Tell me about Piezo transducer rods

[drewl]

like the kind that go under the bridge saddle of acoustic guitars.

Repaired a preamp for an Ovation acoustic, but the transducer is weak.
If you plug it into an amp and crank it way up you can hear some sound, and it makes noise when tapping on it.

So do they become "weak"?
can you measure their resistance, I don't think you can?

I need to pull it out so I can match up a replacement, they're pretty cheap.

[shortcircuit]

The main body of the rod is conductive metal, and onto this metal is glued small ceramic wafer rectangles, across the top of these wafers runs a thin strip of conductive metal, usually copper. Leads are soldered to the ceramic and brass, and when the piezo is excited by changes in pressure (such as string vibrations), very small amounts of electric current are produced by the interaction of the ceramic and brass. Is it measurable ?? good question, never tried it. If the copper gets damaged you can lose signal from the disconnected wafers . If you can bypass the pre amp and connect the piezo straight to a jack ,that would let you know if it's the piezo, or possibly still a problem in the pre amp . silly question your using a good charged battery in the pre amp??

[TomCarlos]

Is this an old Ovation Piezo or something new? I think the old ones had the Piezo as part of the saddle assembly?

The resistance of the piezo's is very high - so you really cannot measure them. But it sounds like you have sound? You fixed the preamp? What was wrong with it? Are you sure you have an issue with the pickup as opposed to the preamp?

I have used cheap piezo saddles (the bar units) and connected them to onboard preamp systems. Sometimes, you have the right size connector, other times you might have to solder them in. There are different sizes - length, width, classical (nylon guitar) vs standard (steel string). The thickness can also be different. So you may need to adjust the overall saddle height.

[shortcircuit]

measuring resistance I get OL , with my meter set on AC at idle a 3 bar piezo measured .007 vac at idle and .070vac when strummed.

[Helmholtz]

A piezo transducer is constructed like a ceramic capacitor. Its resistance should be infinite. It measures as capacitor (e.g. 1.5nF depending on size). As it is a capacitive signal source it needs to be connected to a very high input impedance (preamp) for adequate bass response.

[Mick Bailey]

They can fracture internally and then the only coupling is stray capacitance. Due to the high input impedance of the preamp you still get a small signal being amplified. Piezo transducers usually produce quite a high open-circuit output voltage but you need a meter with a high input impedance to measure it. Most DMMs load the signal down too much. Similar to measuring output from a condenser mic capsule - a 10M Ohm DMM kills the signal that's developed across a 5G Ohm load resistor.

I have a dedicated test box that I use to sub in for a preamp to test transducers. I find that if after eliminating other possibilities and I think the transducer is the problem, it usually is.

[kanks]

https://www.google.co.uk/shopping/pr...500710702,cs:1 The bridge saddle needs a air gap or it will be weak normally happens when people change the saddle

[drewl]

Thanks.

Pretty much what I expected, like similar transducers but when you've never actually taken one apart and studied it you aren't exactly sure.

And yes, the preamp and battery are fine.
it had a bad quad opamp I replaced.
Run a very low (couple millivolt) signal into preamp and there's plenty of voltage and signal out.
Even the tuner works, which the signal from the piezo was not hot enough for the tuner to detect it.

I'll take it apart and study it, see if maybe just a broken wire or connection.

Thanks again.

[Helmholtz]

The bridge saddle needs a air gap or it will be weak normally happens when people change the saddle

Not sure if I understand correctly. How would vibrational pressure be transferred to the transducer with an airgap between?
The bridge needs to be in perfect full length mechanical contact with the transducer.

[vintagekiki]

The piezo transducers (6) are wired in parallel, and each guitar string must be on the middle of the piezo transducer rod.
The piezo transducer rod must have strong contact with bridge saddle, against may be weak output signal and sound sensations are also possible.

[Randall]

I have replaced a few of these, they always seem to go bad where it transitions from the strip to the wire, at least in my case.

[Mick Bailey]

There's a trick I learned a few years ago that uses air-drying modelling clay to bed the transducer and saddle in perfect contact to eliminate any gaps and even-out string pressure. Easily reversible if needed.

[m1989jmp]

like the kind that go under the bridge saddle of acoustic guitars.

Repaired a preamp for an Ovation acoustic, but the transducer is weak.
If you plug it into an amp and crank it way up you can hear some sound, and it makes noise when tapping on it.

So do they become "weak"?
can you measure their resistance, I don't think you can?

I need to pull it out so I can match up a replacement, they're pretty cheap.

Which amp are you plugging it into? I had impedance matching problems when trying to plug those guitars/preamps into tube amplifiers, which would result in weak output.

Plugging it into SS practice amp or PA would increase output significantly.

[vintagekiki]

In order to achieve full mechanical contact saddle with the transducer, may be solution to place a sponge between the transducer and the wood bridge that would press the transducer to the saddle.
In any case, any work on piezo transducers within the transducer rod is a touching work with uncertain result.


https://music-electronics-forum.com/showthread.php?t=39569&p=524144&viewfull=1#post524144
Ovation schematics

[Helmholtz]

I had impedance matching problems when trying to plug those guitars/preamps into tube amplifiers, which would result in weak output.

I doubt you had an impedance matching problem with the tube amp. Real impedance matching is only needed when power has to be transferred (or with RF signals).
For audio signal voltages the rule is that the input impedance of the amp should be at least 10 times higher than the output impedance of the source (preamp).
As the preamp should be very low output impedance (not more than a few Ohms with opamps) and the input impedance of tube amps is very high (typically 1M), there shouldn't be an impedance problem.
So if amp output was low, the tube amp's gain would have been too low.