Under bridge piezos - Best grounding method?

[nekodensha]

Hi all,

I want to fit a couple of piezo disks (to run in parallel) underneath the bridge of a 5-string build I'm working on. I'd like to know the best way to keep the electronics grounded with under bridge piezos installed.

The best solution seems to be to fit a thin wood shim under the bridge baseplate, install the piezo disks on the underside of the shim (routing accordingly), and still have the ground wire contact the bridge baseplate. (I can only assume if under bridge piezo(s) are in contact with the bridge baseplate, along with the ground wire, the results will be, um...undesirable).

I'd like to know if a wood shim will sufficently isolate/insulate the piezo circuit from the bridge ground wire, or whether there are other, better solutions.

The mag pickup is a sole EMG-HZ ...so maybe grounding to the bridge/strings is not necessary as EMG say - but it is still definitely desirable IMO.

Any advice on grounding alternatives, piezo placement, low-cost piezo buffering, materials for holding piezos, etc. is welcome. I want to make the whole installation as quiet as using piezos (and a variable coil split on the mag pup ) will allow.

Cheers, nekodensha

 

[walterw]

is this going to be some sort of floating bridge that rests directly on the piezos with a separate tailpiece? because if you're talking about inlaying piezos under a regular bass bridge that's still bolted straight to the body, i don't see where you're going to get any output from them. piezos work by translating changes in pressure into voltage, so if they're under a bridge that's bolted down rigidly onto the wood, there's not going to be any changes to speak of.

however you set it up, the strings (or whatever the strings are resting on) need to be able to freely vibrate on the piezos to create the signal.

regardless, most piezos i've seen have a hot side and a ground side. just set them so the hot side is insulated by something non-conductive and ground side contacts the bridge and it should work as far as it goes.

 

[nekodensha]

Thanks for the reply Walter.

The bridge is just a regular one-piece "quick-load" Gotoh bridge, mounted slightly recessed into the body.

I'm assuming the hot side of the piezo is the side with the crystal film embedded in the centre (the Australian for 'center'), and the ground side is the 'plain' side. So am I correct in gathering that the piezo ground side can directly contact the bridge base, and the wood shim can be dispensed with? Sounds like a better option to me

From what you said I figure I should rout some very shallow pockets, allowing just enough clearance for the piezos to flex, and use some silicone/ rubber-type glue to push the piezos' ground side up against the bridge. I wouldn't want the piezos totally rigid and unable to flex, but having them too loose also seems like it would be a very bad idea (or else I'd just use pup foam- which is probably the best option for all I know).

In any case I definitely wasn't planning to simply jam the piezos between the bridge and its footprint

I can understand how the saddle is a better place for a piezo. Ideally I'd simply order a set of Ghost saddles and their matching buffer preamp from Graph Tech (this is what a local luthier recommended), but that's basically way too expensive ATM. Even the piezo tabs made by MSI USA are a no-go as their Australian distributor requires a bulk order. I simply haven't been able to track piezo coax cable down at all. So basically under saddle piezos are unfortunately not an option, much as I would prefer them.

Artec makes a cheap piezo coax unit, but I think it's for under the bridge, not the saddles. I'm also not sure it'll offer much better quality than a couple of disks.

Thanks again for the reply Walter, it's my first post here on TB and it's been really good to get some feedback on this .

Cheers, nekodensha

 

[walterw]

welcome to talkbass!

i suspect that it's not going to really matter how these piezos are brought into contact with the bridge, as the bridge itself, being screwed down tight to the body, is not going to vibrate (the very definition of a good bass bridge), and thus is not going to produce anything for the piezos to amplify.

i suppose if you were to rig it up so that the bridge was just held by the screws along the back edge, but could still move around a little, and the piezos were mounted under the front edge, that the string pressure would then push the front edge of the bridge onto the piezos, allowing for some vibration to create signal. i have to think this would compromise the overall tone and sustain of the bass, though.

if you could put a piezo under each saddle, so that the string pressure drove it directly, you might have something.

 

[nekodensha]

I see your point Walter. I don't really want to create a mutitude of compromises by weakening the bridge's attachment to the body. I'll just have to hold out until I can find suitable piezo materials for placing under (or on top of, or inside) bridge saddles.

I'm surprised though that even given enough room to flex, piezos won't pick up anything at all from the vibrations traveling from the strings into the body itself via the bridge (and neck).

So while I'm now convinced that this idea is not worth trying out with my current build, I still might try adding some piezos under the bridge of my mouldy old Samick P-bass copy, just to see what transduces and what doesn't. The body is plywood, and it's shortly to be routed extensively for use as a pup and circuit test bed anyway, so I have very few concerns with irreversible damage.

Cheers,
nekodensha

 

[Koeda]

Walter speaks wisdom. Best advice I can give - discs don't work well under the saddle (been there). You can get a sound - but keping the balance between strings - nearly impossible. I can offer one word of advice as far as effective piezos go - Ghost. Get ghost saddles and a good preamp for them and it will make you smile.
David

 

[nekodensha]

Thanks David. Walter has already convinced me under saddle is the way to go, but it's always good to have things corroborated. I do want to use a Ghost system in a future build if funds permit. (Luthierie ain't cheap- it just creates new GAS problems).

I gather from what you said that other preamps are compatible with the Ghost saddles. Glad to hear it! If you know of (or better still, use) a different pre that works well with Ghost saddles other than the proprietary Ghost pre, I'm all ears.

The Samick is still getting piezo surgery though- absolutely nothing to lose there. (I also don't want to have these piezos just sitting around gathering dust). I'm definitely not expecting spectacular results, far from it. I will try some trimpots in the circuit though in view of what you've said about string imbalance. (And be prepared to change the location of the piezo disks at least half a dozen times...)

Just please don't ask for any soundclips! Thanks guys.


Cheers,
nekodensha

 

[walterw]

any preamp with a super-high input impedance (like 10Mohm) will take piezo signals without low end loss. the catch is that there needs to be nothing in between the piezos and that preamp, or the signal will be emasculated before it gets to the preamp (so no trimpots on passive piezos.)

 

[Koeda]

I gather from what you said that other preamps are compatible with the Ghost saddles. Glad to hear it! If you know of (or better still, use) a different pre that works well with Ghost saddles other than the proprietary Ghost pre, I'm all ears.

I am using a Bartolini MPB2-918 Piezo-Magnetic Pickup Buffer and like it a lot in the Jazz Fretless. Any buffer/preamp made for piezos (high imp) do the trick. Just keep it close as you can to the pickup, gambatte.
David

 

[bassbenj]

Thanks David. Walter has already convinced me under saddle is the way to go, but it's always good to have things corroborated. I do want to use a Ghost system in a future build if funds permit. (Luthierie ain't cheap- it just creates new GAS problems).

The Samick is still getting piezo surgery though- absolutely nothing to lose there. (I also don't want to have these piezos just sitting around gathering dust). I'm definitely not expecting spectacular results, far from it. I will try some trimpots in the circuit though in view of what you've said about string imbalance. (And be prepared to change the location of the piezo disks at least half a dozen times...)

Just please don't ask for any soundclips! Thanks guys.

Yes under saddle is the way to go, but that doesn't mean that under bridge is worthless. I do not recommend ceramic piezo disks, however. They are brittle and the tone is less than great.

What you want to do is shove one or more piezo film tranducers under the bridge. Check here for raw piezo transducers (more in a second)

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If you want a working film piezo with all ground planes taken care of go see the company "pick-up the world", great stuff but take your wallet. Just plug to Hi-Z preamp and go. They sell those too.

If you want to experiment buy a bunch of the above film transducers in various sizes. Note that they are ground on one side and "hot" on the other. Because of the ultra high impedance, the "hot" side will pick up hum like mad. The way to install them under a bridge is to put the "hot" side toward the grounded bridge bottom with a layer of mylar film as insulation. Now the pickup "hot" side is sandwiched between it's own ground side and the grounded bridge bottom. That should control hum.

Then feed to the piezo to a high Z (10 meg!) preamp. DO NOT use a lower impedance! If you try it will drastically roll off the bass response of the pickup. You can get away without a piezo preamp on a guitar where notes don't go so low but on a bass you can't. I recommend the Fishman "power chip" for combining a magnetic and piezo pickup. It's a circuit with a pot that has all sorts of nifty options and features (including stereo out which I LOVE!) It supplies the uber high input impedance. They are a tad expensive but worth it.

http://accessories.musiciansfriend.com/product/Fishman-Powerchip-Preamp?sku=303128

I've done quite a few instruments in various ways including a ceramic wafer (from Radio Shack piezo buzzer) on the resonator cone biscuit to a piezo disk mounted on the UNDERSIDE of the top of the guitar under the bridge. Also the piezo coax might be worth experimenting with as well! Is terrific under the saddle on acoustic instruments. All these things "sort of work" and give passable sound, but the thin film piezos are clearly the best.

Good luck!

 

[walterw]

Check here for raw piezo transducers...

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nice!

now i'm imagining one of those piezo strips under a thin veneer of wood or even a super-thin layer of hard metal, sitting on top of a bridge with a smooth baseplate so as to be under all the saddles.

i like the metal idea the more i think about it, as it would shield the film as well as keep the saddle screws from poking through it.

 

[nekodensha]

Thankyou Bassbenj for the link to the piezo tabs and cable!

FWIW, I found a schematic for a 'no-frills' FET preamp here http://www.till.com/articles/GuitarPreamp/index.html, but the impedance of this device is only 3 meg.

If anyone knows how to modify/add to that circuit to generate a +10 meg ohm impedance value, that info would be most welcome.

The Fishman would offer the best solution to integrating the piezo circuit, since it's set up for piezo/mag blend.

(I'm also a big fan of stereo output and importantly, that unit's not too expensive either )

Small -very small- piezo tabs could work under-saddle with the particular model Gotoh bridge I'm using; it has "half-barrel" saddles which are quite flat on the underside. Just need a tab small enough that it will fit between both saddle screws and not poke out at the front or back of the saddle. The 3/8" by 1" are still too large.

Maybe tapping the saddles at the rear and installing piezo coax cable there is an option, since the intonation screws are mounted asymetrically? I'll need to tap the bridge baseplate in any case, so anyone's thoughts on that idea will be appreciated.

Question 1,373,382:- Is Mylar the only suitable material for fixing the piezo tabs to metal saddles? Are there any other ways of fixing the tab to the saddle that will retain some flexibility? (please don't say gaff tape ) I have no heat gun (or Mylar) and as far as I know, that's how Mylar gets fixed to things.

Walter, I like the sound of that idea of yours BTW. I'm just not sure if you mean having the piezos butt up against the saddle undersides, or mounted on top of the bridge plate and sensing vibrations from the saddle "feet"/screws. The idea of keeping piezo leads away from saddle and intonation screws is good!

Thanks to all of you for posting. I believe my understanding of piezo requirements may have improved a little.

 

[walterw]

FWIW, I found a schematic for a 'no-frills' FET preamp here http://www.till.com/articles/GuitarPreamp/index.html, but the impedance of this device is only 3 meg.

If anyone knows how to modify/add to that circuit to generate a +10 meg ohm impedance value, that info would be most welcome.

i think that all you have to do is replace that 3M resistor at the input with a 10M, but there might be more to it.

Walter, I like the sound of that idea of yours BTW. I'm just not sure if you mean having the piezos butt up against the saddle undersides, or mounted on top of the bridge plate and sensing vibrations from the saddle "feet"/screws. The idea of keeping piezo leads away from saddle and intonation screws is good!

the second one, saddle screws all pressing on one flat plate that's covering a piezo film.