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Old 16th October 2010, 07:49 PM   #1
Alex_77 is offline Alex_77  France
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Default LM3886 question.

Hello, I want to create an amplifier based on the LM3886 chip.

I have some questions about it.

If gain=20: R2 should be equal to R1x19 for non-inverting.

But I can use for example:
R1 = 1 Ohm; R2 = 19 Ohm
R1 = 10 Ohm; R2 = 190 Ohm
R1 = 100 Ohm ......

It is preferable to use big or small values?

Sorry for my really really poor English
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Old 16th October 2010, 09:01 PM   #2
ampimp is offline ampimp  United Kingdom
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Hi Alex
i think you will find your choice of resistors will depend on the input impedence you require. also if the value or R1 is too large it has more chance of picking up noise.
Regards Ian
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Old 16th October 2010, 09:13 PM   #3
gfiandy is offline gfiandy  United Kingdom
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Hi,

See LM3886 data sheet on Nationals web site.

If you don't know how to calculate a value for the design use the application note.

R1 = 1K
R2= 20K

This won't give exactly a gain of 20 but it is close enough and the values are available.

If you use a small R1 the current through it will be very large so it will need to be a power resistor. So I wouldn't use anything much less than 1K for R1.

Andrew
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Old 16th October 2010, 09:46 PM   #4
Alex_77 is offline Alex_77  France
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Quote:
Originally Posted by ampimp View Post
Hi Alex
i think you will find your choice of resistors will depend on the input impedence you require. also if the value or R1 is too large it has more chance of picking up noise.
Regards Ian
When I say "R1", I'm talking about "Ri"


Click the image to open in full size.
In this application, Ri change the input impedance ?

What is the utility of Rb (1KOhm) ?

I think "RL" is the speaker ? "Rin" is a pot ?
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Old 17th October 2010, 09:04 AM   #5
asbjbo is offline asbjbo  Norway
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There is, as always, a tradeoff here. Low values cause high currents. High values cause high noise. Resistor values near the datasheet values give a reasonable compromise. Keep Ri = 1k, for instance, and chose the other resistor value to give the gain you need.
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Old 17th October 2010, 11:44 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
do not use that National schematic.
Many of the optional components are omitted.

Use ALL the optional components and build an AC coupled amplifier not a DC coupled amplifier nor a mixed AC & DC coupled amplifier.
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Old 17th October 2010, 11:50 AM   #7
AndrewT is offline AndrewT  Scotland
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if Ri=1x and Rfl=20x and Rb=1x and Rin=20x, then the overall gain is 20times (+26dB). (Rin has been omitted from the schematic, another bad bit of advice from National).

Note, I have not attached ohms to any values, I am simply quoting resistor ratios.

The chipamp gain is 1+ {20x/1x} = 21times (+26.44dB)

Note also that Ri & Ci control the chipamps bass performance. Ci should be sized so that virtually no AC signal ever appears across it. If Ri is low value than that low AC voltage condition requires Ci to be very large in value, tens of mF!
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Old 17th October 2010, 07:47 PM   #8
Alex_77 is offline Alex_77  France
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Quote:
Originally Posted by AndrewT View Post
Hi,
do not use that National schematic.
Many of the optional components are omitted.

Use ALL the optional components and build an AC coupled amplifier not a DC coupled amplifier nor a mixed AC & DC coupled amplifier.

Many Gainclone schematics works like the National schematic !

Click the image to open in full size.
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Old 17th October 2010, 08:54 PM   #9
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Sure they do, but if you make a DC coupled amplifier like the one shown, without DC correction (servo) and DC detection/protection on the output, you are very likely to fry speakers. Or you could just build an AC coupled amp like AndrewT is suggesting, since it's easier and cheaper.
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Old 17th October 2010, 09:54 PM   #10
Alex_77 is offline Alex_77  France
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Ok thank you, maybe Im going to make the amplifier without the capacitor.

If I have a very high DC offset, I will add the capacitor.

What is the max value for the DC offset?

I'm don't want any capacitor because Im trying to make an amp with less parts as possible.


Sorry for my poor english.
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