LM386 Problems

Thread Starter

shredability

Joined Apr 1, 2012
31


I built this circuit on a breadboard using my 12V supply. When I transferred it over to my proto board it all stopped working. My 8ohm speaking will make a loud gargled frequency and when I check with the scope there is an unstable 5VDC at pin 5.
 

Ron H

Joined Apr 14, 2005
7,014
I just built when the schematic showed me, where would one put the bypass caps? and why would it work on the breadboard but not the protoboard?
Oscillations are very much a function of layout and wiring, especially if you have no bypass caps. See Tracecom's schematic. He did it right.
 

Audioguru

Joined Dec 20, 2007
11,249
If the amplifier is powered from a little battery then a pretty big (470uF to 1000uf) supply bypass capacitor is needed to stop the supply voltage from jumping up and down with the output signal. The jumping creates a motorboat sound.

12V is a very high supply voltage. The max output will be only a little higher than if a 9V supply is used and the extra power just makes the amplifier get very hot.
 

Thread Starter

shredability

Joined Apr 1, 2012
31
So when you say bypass capacitor, to what are you referring to? the 250uf in my schematic? I was told that's called a decoupling capacitor? or a cap across the +12 and gnd pins?
 

tracecom

Joined Apr 16, 2010
3,944
The most important by-pass capacitor is between the power and ground pins. Try adding a .1 μF cap as close to pins 4 and 6 as possible. The next step is to add a .1 μF cap from pin 7 to ground.

And as I previously suggested, photos (top and bottom) of your assembly might help someone to spot problems.

Oh, and the 250 μF cap in your schematic is a coupling capacitor. It is there to connect the output of the amp to the speaker. It allows the AC (sound signal) to pass, but blocks the DC.
 

tracecom

Joined Apr 16, 2010
3,944
Alright guys, thanks for the help all I needed was a cap across the power pins I guess my supply was shaky.
Good. As you saw, it was not included in the schematic in the datasheet. That is because it is standard practice to include it and the publishers of the datasheet took it for granted that you would do that.
 
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