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Chip Amps Amplifiers based on integrated circuits 

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8th September 2011, 08:18 PM  #1 
diyAudio Member
Join Date: Jul 2006

LM 3886 INPUT volatge
Hello guys,
Just want to know what is the maximum PP input volatge which we can give to the LM3886 amplifier while the gain is 21 and the load is 4 ohms and the ouput power is 68W? How to calculate the maximum pp input voltage yet be sure that the amplifier does not go to cliping or saturation? Thanks 
8th September 2011, 08:33 PM  #2 
diyAudio Member

If you have + 35 V DC you can get +33 V out I should think. Divide this by 21. Around 1 Vrms is max.
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8th September 2011, 08:47 PM  #3 
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8th September 2011, 09:01 PM  #4 
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The supply voltage will determine the max output voltage so if you wil have 28 V then the peak voltage will be a couple of volts less.
This is stated in the datasheet under the parameter Vod and note 3 on page 3. http://www.national.com/ds/LM/LM3886.pdf
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8th September 2011, 10:35 PM  #5  
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Quote:


8th September 2011, 11:13 PM  #6  
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Quote:
Ok Let me explain what I mean. I told that the load is 4 ohms, the supply voltage is +28V and of course the output power as the datasheet says for such a load and supply voltage is 68W. Now I calculate the output RMS voltage which is: Vrms=sqrt P x R= sqrt 68 x 4=16.49V, so the PP output voltage will be: Vpp =2(16.49 x 1.414)=46.63V OK now if the gain of the amplifier happens to be known (here as I told is 21) we can have the pp input voltage as: Vin pp =Vout pp / gain so Vin pp = 46.63/21 = 2.22V So I calculated the input pp voltage for the the output pp voltage of 46.63V and the gain of 21. I call that input pp voltage the maximum input voltage which I am able to give to the chip white the load is 4 ohms, gain is 21 and the output power is 68W, right???? It reflects to me that any input pp voltage bigger than 2.22V will cause the output to go to saturation and clip... What is your idea? 

9th September 2011, 12:09 AM  #7 
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Join Date: Jun 2005
Location: Midwest

No, not right. You seem to have the false impression that higher voltage than absolutely necessary causes clipping. Higher does not cause clipping. Clipping is caused by too LOW an input voltage, when input signal x gain > available output voltage after forward drop across the amp chip (loss after PSU input voltage) or current limiting protection.

9th September 2011, 05:16 AM  #8 
diyAudio Member

To simplify for you: A normal signal source is what you can have together with a LM3886 with the gain set to 21.
28 V supply voltage => 26 V peak out => 1.24 V peak in => 875 mVrms is clipping voltage.
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9th September 2011, 06:00 AM  #9 
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Join Date: Jun 2005
Location: Midwest

Maybe I need more sleep, we seem to be talking about two different things so disregard what I wrote above.

9th September 2011, 08:38 AM  #10  
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Join Date: Jul 2006

Quote:
exactly opposite! Suppose you have an ideal opamp which its power supply is 10V and its gain is 5. Any input voltage LARGER than 2V causes the output of the opamp to go to saturation (2 x 5= 10V). In this cause if the input signal happens to be sine, It would be changed to Square wave after clipping. Actually I do not know what you are talking about? An opamp must be able to amplify ANY LOW voltage before it goes to saturation... 

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