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rattletree.com • View topic - Electric mbiras

Electric mbiras

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Electric mbiras

Postby Joel on Wed Dec 10, 2008 4:04 pm

I've been a big fan of the Oyster external peizo pickup. Having been doing the electric mbira thing for many years now, I have found the flexibility of being able to move the pickup around far outweighs the convenience of internally installed pickups.

I am convinced that it the differences in the woods of the mbira body (even in the same species of wood) makes a 'one-size-fits-all' internal pickup not work well. A pickup put in the same place of different mbiras will still accent some notes more than others (and not just the notes directly above the pickup). Some builders put multiple pickups across the whole back of the body to capture everything and seem to just hope that everything will be balanced. I have never found this to be the case. Dan Pauli does something interesting where he uses two different pieces of wood for the left and right side of the mbira-separated by a spacer to isolate each side. He then separately controls each side-even using a different amp for each side. This gives a lot of control, and many mbira players like the results-including Stella Chiweshe and Chris Berry (and of course Dan)who use his mbiras regularly while on tour. Here's a pic of one of Dan's mbiras:
Image

Now Dan is a good friend of mine who I have played music with for many years-so I have great respect for his building. I have a few issues with that method, however. The main issues are: 1)These mbiras tend to not sound like an acoustic mbira to me, and 2)there are a lot of resultant melodies that happen on the mbira that cross over both hands, so I think it takes away some of the inherent depth of the music when the sound is split onto two apms like that (mind you, Dan has very astutely observed that acoustic guitars and electric guitars have very different properties and that he is attempting to build the best 'electric' mbira-therefore, it's not really fair for me to compare his mbira to a purely acoustic mbira). As for the second point, one isn't required to play through two separate amps or speakers, so I suppose that issue can be addressed also.

But for me, having an mbira that sounds great acoustically AND sounds great plugged in is a goal. That is why I have always used the oyster:
Image

But, sadly these things never seem to last and inevitably I end up having to sand off the back of the oyster and resolder the wires every few weeks.

So on my most recent mbira build, I tried something different. I have been thinking for a long time about having a few pickups installed and then having a separate volume control for each pickup. That seemed like it would take care of the balance issues. I could never figure out a method that I liked for installing the pots, though, and I realized that I would probably never change the settings of the pots while playing. I decided to do an internal pickup with three peizos and use a 500K Audio taper pot (like a Les Paul) and figure out the best settings for the mbira. I temporarily wired up each pickup with the pot and came up with the resistances that gave me the best balances. I then measured those resistances and installed resistors and got rid of the pot. I then covered the whole thing with a coverplate with a metal shield. So far I am very happy with the results. I have only used it in a few venues, so I haven't seen yet if there are vast differences from venue to venue (the big advantage of the oyster).

The mbira with installed pickup:
Image
Image
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With love and gratitude,
Joel Laviolette
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Re: Electric mbiras

Postby dezesticks on Thu Jul 09, 2009 9:38 pm

One problem that most people do not take into account with Piezos is that they have random polarity. I often use as many as ten piezos wired together and the trick is to make sure they all have the Ceramic as the +. I use a oscilloscope to test but you may be able to use a good volt meter. If you put piezos of different polarity in line you get automatic cancellation. Recently I have been playing with the accelerometer used in WII remotes as a pickup. The sound is amazingly even but has noise issues as they were not meant as sound devices. I am working on a noise filter/gate to see if they will work better. They do require two AA batteries which look funny on the back of an mbira but hey gotta do what we gotta do.
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Re: Electric mbiras

Postby Joel on Thu Jul 09, 2009 9:48 pm

I don't understand what you mean about random polarity? The main disc should always be negative (or ground) and the attached ceramic should always be positive. unless you mean that the possible variations of the insulating material thickness would be enough to shift the phase between two elements?

If I'm missing something, i'd be interested to throw some piezos up on my scope to see what you mean...
With love and gratitude,
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Re: Electric mbiras

Postby Joel on Sun Jul 12, 2009 12:39 am

Well, I have been learning more! Thanks, Aaron for bringing this to our attention. Here's a post from a friend at a DIY site that I belong to:

At frequencies below resonance their impedance is capacitive...about 20,000pf avarage. Bigger ones are higher.

Easiest way is if you have something like a charge amp. A fet input follower (unity gain) op amp will do. My lab scopes are very high impedance so I can just hook the piezo directly to the scope.

Hook input/ grd to the piezo. Then just lay them on a support (like a little ring about 2/3 the diameter) and press on it with a pencil eraser or something. See if the op amp (or scope) output goes positive or negative. Even with a fairly light pressure you should see 5-20 volts.

For this to work RC has to be a second or so, so that means the op amp (or scope) input has to be at least 50 meg ohms . A bit higher would be better. The input impedance of the amplifier and the capacitance of the element form a high pass filter.


Take care soldering to these things. High heat will depole them.

I buy the PZT material in large sheets with the poling direction marked with a dot on one side. I still double check them though.

Digi key sells the buzzer elements alone. They used to be 21 cents, but the price just went up to about 90 cents. Buy a bag of the ones that resonate at 7 or 9 kHz....They are thinner and make better pickups.

I design special microphones with similar materials used for energy harvesting....using sound and vibration to generate power to run electronics.
With love and gratitude,
Joel Laviolette
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Re: Electric mbiras

Postby Robert on Sat Oct 17, 2009 4:35 am

A few beginner questions:
How are the Oyster pickups mounted into the mbira? (poster gum, wood glue, or just pressure from the metal plate, etc.)
Does the 1/4" plug go into the side of the instrument?
Did your fixed resistor values work for all amplified playing situations?

And regarding your paste from a DIY forum, Digikey has both 1) "Buzzer Elements, Piezo Benders" as well as 2) "Buzzers". Which is the one to try? Also, there are lots of other variables (click the links to see) that have me stumped as to what to try, such as voltage and current ratings, impedance, etc. Suggestions on where to start? I also wonder if the multiple pickup approach would be necessary at any rate.

I've tried in the past with using K&K external piezo pickups without too much success. The adhesive eventually stopped working, so I tried to mount a piece of plastic to press the pickup into the wood at the rear of the body (it's paper-thin btw), or just poor-manning it with duct tape, but the pickups required a preamp (I've tried both K&K and L.R. Baggs), and due to its low signal I found it was prone to feedback with even medium volume situations. There was also the problem with the uneven match of low vs. high frequency sounds, which the tone controls on the pickup preamps could only partially remedy, so I gave up on this method except for very small situations.

I've heard of using a very cheap ($2) Radio Shack piezo element (probably this one since that is all they have), and having it mounted within a drilled hole somehow. Have you heard of this? I believe it was only with a single pickup, but I'll have to ask for more details on this method, although the sound seemed very natural and balanced to me.

And in your experience how prone to acoustic feedback is a deze-mounted directional microphone (on an all-acoustic mbira) at louder monitor volumes? I've seen it done, but don't know details.
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Re: Electric mbiras

Postby Joel on Sun Oct 18, 2009 11:27 pm

Hi Robert,

Robert wrote:How are the Oyster pickups mounted into the mbira? (poster gum, wood glue, or just pressure from the metal plate, etc.)

With poster tack.

Does the 1/4" plug go into the side of the instrument?

If you're taliing about the mbira up there with the resistor network-yes it is on the side.

Did your fixed resistor values work for all amplified playing situations?


So far so good.

re: which buzzers...I use these: http://www.mouser.com/Search/ProductDetail.aspx?R=7BB-20-6virtualkey64800000virtualkey81-7BB-20-6

I've tried in the past with using K&K external piezo pickups without too much success. The adhesive eventually stopped working, so I tried to mount a piece of plastic to press the pickup into the wood at the rear of the body (it's paper-thin btw), or just poor-manning it with duct tape, but the pickups required a preamp (I've tried both K&K and L.R. Baggs), and due to its low signal I found it was prone to feedback with even medium volume situations. There was also the problem with the uneven match of low vs. high frequency sounds, which the tone controls on the pickup preamps could only partially remedy, so I gave up on this method except for very small situations.

The piezo will need some sort of high impedance pre-amp. Some people like fancy DIs. I like tube amps. A tube preamp can handle high input impedance well.

I've heard of using a very cheap ($2) Radio Shack piezo element (probably this one since that is all they have), and having it mounted within a drilled hole somehow. Have you heard of this? I believe it was only with a single pickup, but I'll have to ask for more details on this method, although the sound seemed very natural and balanced to me.

The above mouser link is essentially the same thing without the plastic housing...and are $.50 each. Yes the balance can be great...or not...depending on lot's of factors. That's why I like the oyster for that reason-it can be moved to the sweet spot.

And in your experience how prone to acoustic feedback is a deze-mounted directional microphone (on an all-acoustic mbira) at louder monitor volumes? I've seen it done, but don't know details.

[/quote]
In my experience...it's very prone to feedback. That's essentially the same as an acoustic guitar miced up on stage.
With love and gratitude,
Joel Laviolette
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Re: Electric mbiras

Postby Joel on Mon Apr 11, 2011 11:19 am

Just a short picture update of a few electric mbira I have completed lately:

Electric matepe:

Image
Image

Electric mbira with integrated tonestack:
Image
Image

Electric mbira with integrated tone stack point-to-point:
Image
Image
With love and gratitude,
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