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Old 30th November 2012, 09:47 AM   #11
tinitus is offline tinitus  Europe
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actually, it's even possible to place a small preamp inside a jack connector
but just slightly bigger might be more 'practical'

http://www.albertkreuzer.com/index.htm

Discrete FET Guitar Preamp

FET Preamp Cable

3M ohm it seems to be


oh, and btw, with a violin especially, I would instead consider a footpedal for easy volume control
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Old 30th November 2012, 02:43 PM   #12
JMFahey is online now JMFahey  Argentina
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Yes, cool idea.
If a volume pedal is too expensive, at least a footswitch mute is useful, so you can get close to the amp without squealing, or lay the violin on a table or somewhere.
If you dare to build it, I can post the Schematic and PCB of the Violin Preamp I made, with Volume, Treble and Bass controls.
You can build it in a small cigarette pack sized box, clipped to your belt.
I used a clock buzzer type piezo disk as a microphone, pressure fit between bridge and body, for *strong* signal.
That disk has quite a large capacitance, so a 1M input was enough, but if you want, you can rise the input resistor to 3M3 and even 10M (usually not needed, unless it's a Double Bass).
A Luthier made the solid bridges for me, to replace the original ones.
I sold many such amps, we have a lot of Tango and Argentine Folklore groups here.
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Old 30th November 2012, 03:24 PM   #13
JMFahey is online now JMFahey  Argentina
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Just a practical example: one of my "Fahey 4409" 60W 1x10"+Tweeter Double Bass amps.
5M input impedance, because this particular Jazz and Tango Musician does not use an external preamp.
He connects his "double button" Piezo pickup straight to the amplifier, with a very good quality 8 or 9 foot cable.
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Old 6th December 2012, 01:08 AM   #14
Struth is offline Struth  Canada
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Hi Guys

For a raw peizo pickup an input impedance of at least 10M is required. The standard 1M input for a tube guitar amp will produce a thin bass-less tone. Fortunately, when using either a tube, jfet or jfet opamp, a 10M leak resistor is fine.

There is no need to contour the input or shape the frequency response if you interface the pickup with the correct very-high impedance. This is a common error in dealing with peizos.

One thing to be wary of is having too much treble response inasmuch as you really do not need a crossover and tweeter. This is an identical issue with acoustic guitar. Again, the industry has almost completely paraded down the wrong path with respect to development of dedicated amplifiers for acoustic instruments. The extended treble response will in fact be very accurate BUT it will be entirely unlistenable. The ice-pick attack of notes, string noise, finger scrub, fret rattle etc all come screaming thorough and detract greatly from what would otherwise be a sweet performance.

If you are building a small amp, as this seems to be the goal using ICs, then use a small nominally fullrange driver. Even with this, you might want to experiment with an acoustic diffusor in front of the driver to block the central on-axis sound. This will smooth the response so that all of the tone will be like that of the off-axis response.

Have fun
Kevin O'Connor
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