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Old 26th December 2013, 10:07 AM   #1
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Default LME49720 preamp for F5

i start to bouild the F5 amplifier by nelson. the input impedance of the amp is 100K and the gain is 15db, so i feel that is need preamp.

my audio surce is sound card so in the output it have ~400mv.

i want to bay this preamp(the LME49720 is very quality specsification)

do you think its 20db gain is enough ? maybe i need less gain?

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Old 26th December 2013, 10:21 AM   #2
AndrewT is offline AndrewT  Scotland
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What is Gain Structure?
regards Andrew T.
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Old 26th December 2013, 10:54 AM   #3
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thanks, its good article.
now im sure that if i want 25w on the F5 output, i have to amplified the computer signal.
but how much? i thins 20 db its ok, im worng? 0.3v *30 (F5)=9=>10W
so what i need is 0.8v in the entrance of the F5 to get 25w on the speaker !?
if so, the preamp in need to amplified by ~2 ! ?
i just want to be sure before i buy this preamp
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Old 26th December 2013, 11:32 AM   #4
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With the F5's voltage gain of 15.3dB=20*log(x) and a maximum output voltage of +/-20V (from manual), the amplification factor x=10^(15.3/20)=5.82 and therefore the input voltage required to drive it to full output should be 20V/5.82=3.42V. Compare this to what your source can deliver, and decide on how much gain you'll need in the preamp stage.
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Old 26th December 2013, 02:51 PM   #5
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oppsss mistake i use this equation: db=10^(n/10). and 10log(x). sorry.

thanks for the info: i gain the audio source to follow ~3.4v
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