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Thanks, those are both handy bits of information, and useful link, David. The build I had in mind when I asked this already has 2 magnetic pickups, but i want to make it a piezo hybrid, so I think I will wire two piezos in parallel with an on/off switch to the jack, thanks for both of your advice.
If you don't mind, can you explain why you'd want both piezo and magnetic pick-ups?
Thanks,
Eli
(Un-)solicited electronics rambling...
Evidently putting piezos in series must work, based on what others have said, but not having tried it, based on my initially frustrating experience making an overdrive 'unstomp' box for a violin with naive assumptions about piezo impedance, I will never try series piezos...I had some really bad hum, noise and frequency response issues with the very highest impedance ones (75 mm length of piezo polymer coax), and can't believe how agreeable a large polymer ribbon (~3/4" x 3") piezo has been with everything I plug it into....
A magnetic pickup is (simplified), a coil of wire or inductor. An inductor's electrical property 'L' (inductance) is essentially a constant and independent of frequency. It's impedance (XL=2 x Pi x f x L) however is proportional to (increases) with frequency, so the combination of a magnetic pickup's impedance and a conventional amplifier's input impedance has a general characteristic of a low-pass filter. If you put two identical magnetic pickups in series the combined impedance at a particular frequency doubles in a way that the high frequency rolloff point is cut in half...it may sound less bright, depending on the amp impedance. Inductance and impedance of inductors, and (magnetic pickups) in parallel reduce in a manner similar to resistors in parallel...it is reduced. Paralleled magnetic pickups generally sound brighter. Cable length complicates the issue, but that's a summary.
A piezoelectric sensor (and an electret for that matter) is (simplified), a capacitor. A capacitor's electrical property called capacitance is essentially a constant and independent of frequency but it's impedance is inversely proportional to frequency (decreases with increasing frequency). Because of that inverse dependence of frequency, a capacitor's impedance reduces with increasing frequency Capacitors in parallel ADD in capacitance,the way inductors in series add.
Xc = 1/(2 x Pi x f x C). (Pi, of course being that weird little Greek letter that looks like a Spanish tilda on crutches, that you needed in school to figure out area and circumference of circles before you decided being a professional musician sounded better than geometry.)
When you put capacitors in series their combined capacitance is reduced, and there are not a lot of times it's done. The worst effect this has is that a piezo pickup and the input impedance of an amplifier behave as a high-pass filter. If your piezo impedance is too high for your amp impedance, you will have reduced or poor low frequency response...this is used to advantage with string instruments like bass, cello, etc, in a controlled way, so excessive 'thumpiness' can be controlled. If it's done in an uncontrolled way (like my attempt to use 3"/75 mm of piezo coax (plus regular coax, total capacitance only 100 pF), feeding a 1M ohm guitar amp, the bass cut was ridiculous, even on a violin...so I made sure my first JFET preamp had the highest input resistors I could find...not aware that circuit noise tends to be proportional to resistance...the hiss was bad, the low end was gone, and it was a bear to shield, even with the coax braid already on it. Obviously Fishman had that figured out, and his stuff works...so much for the DIY'er.
The large ribbon piezo is about 3000 pF all cabled up & shielded & is happy with a 1M ohm guitar amp.
Your mileage and language may vary...Series pickups CAN work, but the signal isn't equal, in part because they are often at two different locations along the strings and can also be unequal if the pickups are drastically different.
An example is paralleling a high-output magnetic humbucker (they usually have higher resistance due to more windings, so people equate magnetic pickup resistance to output) with a single coil magnetic pickup...the single coil one is usually a lot lower impedance and drags the humbucker 'down to its level' significantly...not a dramatically worthwhile combination (see deafeddie.net for more useful conbinations).
If no one else answered yet why one might combine a piezo and a magnetic pickup on the same instrument, the one response of "more tones" can be added to by saying that the piezo picks up the physical vibrations of the body and string for a more acoustic sound (unless you can overdrive electronics with it), and a magnetic pickup picks up primarily the string movement in the pickup's magnetic field. So why do solid and hollow bodied instruments sound so different with similar pickups (implying they seem electrically similar)...my best attempt at a guess is that the string vibration interacts with the physical/acoustical properties of the very different bodies...so the string vibrates differently with the same magnetic pickup on two different instruments.
I've read that a piezo under the top of an instrument (say a violin) picks up string and body vibration, but a sensor in a bridge or under the foot of a bridge gets mostly string vibration...but you can't always get inside an instrument to mount a pickup...except on this website...so you stick it, wrap it, whatever, on the outside.
Hopefully all my rambling is helpful to someone who's still awake after reading it.
I will definitely be posting some my own questions shortly, and trust me, some of them will seem clueless, so maybe that will balance out...
Murray
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