maximum inputsignal strength(V) on the lm3886? - diyAudio
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Old 8th December 2004, 08:28 PM   #1
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Default maximum inputsignal strength(V) on the lm3886?

Hi guys!

can anyone tell me how high the input signal can be on the lm 3886 and\or the lm4700 before clipping?
In the specs i belive i'v seen a example of maks 1V, but that was only a example?

i have a gain of 30, but thats not enough for regular music, the amp wont alow me to cranc up the volume, and so i need to either have my preamp pop up the signal strength 2 or three times or increase the gain of the power amp.

I am reluctant to increase gain as this would result in increased noise in the amp, and i really dont want that..

Oh, and by the way, I'v heard that the industrial standard of regular audio cources are 0,775V peak to peak, with the rms being about 35% of this, wich gives us about 0.23Vrms as a input signal.

could someone confirm this?

thanks
marius
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Old 9th December 2004, 04:48 PM   #2
macboy is offline macboy  Canada
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Quote:
Oh, and by the way, I'v heard that the industrial standard of regular audio cources are 0,775V peak to peak, with the rms being about 35% of this, wich gives us about 0.23Vrms as a input signal.
I think that you are mistaken. Line-level is (nominally) 2 Vp-p, or 0.71 Vrms. Of course, this is a sort of de-facto standard, and nobody is required to meet it. Most equipment has a lower maximum output voltage.

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can anyone tell me how high the input signal can be on the lm 3886 and\or the lm4700 before clipping?
You have not provided enough information. What is your power supply voltage? Load impedance? These determine the output clipping voltage, which is what you need to know to determine the corresponding input voltage.

Your amp should clip about 3 or 4 V less than the rail voltage (but this varies with load impedance... see the datasheets). So let's assume that you have +-35 V rails. The amp will clip at about 32 V peak output (either + or -). So with a gain of 30, you will need about 1.067 V (peak) input. That's a little higher than nominal line level. And remember that most sources are even weaker, so you'll need more gain to overcome that. That means you'll need some kind of pre-amp.

I'd recommend adding an input stage to your amp. Use a good quality op-amp, and run it with a gain of about 4. Now reduce the gain of your chip amp to about 15. That gives an overall gain of 60 which should be sufficient to achieve maximum output. Since you have a lower gain in your first stage, it's feedback resistor can be much lower than before. And since your chip amp is now driven by an op-amp, you can use much lower gain-setting resistors here too. The combination of this may result in a lower noise floor than before, even with the overall higher gain.
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Old 9th December 2004, 08:11 PM   #3
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Thank you mack boy! grate to get some help when stuck for shure.

so not really much of an input signal limit at all then i guess, which makes for grate flexibility.

reducing the gain and lowering the value of the feedback resistors, then adding a current buffer seems a solid bet now.. guess i'l be doing this as well now. (this gainclone was originally meant as a pure power amp with nothing else to it.. now it has a snubber, 40k uf per rail and all teh tweakes i'v heard is possible for this thing.. truly addictive..)
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