Funny impedance mismatch between magnetic and piezo?

[David Schwab]

David that's very true but I was trying to preserve the tone of the magnetic pickup in as simple a passive way I could think of. You can't have everything all the time (until you are willing to drop a 9V into the mix).

I guess it would add some piezo sheen to the mix.

There's a builder who's name escapes me, that makes guitars and basses with a piezo bridge pickup, and a magnetic neck pickup. He runs the piezo in series with the magnetic pickup. He says that makes them work together and reduces the piezo quack.

Could be worth trying out.

[David King]

George Fulanetto (F-Basses) would do this with RMC saddles and his magnetic pickups on the Alan Caron model basses. It does add a sheen to the magnetic tone. Graphtech's piezo saddles can be ordered with a 1meg pot to act as a passive blender of sorts.

[madzub]

I guess it would add some piezo sheen to the mix.

There's a builder who's name escapes me, that makes guitars and basses with a piezo bridge pickup, and a magnetic neck pickup. He runs the piezo in series with the magnetic pickup. He says that makes them work together and reduces the piezo quack.

Could be worth trying out.

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. This way the magnetic pickup and the piezo are loading each other, if i understand correctly. I tried this briefly with a disc piezo, it worked, sounded as expected, added some acousticy shimmer, but it removed the mid-hi resonance of the magnetic pickup, replacing it with more high end. Not a bad sound at all, just different. There seemed to be some phase cancellation going on, maybe on the crossover frequencies between the piezo and the pickup, and would sound very different depending where the piezo was placed, giving different phasing effects. It also makes a lot of noise if you hit it directly.

[Peter Naglitsch]

I guess it would add some piezo sheen to the mix.

There's a builder who's name escapes me, that makes guitars and basses with a piezo bridge pickup, and a magnetic neck pickup. He runs the piezo in series with the magnetic pickup. He says that makes them work together and reduces the piezo quack.

Could be worth trying out.

Check out the Wendler MagPi system. He uses a passive HB together with a passive Piezo for acoustic guitars:
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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...

[MarkusBass]

Fender makes on of their bass guitars with piezo bridge and the transducer is connected in parallel to magnetic pickups (without any buffer). But the problem is that it sounds very bad and what is even more painful, the signal from piezo is so strong that it can overdrive the preamp. I've got such a bass with a request to remove the piezo bridge - considered as failed. I removed it and put it into another bass with a buffer and learned that there is nothing wrong with it. It only needed the buffer and proper balancing of signal volume between piezo and magnetic pickups. I was happy because I got this piezo bridge for free. And the customer was happy because he had his bass sounding good. So piezo is very difficult to be used in a bass; it can make it sound very poor if used incorrectly and it can make it sound great if you know how to use it.

Mark

[J M Fahey]

Not a Pickup maker, even less a Luthier, but I have a "Shop Bass" for customers who don't bring their own to test my amps.
Years ago I added a piezo disk sandwiched between the bridge and the body.
Sounds *very* good, customers love it and want to add same to their regular basses .
I *did* use a buffer, of course
It does not only add "sheen", it's not just a high frequency increase, the main effect (in my view) is that a whole lot of new harmonics appears.
A magnetic pickup "hears" those harmonics vibrating the strings a lot just above them, mising many others in the process; a bridge pickup catches *all* of them and at an even level.
Typical reaction after the Piezo is turned down is "now it sounds muddy".

[Triad]

A passive piezo (unbuffered) isn't good if used alone... I tried. But I think it's pretty nice if you already have a good magnetic pickup. Yes, the piezo adds some extra articulation... the effect is very strong expecially on fretless, where it can capture something more than the mag. Some sort of doublebass-like attack.

[Jazz P Bass]

Have you Googled this issue?
(Not that I am not enjoying the back & forth of the post)
You are not the first to attempt this.
The answer, more often than not, is some type of buffer on the piezo.
The impedance mismatch must be addressed.

[Triad]

Yes, I googled a bit. Do you think that if I put both the signals into a buffer I'd solve everything? Or I should only buffer the piezo and leave the magnetic signal alone?

[David King]

What about a stack of 1:64 audio transformers to drop the piezo voltage output and boost the current.

[Triad]

Audio transformers? Looks a bit too complicated for this application.

[MarkusBass]

Such an audio transformer would need to have input impedance of at least 3 Mega Ohms and I haven't seen such a transformer in my life. Have you?
Talking seriously there are several options:
1. a buffer for piezo and a passive mixer (with resistors only) - this is the cheapest one,
2. a buffer for piezo and a buffer for magnetic pickups and a passive mixer (again resistors),
3. a buffer for piezo and an active mixer to mix the signal with magnetic pickups,
4. a buffer for piezo and an active mixer to mix the signal with magnetic pickups, and a phase-switch for piezo - this is Fishman's buffer.
You can choose any one of these options depending on your budget. I used option #1 but sometimes it was diffucult to match signal levels (but I always managed to do it). I have somewhere this buffer - if I find it, I'll post a photo.

Mark

[Mike Sulzer]

There is another option which is better than the usual implementation of 1. Use a simple buffer on the piezo. This can be nothing more than a FET source follower. This buffer must have a low output impedance, such as a follower or an op amp. The follower needs a coupling capacitor at its output. Remove the ground lead of the mag. pickup and connect it to the cap. The mag. pickup is now grounded through the low impedance of the source follower, and so its sound is affected almost not at all. You can come up with a scheme for reducing the level of the piezo if it is too high.

Such an audio transformer would need to have input impedance of at least 3 Mega Ohms and I haven't seen such a transformer in my life. Have you?
Talking seriously there are several options:
1. a buffer for piezo and a passive mixer (with resistors only) - this is the cheapest one,
2. a buffer for piezo and a buffer for magnetic pickups and a passive mixer (again resistors),
3. a buffer for piezo and an active mixer to mix the signal with magnetic pickups,
4. a buffer for piezo and an active mixer to mix the signal with magnetic pickups, and a phase-switch for piezo - this is Fishman's buffer.
You can choose any one of these options depending on your budget. I used option #1 but sometimes it was diffucult to match signal levels (but I always managed to do it). I have somewhere this buffer - if I find it, I'll post a photo.

Mark

[MarkusBass]

Interesting idea. I have to try it. But I think that usually the bass player would like to be able to control the balance between piezo and magnetic pickups. Here is the buffer that I use. The board is 18x18 mm and the current consumption is 0.3 mA. On the other side of the board there is a trimmer to decrease the signal from piezo if it's needed (and usually it is needed).

Mark

[Mike Sulzer]

You can connect the output of the buffer to the top of a pot, bottom grounded, slider to the mag. pickup low side. I have used this arrangement for adding in a dummy coil output.

Interesting idea. I have to try it. But I think that usually the bass player would like to be able to control the balance between piezo and magnetic pickups. Here is the buffer that I use. The board is 18x18 mm and the current consumption is 0.3 mA. On the other side of the board there is a trimmer to decrease the signal from piezo if it's needed (and usually it is needed).

Mark
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