Piezo preamp for piezo bridge - diyAudio
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Old 25th October 2011, 10:33 AM   #1
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Default Piezo preamp for piezo bridge

Hi there. I searched the articles but nothing found about piezo bridge for electric guitars.Anyway i have a LesPaul guitar with piezo bridge. Bought it from ebay. I need some advice for preamp. This is piezo bridge + boss me50.

Les Paul Piezo Test - YouTube

So not exacly the sound of lrbaggs or fishman like sounds. I need a compact preamp for better volume and tone. I installed 9v battery box and 3 way(on-on-on) switch for select piezo / pickup / both. So all i need is a preamp that increases volume simply. Sorry for my english.
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Old 25th October 2011, 03:32 PM   #2
Bone is offline Bone  United Kingdom
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This is what you want. < Discrete FET Guitar Preamp > To get a proper bass response the Piezo device needs to be fed into a high impedance.
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Old 25th October 2011, 03:54 PM   #3
kevinkr is offline kevinkr  United States
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^^Looks like a pretty good choice, note also that you can place capacitance across the input to control output level and LF response. Start as is and add a few hundred pF at a time until you get the bass response you want. (This will also help tame the potentially high peak output of the pickup when you play hard, and also lowers the general output level.) If you cannot get the sound you want you may also increase/decrease the input resistor value, lower values of resistance will push the LF corner up, slightly reduce output levels, higher the reverse. Large values of capacitance reduce output levels, but extend the LF response. You may have to fiddle with both to get it right.

If you use LTspice or other similator, measure the capacitance of your pick up and model it as a voltage source in series with that value of capacitance, that will help you to understand the relationship between input resistance, capacitance, gain and LF extension. (I would expect something like -3dB @ 80Hz -100Hz might be a reasonable starting point, and it could end up significantly higher than that) Excessively extended LF response may result in thumping when you handle the guitar or play it. It's all about the right tradeoffs.

I designed piezo pick up pre-amps (amongst other things) at Fishman for a number of years.
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Old 29th October 2011, 10:54 AM   #4
tinitus is offline tinitus  Europe
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PZP-1 Piezo Pickup Buffer
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Old 24th November 2011, 07:05 PM   #5
Lid55 is offline Lid55  Canada
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A newbie question regarding this type of construction:

I created the pre-amp using this design
http://scotthelmke.com/Mint-box-buffer.html

- (I did change two components back so they were applicable for guitar rather than bass)
- my first try... ended up in pyrotechnics due to soldering the one capacitor in reverse phase
- after correcting this... I end up with a strange result... when I place the battery in... the guitar signal goes through... but it sounds identical to a standard cable... as if the pre-amp does nothing at all... but... if the battery is out then there is no signal

Did I make the wrong assumption to solder all grounds together? (including the negative 9V clip wire)

Also... since that first capacitor blew its top... could any of the other components have been damaged? They look fine.

-Lid55
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Old 24th November 2011, 07:20 PM   #6
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Hi Lid55, Were you expecting some gain? A buffer does not have any gain, what you describe is excactly what a buffer should do It is for matching impedances, has a high input impedance with a low output impedance, to keep both source and destination happy. It should not alter the signal in any way.

Tony.
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Old 24th November 2011, 09:58 PM   #7
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Exactly, a buffer like that provides no gain, and shouldn't affect the sound in any way - apart from restoring the missing bass, if the existing preamp was too low an impedance (which is VERY likely to be the case with a piezo).
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Old 24th November 2011, 10:08 PM   #8
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by Nigel Goodwin View Post
Exactly, a buffer like that provides no gain, and shouldn't affect the sound in any way
well, if like in this case doesnt affect the sound, maybe it isnt needed

or the buffer isnt working good enough
still too low input Z, maybe

btw, if you guitar has onboard active preamp, which I suppose, that is where your problem might be
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Old 25th November 2011, 01:04 AM   #9
Lid55 is offline Lid55  Canada
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I don't have an onboard active preamp in my guitar... though this is eventually what I am intending to do with this. I have modified my guitar (came stock with three magnetic pickups) to now include a piezo pickup (I installed a large piezo underneath the metal bridge piece). I do like the extended frequency range of the piezo, but the piezo by itself is thin sounding in the lows and is unbalanced in level compared to the magnetic pickups. A problem for a later date probably is that there is bleed from the magnetic pickups in the blend pot (at 0% magnetic and 100% piezo... I can still hear the mags a bit).

So my goal then is to use this buffer to increase the lows, and if it's possible, to increase the gain to match the magnetic pickup level. I guess I have confused these two issues and assumed the circuit would address both. Does anyone know if this kind of circuit can be easily modified and used as an amplifier as well as a buffer?

But besides that... the lows are still unaffected... the piezo alone still sounds thin.

Here are a few facts/details about the situation that might or might not be problematic:
- I soldered the negative/ground of the input, output, and the 9V clip wires together
- the transistor's "ground" lead was not wired to the ground (assumed that this is just a naming convention)
- I am using mono 1/4" cable connectors on both input and output

any ideas?
- BrettC
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Old 25th November 2011, 01:34 AM   #10
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Quote:
Originally Posted by wintermute View Post
Hi Lid55, Were you expecting some gain? A buffer does not have any gain...
Tony.
It's current gain rather than voltage gain but with the 220K resistor he's providing very little current gain either.

G
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