DIY project: The Mint-box Piezo Buffer

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Scodiddly
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DIY project: The Mint-box Piezo Buffer

Post by Scodiddly » Mon Jun 21, 2004 3:20 pm

Image

Piezo pickups are definitely useful, but they have one really serious problem:
Impedance matching. Having an impedance mismatch is like having your car in the
wrong gear, and piezos really need to see a very high input impedance to sound
good. If a piezo has to drive a low impedance the sound will lose a lot of
bass/warmth. There are a variety of good "acoustic" preamps available to help with
this, including built-in electronics and floor units. Trouble is, they're
expensive and usually include EQ and effects you may not need.

Often enough I've done sound for musicians with just this problem, so I finally
ended up building a few very simple, very cheap buffer boxes that I could carry
in my bag and even sell cheaply if a musician wanted to have one.
It runs off a 9 volt battery, uses a single FET (field effect transistor) and a
few other parts, and can be mounted inside those little tin mint boxes.

Here's the schematic:
Image

For the FET I use a 2N5457, which is a fairly common part. It's the same FET
used in the Scott Hampton "J-FET Mic Preamp" kit. The resistors can be 1/8 or
1/4 watt. The input and output capacitors (0.033uF and 0.1uF) should be
something good for audio - I use metallized polyester caps. The 20pF capacitor
can be just a ceramic disc. The 4.7uF capacitor can be tantalum or
electrolytic, doesn't have to be anything fancy.


I'm not going to write too much about the construction, other than to give a few
tips.
1. I just use classic perfboard for the layout, which is probably the
easiest given how simple this circuit is.
2. The tin mint box provides good RF shielding if you actually solder a ground
wire to the box and to the lid. For some reason the mechanical ground
connection from a jack doesn't work very well on its own.
3. A 9v battery will last an amazingly long time in this box, but to save power
you can make the input jack work like a power switch. Simply purchase stereo
(Tip-Ring-Sleeve) jacks, and connect the negative (black) battery lead to the
ring contact on the input jack. When a plug is inserted it will make a
connection between ring and ground, completing the battery circuit.
4. The "gain" switch could be just about any kind of switch. I use one of
those little jumpers found on computer cards, but a SPDT switch would work, or
just make only the connection you need. Gain reduction is really only needed
for pickups with a very hot output, such as upright bass.
5. I used Re'an 1/4" jacks because I have a big bag of them. They're cheaper
than the Switchcraft jacks listed below, but I have to carve some plastic off
to make them fit in the box. I also have to do a couple other hacks to get the
lid to close with the Re'an jacks.

Suggested parts list:
You can buy all these parts from any decent supplier, but here's a list of
Mouser (http://www.mouser.com) part numbers.

512-2N5457 (2N5457)
140-PF1H303K (0.033uF polyester)
140-PF1H104K (0.1uF polyester)
74-199D10V4.7A (4.7uF tantalum)
140-50N5-200J (20pF ceramic)
291-4.7M (4M7 resistor)
291-10M (10M resistor)
291-220K (220K resistor)
291-10K (10K resistor)
502-112B (Stereo jack)
530-CP-1094-AST (9v battery snap)

Many thanks to:
Walter Harley (http://www.cafewalter.com), for providing the original
circuit and helping me tweak it for my upright bass pickup a few years ago.
Dave Latchaw (http://www.mp3forkidz.com/mic), for providing a wealth
of DIY microphone inspiration including the use of mint boxes for prototyping.

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Re: DIY project: The Mint-box Piezo Buffer

Post by dayvel » Mon Jun 21, 2004 3:54 pm

Now you've done it. I *really* have to update my site. That looks really good. I think the insulated jacks are a good idea, helps with little ground loops.

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Re: DIY project: The Mint-box Piezo Buffer

Post by aurelialuz » Mon Jun 21, 2004 4:25 pm

thanks VERY much for posting this.

alex
"While every effort has been made to ensure optimum sound quality, priority has been given to historic content and importance."

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Re: DIY project: The Mint-box Piezo Buffer

Post by Scodiddly » Thu Jun 24, 2004 3:35 pm

Built a couple more, and took the opportunity to scan in the little circuit board:

Image
Image

I played around a bit with trying to combine the two images so the layout could be seen. The component side has been flipped and then "mixed" in with the solder side.
Image

Edit: I fixed the discrepancy between the schematic and these scans.

I'm doing one little cheat to make the layout cleaner. Many FETs allow the drain and source to be reversed in a circuit, so I've done that here. The important thing is that the input does go to the gate terminal.

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Re: DIY project: The Mint-box Piezo Buffer

Post by Flight Feathers » Sat Feb 12, 2005 7:49 pm

thanks a million for posting this. i just put in a fishman pickup in my upright, and have been searching google for hours trying to find EXACTLY this. this post is by far the most thorough explanation out there. thanks again.

ayan
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Re: DIY project: The Mint-box Piezo Buffer

Post by psychicoctopus » Sat Feb 12, 2005 8:22 pm

What piezo elements do you recommend using? Have you tried the flexible film ones from EMI?
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Re: DIY project: The Mint-box Piezo Buffer

Post by radiationroom » Sat Feb 12, 2005 9:20 pm

Thanks for posting this. One of the more useful ideas I've seen in a while.

73's - Carli

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Re: DIY project: The Mint-box Piezo Buffer

Post by standup » Sat Feb 12, 2005 9:34 pm

I recently bought a Chinese ("Eastwood") electric resonator guitar... The P90 style pickup sounds pretty good but not very dobrolike, the piezo is weak, nasal, and thin.

Thiis sucks. If I had used the FSF I may have found this. Instead, on Thursday I ordered a $40 buffering preamp for internal mounting from Pick up the World. Live n learn I suppose.

But hopefully the electro-dobro will sound better when the piezo is fuller and at the same volume as the pickup.

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Re: DIY project: The Mint-box Piezo Buffer

Post by Scodiddly » Sat Feb 12, 2005 10:18 pm

On my To-Do list - get some piezo elements and experiment. You can even use the ones inside $3 Radio Shack piezo buzzers, although I wonder what the sound quality is like.

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Re: DIY project: The Mint-box Piezo Buffer

Post by psychicoctopus » Sat Feb 12, 2005 11:06 pm

Here's a source for flexible piezo film pickup material. cheep! I've tried it so I can attest that it does work, but I wasn't equipped with a proper Hi-Z preamp to make a valid tone judgement.

http://www.windworld.com/products/catalog.htm#hardware
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Re: DIY project: The Mint-box Piezo Buffer

Post by Flight Feathers » Sat Feb 12, 2005 11:13 pm

Scodiddly wrote:On my To-Do list - get some piezo elements and experiment. You can even use the ones inside $3 Radio Shack piezo buzzers, although I wonder what the sound quality is like.
the other guy in my band has a really cheap ibanez acoustic that sounds incredible, that came with a pickup built in. it has always sounded thin, brittle and mettalic, and super feedback prone. of course, we never used a preamp with it. i just installed a fishman blender into it, and discoverd the pickup wasn't even an under the saddle piezo type, it was a piezo disc mounted under the bridge inside the body. the whole electronics of the guitar probably cost them $1. it sounded pretty terrible even plugged into the fishman preamp.
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Re: DIY project: The Mint-box Piezo Buffer

Post by inverseroom » Sun Feb 13, 2005 6:56 am

Neato. Someone--me?--ought to do a series of simple effects built into Altoids boxes...

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