I was asked to comment on whether the Phantom Piezo Preamp (available in the shop here) is suitable for connecting to a balanced piezo transducer as covered by Chrisitan Liljedahl here.
The short answers is yes, absolutely – the Phantom Piezo Preamp has balanced input and output and effectively you connect a single or double piezo transducer in a balanced configuration.
Piezo transducers
Bare piezo transducers generally consist of a brass disc onto which a thin slice of piezo crystal is bonded. The top of the piezo crystal is covered with a layer of silver-loaded paint which may be soldered to (however, if you do solder to this paint, you must use silver-loaded solder).
The transducer has a free-air resonant frequency in the region of 1 ~ 6 kHz depending on the diameter of the transducer – the larger the diameter of the transducer, the lower the resonant frequency. (Note that ultrasonic transducers have much higher resonant frequency but we are not considering those here).
As the disc is caused to vibrate, so the piezo crystal vibrates in sympathy and produces an output voltage between its upper and lower surfaces in accordance with the piezoelectric effect. This voltage varies according the sound pressure acting on the disc and hence the crystal and thus the transducer acts as a microphone. The reason why this device is called a piezoelectric transducer and not a piezoelectric microphone is that it works both ways – if you apply an alternating voltage to the electrical connections, the transducer will vibrate in sympathy.
In order to get a good frequency response from the transducer, it needs to be mechanically coupled to a vibrating surface of considerably higher surface area than the brass disc. This increased surface area will considerably lower the low-frequency cut off of the system. Typical old-school PZM contact microphones, such as the Realistic (Radio Shack/Tandy) and Crown Audio types simply consist of a piezo element bonded to a piece of steel plate. The microphone itself is then expected to be placed on a hard surface such as a table-top. The table top has a very large surface area and thus the system comprising the piezo element, the steel pate and the table top will have a system frequency response that covers the entire audio band.
The piezoelectric effect is able to deliver a significant voltage (compared e.g. with a dynamic or condenser microphone) but little current. Consequently, to get a usable signal from a piezo element a high-impedance pre-amplifer must be situated very close to the transducer. If there is any great distance between transducer and pre-amplifier the capacitance of the connecting cable will greatly reduce the high-frequency cut off and lead to a dull, lifeless sound (tone suckage).
Isolated, unbalanced and balanced connection
The fundamental idea of electricity is that a closed conducting loop is required to complete a functioning electric circuit. When the switch is closed, the circuit is completed, current flows and the lamp lights. Conversely, when the switch is open, circuit continuity is broken, no current flows and the lamp is extinguished:
The lamp has two wires to connect it to the rest of the circuit. In the above schematic, the two wires are of equal status and the switch could just have easily (and with the same effect) have been situated on the negative side of the battery. This is an isolated circuit (i.e. no part of the circuit is connected to ground).
However, we could use a common ground to connect the lamp to the battery thus:
By convention, the ground is at the same electric potential as the Earth (i.e. zero volts). The above circuit is unbalanced as one side of the lamp is connected to ground and the other side is isolated from ground.
A third option would be to ground the mid-point of the battery (we will overlook the fact that you can’t actually do this with a single 9V battery):
This circuit is balanced in the sense that one side of the lamp is at a positive potential (+4.5V) with respect to ground and the other side of the lamp is at an equal but opposite negative potential (-4.5V) with respect to ground.
Unbalanced and balanced audio
When an audio connection is unbalanced, one of the signal wires is referenced (i.e. connected directly) to ground. Examples are phono/RCA and mono jack/TRS. Guitar leads are unbalanced.
When an audio connection is balanced, neither of the signal wires is referenced directly to ground. However a ground connection is usually provided for shielding purposes and/or phantom powering. Examples are XLR leads for microphone and line-level use.
Some types of transducer which generate a voltage or current such as a a dynamic (moving coil) microphone capsule or indeed a piezo element may be connected balanced or unbalanced. However, in this case a balanced connection has an isolated topology not a balanced topology .
A genuine balanced connection requires a pair of equal and opposite signals which have the same magnitude but opposing polarity and include a ground reference.
The purpose of balanced audio
So, what is the actual purpose of balanced audio? Well the answer lies in a characteristic of audio pre-amplifier balanced-input circuits called Common Mode Rejection Ratio (CMRR).
Essentially, any audio cable connecting two devices will pick up interference. Interference is any unwanted voltage referenced to ground.
With an unbalanced signal the interference will be picked up by the un-grounded side of the signal and be amplified by the following circuitry.
With a balanced signal the interference will affect both signal wires equally – However a balanced input stage amplifies the difference between the signal voltages and thus the “common mode” signal which is the same on each signal wire is rejected.
Whilst a balanced connection is no more immune to picking up interference than an unbalanced connection, the balanced input stage rejects interference whilst the unbalanced stage amplifies it.
Connecting piezo transducers
We can connect a single piezo transducer unbalanced or isolated (don’t forget you need to connect to a pre-amp with a very short cord):
So now we finally get to the interesting part. Let us assume that the manufacturer of the piezo element has arranged it so that if we squeeze the crystal in a particular direction we get a positive voltage at the silver connection with respect to the brass plate and that this is true for every sample of piezo transducer of a given type. It stands to reason that if we connect two transducers back-to-back then the voltages from the two crystals will be of equal magnitude and opposite polarity. Thus if we ground the common connection between the piezo elements, we get a truly balanced connection.
We could connect the two transducers with the brass discs inward or outward and get the same balanced effect. However, if we connect them with the brass discs outward and ground the brass discs, we get the added benefit that the discs provide electrical shielding. This should lead to a super low-noise connection to a balanced input pre-amplifier.
This sounds like a great idea in principle and if a piezo manufacturer would like to get on board and develop and manufacture such a device I will be happy to buy it. In reality, there is a problem in that to get an accurate signal, you need to couple the transducers in such a way that the vibration due to the incident sound affects both crystals in the exact same way. This is not easy to achieve as you have to make a connection to the crystals and also ensure that they do not short to each other:
As may be seen, there is a practical limit as to how close you can couple the two elements.
Conclusion
After a fairly modest amount of experimentation I abandoned the idea of making back-to-back piezo elements because of the practicalities. I don’t believe that the back-to-back arrangement will give a better sound than a single element and if the single element is suitably shielded with copper-foil tape then the noise should not be a problem. Given that you need to have a pre-amp close to your transducer to get proper high-frequency response and you have to shield your preamp as well, I don’t think there are any practical benefits to double transducers. 
Thanks for you article. I tried to make some back-to-back balanced piezo pickups also, but shielding one (or more in parallel) discs works the easiest and best.