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Author Topic: a little question about lm386  (Read 2678 times)
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jeanninemtv
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« on: April 08, 2012, 09:44:36 21:44 »

hello, i have today a little (stupid?) question,
in fact i want to do a simple audio amp with a lm386 , but as i want to put it in a reduced space, i'll use a 250mW lm386 in smd format,  (to amplify output sounds from the pic after a rc filter , to test signals like hpwm or sound or even the 1bit audio of roman black)

the question is, what wattage do i need for the smd components , 1/2 w , 1/4 W, 1/8 W 

thanks in advance
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CocaCola
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« Reply #1 on: April 09, 2012, 03:01:10 03:01 »

Resistors are rated in watts, and since they are all on the input side even 1/8W will work as there is nil watts going in, if all you are amplifying to is 1/4 watt output in the end...  On the output side you only have a single cap, caps are rated for voltage not wattage, and the output voltage of a lm386 will never exceed the supply voltage, in fact it should be about 1/2 the supply voltage since the data sheet shows it as a peak-to-peak output based on speaker resistance...  SMD ceramics at 100 can be costly, smd or regular (lay a through hole on it's side to save height) electrolytic 100 can be had in very small 16v or 25v package (plenty enough rating) that is cost effective and also size conservative, you might even find a larger value electrolytic in the same package size as a 100 that might give better filtering on the output...
« Last Edit: April 09, 2012, 03:08:33 03:08 by CocaCola » Logged
solutions
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« Reply #2 on: April 09, 2012, 09:22:11 09:22 »

In the real world, unlike academia, there's no such thing as a stupid question, since it prevents a stupid (read that as expensive, time consuming, project killing) mistake. 

What CocaCola said - 1/8W will be fine for the small signal path, though your largest component will be that volume control.  Maybe put in a digital pot? Got cool factor and probably will take up less space.

Also, to get good habits and sound professional, there's no such word as "wattage"...it's "power" or "power dissipation" or "power rating", depending on context.

Good practice is to rate your caps based on fault conditions, like surge, back-emf, etc, and not just the size of the battery or expected large signal condition.
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jeanninemtv
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« Reply #3 on: April 09, 2012, 09:23:59 21:23 »

ok, so i learned a very good new lesson today Smiley  btw i put all in 1/8 W  with  0805 package (except a 10 ohms resistors and added in the output a tiny 8ohms speaker (as i just need some sound boost not a screamming device ^^ so i'll put a little trimmer just to calibrate the adequate 'voice volume' and then to left it forever in that level ^^ )  btw i added a rc filter that improves a little my output Smiley

the 2 input bits are intended to be used with sound 2 or with the btc 1.7 output


 

« Last Edit: April 09, 2012, 09:27:29 21:27 by jeanninemtv » Logged
CocaCola
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« Reply #4 on: April 10, 2012, 01:03:14 01:03 »

Might be lost in translation, but once you establish the value of the trimmer, you can simply replace it with a static resistor divider of the same value and get rid of the trimmer all together if you don't need adjustment, and want to reduce size...
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jeanninemtv
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« Reply #5 on: April 10, 2012, 10:47:25 10:47 »

Might be lost in translation, but once you establish the value of the trimmer, you can simply replace it with a static resistor divider of the same value and get rid of the trimmer all together if you don't need adjustment, and want to reduce size...

yes, in fact a little adjustement between each final test would be enough , that's why i put a cheapo trimmer instead of resistors ^^ ...

 
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