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Old 20th September 2008, 07:27 AM   #1
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Default LM386 Oscillation

I have built the LM386 circuit based on a gain of 200 (as seen on datasheet) multiple times but my problem is that I keep getting bad buzzing and oscillation. I believe the problem could be that I'm making my own power supply for amp rather than battery operation. Also I don't have an earth ground. I'm just using a hot and neutral, but don't have a choice. I have no earth ground as I live in an older home with no good ground. Any ideas ???
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Old 20th September 2008, 07:51 AM   #2
Dxvideo is offline Dxvideo  Turkey
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With 200 gain?
You must have 20 X gain not 200! The reason of the oscillation must be this.
Rf=20K Ri=1K makes 21X gain.. If you use a CDP as the soruce, this raito is fairly enough.. But if you will use your MP3 player also then 22K/680R will be better.
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Old 20th September 2008, 08:43 AM   #3
lineup is offline lineup  Sweden
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Well, it is all in manual = datasheet.

Those who use any IC,
Without consulting & reading DATASHEET PDF
for the chip intended to use,
should do something else than DiyAudio

Datasheet tells LM386 is preset with voltage Gain = x20
I do not think we can go lower than this.
(of course we can use resistors, attenuators before input)
But we can go higher, upto 200 like. They say.

But of course, Dxvideo is right:
we should NEVER, in any amplifier, Use more gain, than we need.
This is stupid, foolish, crazy

and will only make amplifier more BAD than it can be.

Seems Dxvideo is right on the in-stability issue, too.
Might be fixed with one capacitor. Unless you have tried this already.
Quote:
DATASHEET Tells:

When using the LM386 with higher gains (bypassing the
1.35 kX resistor between pins 1 and 8)
it is necessary to bypass the unused input,
preventing degradation of gain and possible instabilities.

This is done with a 0.1 mF capacitor (=100uF) or a short to ground
depending on the dc source resistance on the driven input.
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Old 20th September 2008, 09:03 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
with the gain at 200times and the max output at 1W you need just 14mVac to drive this amp to peak output.
For a normal noise signal of -100dB the input should see no more than 0.2uV.
That is an indication of how low the noise should be on your power supply.
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Old 20th September 2008, 02:45 PM   #5
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Quote:
Originally posted by lineup
Well, it is all in manual = datasheet.

Those who use any IC,
Without consulting & reading DATASHEET PDF
for the chip intended to use,
should do something else than DiyAudio

Datasheet tells LM386 is preset with voltage Gain = x20
I do not think we can go lower than this.
(of course we can use resistors, attenuators before input)
But we can go higher, upto 200 like. They say.

But of course, Dxvideo is right:
we should NEVER, in any amplifier, Use more gain, than we need.
This is stupid, foolish, crazy

and will only make amplifier more BAD than it can be.

Seems Dxvideo is right on the in-stability issue, too.
Might be fixed with one capacitor. Unless you have tried this already.
I have not yet tried bypassing the unused input on the IC. I noticed it gave this as an option but I must have missed the part where it said it was necessary when applying more gain. I will definitely try this and only attempt a x20 gain. Thanks a bunch for everybody's help.
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Old 20th September 2008, 06:45 PM   #6
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Default Re: LM386 Oscillation

Quote:
Originally posted by kucharcity
I have built the LM386 circuit based on a gain of 200 (as seen on datasheet) multiple times but my problem is that I keep getting bad buzzing and oscillation. I believe the problem could be that I'm making my own power supply for amp rather than battery operation. Also I don't have an earth ground. I'm just using a hot and neutral, but don't have a choice. I have no earth ground as I live in an older home with no good ground. Any ideas ???
you need to read more and find out how earth in sound systems genneraly operates .....

operating in a house with no ground has absolutelly nothing to do with your sound systems .....

also the earth you have allready is coming either from the - of your power supply or thye center tap of your trafo ....

posting pictures and a complete schematic of what you ve done may solve your problems that in my opinion lay in your power supply or general set up....

as about the gain issue i think that you have to double check whayou ve done or what have writen in your post

regards sakis
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