The pz signal is then routed through a "bias pot" along with the mag signal. The capacitance of the piezo helps to smooth out the mag side blossom, while the inductance of the magnetic helps to realign the attack transient of the piezo...it's a win/win situation, and totally passive. .
If you measure the resistance of a piezo element it turns out ... to have none. Any talk of "proper impedance" is a little like talking of the "proper impedance" of a piece of cheddar. It's actually best modeled as a capacitor with a built-in AC signal source. And here we are with all the caps we use in filters with traditional mag pup ckts! There are countless ways to make use of such a "living capacitor" in various topologies, active OR passive, it's just a question of experimenting, as Dave has done. It would be cool to gradually assemble a list (with example sound clips) of different ways to insert a piezo in a traditional circuit with minimum modifications. The challenge is coming up with the most flexible circuit, ideally without having to drill any additional holes. In time, the piezo-augmented 2 mag pup setup might come to be as standard as the Jazz plate.
So I'll go first, this is a twist on the stereo jack dual output. One topology I like is having the piezo on its own output, which is easily done using Ring-Sleeve of a stereo jack, with the Mag signal on Tip-Sleeve, allowing you to blend in as much of it as you want at the mixer, before or after recording. On a passive bass, this only requires replacing the mono jack with a stereo one, using a stereo cable, and splitting the "right" and "left" at the mixer into two inputs. If you pan one hard right and the other hard left, you can record the two signals on one stereo track and defer any blending until mixdown.
On an active bass, with an internal preamp-tone control, the Ring lug is normally reserved, so you can't use it? Wrong. Here it gets interesting. The Ring terminal is used to ground the negative battery terminal (which is left floating at rest) and thus to activate the preamp whenever you plug in a mono plug that shorts the Ring to Sleeve. If you add a switch to power the preamp on and off, or just leave it on all the time, you can unsolder the battery negative wire and reclaim the Ring terminal to run Piezo signal on it.
But here is an even better approach. I connect the piezo across Ring-Sleeve in the bass,
and leave the battery negative connected to Ring as well. At the mixer end I split the stereo cable with the Tip-Sleeve as "Mag" and Ring-Sleeve as "Piezo", like in the passive bass case. In addition I use an impedance-matching transformer (e.g. Radio Shack 2740017 = US $16.49) at the console end to receive the "Piezo" plug. That's not a bad DI for the Mag signal either, BTW. In that case we use two transformer/adapters.
What happens then is ideal. An additional benefit of using the transformer will become apparent.
If we plug a normal mono cable from the bass to an amp in this situation, it acts like before, Ring is shorted to Sleeve/ground, the preamp powers up, the Mag signal presents at the Tip-Sleeve pair. The Piezo is shorted out, it's out of the equation.
If we plug in a stereo plug, with an impedance-matching transformer on at least the Piezo plug at the mixer end, the transformer primary gets connected across Ring and Sleeve. It looks like a DC "short" to the battery circuit and the preamp powers up normally, presenting Mag signal at Tip-Sleeve. The transformer primary looks like a 1k resistor and that is plenty stiff enough for the preamp (which only draws a little current) to turn on. Meanwhile in the AC realm, the Piezo is feeding the transformer primary across this same pair, and the secondary of the transformer presents a low impedance Piezo signal to the mixer, as if the battery circuit weren't even there. If this is unclear, I'll post a diagram later, but it's pretty obvious.
Even if the preamp draws 3 mA (which is a lot by preamp standards) its effective resistance is 9 volts / 0.003 A = 3000 ohms. The DC resistance of the Radio Shack transformer "high Z" side is about 1 kOhm. So across the transformer the preamp will lose
9 V (1000 Ohms) / (1000 + 3000 Ohms) = 2.25 Volts
or about 1/4 of the voltage, cutting the preamp's 9 volts down to 6.75 volts, plenty good enough in 99% of the cases, as the preamps are designed to run on depleted batteries down to 5 volts or less. If the preamp draws a more typical 1 mA, the loss drops to 10%, even more negligible. Worst case, if the preamp is especially hungry, or the transformer you use has greater DC resistance, you can boost the battery to 18 volts. In practice this has worked with 9 volt circuits just fine. The loading of the piezo by the transformer incidentally changes its frequency response for the better, attenuating excessive high fequencies.
The only caveat is not to plug the Piezo Ring-Sleeve signal directly into the mixer without the transformer, as that will place part of the 9 volts DC from the bass directly across the mixer input. On some designs this is harmless. Many of the older vintage mixers, like my big English console, have transformers on the inputs already and might not even require an external transformer. On others it is met with less favorable response. If you are the distracted type, and don't want to be known as the intrepid console smoker, tape the transformer adapter(s) to the cable so you don't ever forget. A nice cable can be made with a male stereo 1/4" plug at one end for the bass and, at the other end, two transformer adapters with male XLR plugs for the mixer. On a bass with no Piezo, a stereo jack can still be useful for bringing back e.g. one of the mag pickups unattenuated, with an eye to recording it separately and having more flexibility at mixdown time.
So, anyway, there are many other ways, but this is one scenario for integrating a Piezo transducer with a bass setup, the "separate pair" wiring scheme. It works for both active and passive basses, the bass is unaltered for use with a mono amp cable, and it is especially well-suited to the recording situation, where using the special cable automatically turns it into a more adventurous instrument that lets the mixer blend Piezo and Mag signals or record them separately.
Next, we will examine a mounting strategy for the $1.99 Radio Shack transducer
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