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Old 7th January 2013, 05:07 PM   #11
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@JMFahey sorry, i hadn't slept for hours when i posted those frequencies. they should have been 20 - 1400 hz.... duh... my bad!

the instrument is a new version of this acoustic electromagnetic harp i designed last year - as seen here :: ÚLFUR

i used LM386-4n's for the old version, but i want to go for something louder like this guy - Magnetic Resonator Piano | Music . Entertainment . Technology

he uses the LM1876, thats why im opting for that chip. the chip doesn't heat up at all with proper heatsinking.

i would like to use piezos for the feedback circuit instead of magnetic pickups because of interference from the heavy magnetic actuation fields going on. thats why im looking for a pre-amp for the piezo! any thoughts on that?

also - to drive 10x LM1876 at once, do i need an insane power supply system? or should i just wing it?

thanks guys!
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Old 7th January 2013, 07:20 PM   #12
JMFahey is offline JMFahey  Argentina
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1) I loved the concepts at ulfur.com
Is it some Art Cooperative or something?
Cool pictures too.

2) draw a simple diagram of your instrument.
I still don't know *what* are you driving.
Also to what levels.
Are you picking vibrations and amplifying elsewhere (amps+speakers) or trying to have loud acoustic output out of the instrument itself?
Because in that case you might need hundreds of watts.

3) a simple piezo preamp is easy, later I'll draw some.
You can build it for peanuts, standard TL072 and common parts are more than enough.

4) you must collect some needed data yourself.
As said earlier, build *one* channel and experiment.
Measure actual power needed (we don't want to kill mosquitoes with grenades).
Also measure voltage and currents involved (remember this is *not* a resistive load), draw a voltage/current graph relative to frequency.

Last edited by JMFahey; 7th January 2013 at 07:26 PM.
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Old 7th January 2013, 09:06 PM   #13
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thanks! my name is úlfur, and the website is a portfolio of my work.

i am driving guitar strings. so basically, its an acoustic electronic instrument. i drive them via feedback, but another feature i have in mind and have tested with great results is having an "outboard feature" - where you can route any kind of audio through the instrument for a resonant "string reverb" effect.

thanks!!! i really appreciate that! would i be able to use one preamp for all the LM1876 inputs?

yeah today i successfully made a neat one channel system with an LM1876! im working day and night on this for the next couple of months..... i think ill use the power supply i made until it decides otherwise. we will see


i am finishing my order of solenoids online as we speak here :: Tubular Electromagnet *** E-66-75 *** Magnetic Sensor Systems.

do you think i should go for the 28AWG with a nominal internal resistance of 2.9 Ohms, or the 33 AWG with a nominal internal resistance of 30.7 Ohms? maybe the middle road makes sense - 30 AWG... ahrg! i just dont know!
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Old 9th January 2013, 01:56 PM   #14
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@JMFahey any ideas on preamp schematics?
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Old 9th January 2013, 03:16 PM   #15
JMFahey is offline JMFahey  Argentina
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This is as simple as can be.
Gain depends on R3 / R6
10K for 10X gain ; 1K for 100X gain.
Set as needed .
If you use very small Piezos, increase R1 / R4 to 3M3 or even 10M, but you'll have to be very careful with input wiring shielding and ground, place preamp as close to pickups as possible.
Good luck.
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Old 9th January 2013, 03:48 PM   #16
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thank you!
will keep you updated on my progress!!
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