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Piezo Questions...

The Ethanator

The Ethanator

Jul 28, 2008
517
68
Illinois, except like 6 hours from Chicago
#1
A few months ago, I picked up a Charvel ATX from a friend in a trade. The bass has no knobs on the front, just a PS-900 tone control in the same spot as an acoustic preamp, linked up to a piezo transducer which rests underneath the bone saddle. My friend said that the previous owner upgraded the preamp in some way - he is not sure of how, and there is very little information on these basses, which makes it hard to find specifications and hence, what, if anything, was done to the circuit. Well, when I took the strings off (which held the saddle down), I took a first look at the transducer and noticed there are 6 elements in a row on the transducer, spaced to guitar string spacing. It's not so much out of line on the edge strings, but the middle two are out of line and the D string even passes in between two of them rather than over. If a piezo system is made to detect vibrations and all of the sensors sit underneath one single saddle, would I gain anything by replacing the transducer with one with 4 elements spaced for bass, or am I fine since all 6 are (theoretically) picking up the vibration from all 4 strings? Secondly, I'm reading a lot about 'buffer circuits', since piezos apparently have a higher output. How do I tell if my bass has a buffer circuit installed? Is there any acoustic guitar preamps made for piezo pickups that have a buffer circuit built in, or should I look into getting a buffer circuit in addition to a nice acoustic bass preamp?
 
#2
1. You're fine. The strings make the entire bridge vibrate, as well as the body.
2. Piezo's don't have a higher output, they have a higher output impedance, leading to signal degradation between the piezos and the amp. It makes them sound thin and lifeless. If you have a preamp already installed, then you don't really *need* a buffer circuit. Usually a buffer circuit is what is in place instead of a preamp, especially if you have passive pickups combined with the piezos.

To sum it up, if it doesn't sound like crap you're good to go
 
#3
Piezo buffers are desirable for the sole purpose of lowering the output impedance of piezo elements. It has nothing to do with output levels, unless you're trying to match piezo and magnetic sources. The difference between piezo buffers and traditional preamps is that piezo buffers have a very high input impedance. If you run piezo elements into traditional 1M Ohm inputs, you will load down the piezos. Usually, the higher the input impedance, the better, but the reason you don't see traditional preamps with input impedances as high as piezo buffers is that higher input impedances create noisier signals.

The way you tell if the piezos are buffered is to see if they are connected to something battery-powered. If they are, and they sound good, then there is no reason to worry about any of this.
 
#4
BioWeapon said:
1. You're fine. The strings make the entire bridge vibrate, as well as the body.
2. Piezo's don't have a higher output, they have a higher output impedance, leading to signal degradation between the piezos and the amp. It makes them sound thin and lifeless. If you have a preamp already installed, then you don't really *need* a buffer circuit. Usually a buffer circuit is what is in place instead of a preamp, especially if you have passive pickups combined with the piezos.

To sum it up, if it doesn't sound like crap you're good to go
Click to expand...

Beat me to it. :hyper:
 
#6
The Ethanator said:
There does not appear to be any sort of battery in the cavity - the piezo is hooked up to a passive tone control, and I do feel like the tone is way too treble-y even with the tone know all the way down. So if I were to replace the tone control with an active preamp, would I need to include a buffer circuit between the two?
Click to expand...

A preamp is a buffer. It is neither necessary, nor a good idea, to have multiple preamps/buffers chained together.

What you want is basically any sort of transistor or opamp based circuit with an input impedance that is optimized for piezo elements, and an output impedance in the usual range for signals that feed standard 1M Ohm inputs on amps/effects/etc. A standard bass preamp is not going to work well, as the input impedance is not high enough. There are preamps/buffers made specifically for piezos. That's what you want.

Once the piezos feed into a piezo buffer, the signal at its output can be treated just like any other signal from an active bass. If you want a tone control, you can add one.
 
#7
If a preamp/ buffer is optimized for the piezo's, won't magnetic pickups then not have optimal input to the pre?
What would be a good compromise for combined piezo/ magnetics with an active pre?
Poss to run 2 separate buffer/pre's, then blend their signals?
 
#8
I've worked with piezos since the late 70's. To get the full range a piezo can produce, it needs to see an input impedance of 5-10meg ohms. Reduce that input impedance, and you'll lose low end and due to the loss, it will appear to have less volume.

A buffer will have a high (10meg or so) input impedance and a low output impedance. Although a buffer doesn't amplify (it's actually very slightly less than unity gain) it will increase the volume of most piezos due to the now produced low end. Without a buffer the tone and output of the piezo will vary depending on what it is plugged into.

There is no good way to passively mix magnetic pickups (which need to see 500k to 1meg impedance) and piezos (which need 10 times that.)

Using two buffers, one for the magnetic pickups and one for the piezo, is a pretty good plan. The magnetic pickup should use a buffer with a 1meg input impedance, and the piezo at least 5meg.

Getting even string to string volume depends on making sure the bottom of the saddle, and the bottom of the saddle slot is absolutely flat. Plus, the saddle cannot bind in the slot.

Piezos that go under saddles come in basically three types, discrete piezo pieces (one per string), piezo film (B-band for example) and piezo rope.

I've enough experience with piezos to know that getting even response takes some work. I would not use a 6 element piezo strip with a 4 or 5 string bass. It may work though, and won't hurt to try. Even very slight gaps in the piezo/saddle/slot bottom will result in weak strings, and using a 6 element pickup with 4 or 5 strings assures gaps.

Similar piezo strips for 4 or 5 string basses are cheap on e-bay. I like piezo rope pickups and they are cheap as well.
 
#10
JustForSport said:
If a preamp/ buffer is optimized for the piezo's, won't magnetic pickups then not have optimal input to the pre?
What would be a good compromise for combined piezo/ magnetics with an active pre?
Poss to run 2 separate buffer/pre's, then blend their signals?
Click to expand...

Piezos and magnetics don't mix, period. You always buffer them separately before combining signals.

The piezos must be buffered to provide appropriate input and output impedances, and then the magnetics must be buffered, because the piezo buffer has a lower output impedance than they do, and thus, they would be loaded down without their own buffer.
 
#11
Thanks, 'Steve Dallman' and 'line6man' for the detailed explanations-
awaiting a 'NBD' (to me) which includes a piezo bridge combined with magnetic pickups.
Hope these explanations help OP and others, as I've only gotten bits-n-pieces to attempt put together before.
 
#12
Grissle said:
Try this for piezo only:
http://www.graphtech.com/products/b...e-kit?id=e9085f46-5ed6-4a03-b83b-1d4ba296a7c0

Or this for piezo plus magnetic:
http://www.graphtech.com/products/b...nced)?id=f619a825-6902-460c-be5f-fe86687f1c57
Click to expand...

You couldn't use that first kit for a piezo pickup, as it doesn't process the signal at all. It would still sound bad.

The second one is rather expensive at 150$.

You could instead, for less than 10$ of parts, and even without a board to solder parts to, a Craig Anderton buffer for the piezos.
Diagram for those interested:
Link Removed
 

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