Funny impedance mismatch between magnetic and piezo?

[J M Fahey]

I have used a *very* simple single FET buffer (no PCB involved) and added a 47K resistor in series with its output (you can experiment with its value) , so "now it behaves as if it were a magnetic pickup" and can be used in a standard otherwise passive circuit.
I mean classic volume and tone controls, mixing with other pickups, etc.
If you want to carry this "simulation" to the end, choose a resistor value such as when connecting the piezo in parallel with the magnetic, the end result is *roughly* what you would hear if you had just connected 2 magnetics in parallel.

I made it this way, so it works with *any* previous magnetic PUp or circuit you find there.
Series connection is very interesting, but not all pickups have the "floating" winding (independent from ground or shield) to achieve it.

Of course, the "best" solution is to add a dedicated buffer to every pickup (easy and cheap with TL064) or at least one for the Piezo and another for "everything else", in which case a TL062 is enough.
Although "mixing with resistors" sounds backwards in a regular amplifier, compared to what we usually find inside an Instrument is NASA/Nobel Prize Technology

[madzub]

Check out the Wendler MagPi system. He uses a passive HB together with a passive Piezo for acoustic guitars:
New Page 2
And that is without a buffer...

I had a look into this a while ago when a customer wanted me to build a five string acoustic archtop bass for him using the MagPi system. I wasn't able to find exact info about how this was built (series or parallel, any additional components) and the customer changed his mind in the end so I never started to experiment with it. However the inventor claim he get "true" acoustic sound using only passive components. Personally I don't find his sound samples to convincing...

Wendler, that's the W I was thinking of!

[Triad]

Yeah, it's an option. But wiring more piezos together in parallel would make things better or worse? I'd guess "better" 'cause the impedance should be lowered.

[Mike Sulzer]

Better for what? Not putting in parallel with the mag. That would lower the resonant frequency of the mag. even more. But if you use enough piezos, maybe putting them in parallel and in series with the mag would be OK.

Yeah, it's an option. But wiring more piezos together in parallel would make things better or worse? I'd guess "better" 'cause the impedance should be lowered.

[Triad]

"If I used enough piezos in parallel AND in series with the mag"... not sure about what you mean here. You're recommending to wire the piezos in parallel between them and in series to the mag, right? I don't remember if I tried to connect them in series to the mag before, maybe, and if I did it didn't make a difference.

[Mike Sulzer]

"If I used enough piezos in parallel AND in series with the mag"... not sure about what you mean here. You're recommending to wire the piezos in parallel between them and in series to the mag, right? I don't remember if I tried to connect them in series to the mag before, maybe, and if I did it didn't make a difference.

Well, somebody suggested it earlier, and it seems like a good idea if the capacitance of the piezo system is high enough. But this does not change one simple fact: The right way is to use a buffer.

[Triad]

Tried but without getting better results... oh well, it was worth a try anyway! Thanx again, guys!

[David Schwab]

I've seen that guy too, forget his name also, i think it starts with a W. But I thought he wired the piezo in parallel, so it acts like like a tone cap, except rather than just bleeding highs to ground, it adds the piezo shimmer in place.

No, I'm pretty sure it was in series. I remembered it because it was unusual. It claimed that wiring fixed the problem with using piezos and magnetic pickups together, and the magnetic pickup acted like a buffer of sorts for the piezo.

We already know you can't use them together well in parallel.

[edit]

The builder is named Wendler:

MagPi | Guitar Repair Kansas City | Luthier Acoustic Electric Guitar

Here's a post at TalkBass where he was discussing it:

Do you HAVE to power a piezo pickup? - Page 2 - TalkBass Forums

Let's talk about the piezo. It has been accurately described above as to the function...but I'll add this. A piezo can produce a TON of voltage (that's the device they use to start your backyard barbeque grill...that little "clicker" button). Voltage is what allows that spark to travel an inch or so to ignite the propane. However....there is very little current because of the ultra high resistance (impedance). During my study of the piezo, I found that the device actually OVERPRODUCES the attack transient, in otherwords...1X in = about 2X out (the inductance of the magnetic pickup does a bit of a reverse of this...1X in = about .666X out).

So the piezo can be EXTREMELY difficult to learn to use. Typically, this is fixed "in the mix" through the buffering preamp. Rick Turner (among others) has gone a long way in pioneering a buffering preamp circuit that actually works...however, in using the piezo by itself the basic problem still remains in how to fix that "funny" feel that all piezo devices have...just a bit to strong of an attack from your hands, and you end up with either the dreaded "quack"...or a compressed sounding midband that just doesn't cut it dynamically.

I realized that I could use the highly inductive coil in the magnetic pickup to "absorb" (act like a shock absorber) some of the dynamic overreach of the piezo. Conversely, the piezo, with it's HUGE frequency response, could fill in the low end and the high end the magnetic pup just doesn't have (most magnetics are essentially a midband device...listen to a Jbass pup by itself compared to a string bass and you get it).

This is done, in my basses, by "buffering" the piezo THROUGH the magnetic pickup....it's an EXTREMELY simple circuit (aahhh...just like I like it!!) that begins to allow the player GREAT feel, dynamics and frequency response...in otherwords great articulation...along with a very natural feel.

So NO, a piezo doesn't really need a "buffering preamp" to function...but it won't function BY ITSELF...there needs to be another device in the circuit (in my design, a humcancelling magnetic string pickup) to get the most from the device.

Here's a photo of the complete MagPi pickup system. The only thing missing is the aluminum saddle I use to ground the system.

[Mike Sulzer]

"I realized that I could use the highly inductive coil in the magnetic pickup to "absorb" (act like a shock absorber) some of the dynamic overreach of the piezo."

What an idiot! He talks about ultra high frequencies taken out by the inductance. But the ultra high frequencies have a path around the inductor because the coil has capacitance in parallel with the inductance. However, this forms a capacitive voltage divider with the cable capacitance, removing many of the high frequencies. He should be happy to get the sound he wants and not worry about the explanation.