasotaSlim

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1,569
OK - time for a looooong over due update! I finally fixed the pickup-piezo issue and got it working. I broke down a bought a cheap chrome P90 from GuitarFetish.com and managed to stuff it in the same spot as the other pickup with a little bit more filing to make it bigger. It's a tad off kilter and tilts a little - oops. The problem was that the piezo was somehow canceling out the signal from the pickup when they were wired using a jazzbass minus the tone knob. I think it was shorting out because the piezo was screwed to the bottom of the cone in the center and it was all somehow grounding out via the metal body or something. I went back to the drawing board and isolated the piezo with a plastic washer so it doesn't touch the cone and VIOLA! it works. 2 knob passive jazz bass wiring minus the tone knob - one for the pickup and one for the piezo - they work independently and it gets pretty ding dang loud too - yeah! I'll try to post a clip soon.
 

SarasotaSlim

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1,569
OK finally - got a video of my new Republic Hwy 61 reso with the jazz bass style wiring. 1st knob is 250K to the P90 pickup (a cheap chrome type from GuitarFetish.com) and the 2nd knob is 500K to the piezo transducer mounted under the cone. This is what they sound like together through a small 10 watt Allen Amp (Chihuahua)

 






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