lm386 oscillation

noobman92

Member
2010-11-26 7:33 pm
hey guys
i built myself an amplifier for a speaker (4ohms, 5 watts) using LM386, according to the schematic in the datasheet (attached to post), with gain 20.the problem is that as soon as i turn it on, it oscillates.when the input pot is at the lowest level, i can just hear the music playing on the speaker (using ipod for signal into amplifier).it oscillates all the way to max volume through pot on the amplifier.also, inplace of the 0.05microfarad cap, i used a .1microfarad cap as it wasnt available.rest of the circuit is same.

i even changed the gain to 200, connecting a cap between pin 1 and 8, and then adding a bypass cap between pin 7 and ground, intrestingly, the pot, when moved by a very small amount, doesnt produce oscillations now.above the certain value, again oscillations.

i think its a problem of grounding.as in the gain 20 mode, it stops oscillation when i touch come of the wires, but once i move the pot's control, oscillation starts again.

please help guys.
 

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Hi
It could be a layout/wiring problem. Ideally the loudspeaker wires should be twisted together, the power supply wires should be twisted together, the input signal should use screened cable, and the loudspeaker wires should be kept away from the input signal wiring.

It might also help to have a capacitor between pins 4 and 6 close to the chip, to decouple the supply rail to ground.

btw: Using 0.1uF instead of 0.05uF is fine.

I assume it's RF oscillation, or is it audible?
What sort of power supply are you using, and how is it built - PCB or point-to point wiring or...?

Cheers - Godfrey
 

noobman92

Member
2010-11-26 7:33 pm
Thanx everyone for the replies.also, i'd try the twisted wire thng.the amp is picking up noise frm my cellphone.also,in my circuit,i had a particular place for grounding connections (its a conducting strip on the board), but the connections are at the opposite ends.can it create problems?also, do capacitor and resistor legs protruding out need to b chipped off?and wht shud be the reading of the decoupling cap between V+ & ground?electrolytic or ceramic?also, please indicate al the places in the circut where these caps are necessary.

Hey Godfrey,
its not a PCB.and the power supply is 9VDC Battery. Also, what should be the value of cap between pin 6 and 4?my layout isnt very planned and clean is a fact tht is very true.thanx.
 
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Watch Your Gain

The LM386 has been out of production for many years, most likely because of oscillation issues. The TDA2003 is a good, cost effective, replacement. I've had many LM386's self-destruct on me, as their overcurrent and overtemp protection systems will not stop oscillation. Many people run these amplifiers above the maximum guaranteed stable gain of V X 10 and oscillation inevitably results. Place an op amp ahead of the LM386 if voltage gain above 10 is needed.
 

noobman92

Member
2010-11-26 7:33 pm
its working totally fine with the headphones (no oscillation, no distortion, awesome bass on the HD212 Pros, super-awesome), cool results, the speaker i was previously using was 4ohms 5 watts, my headphones are working cool.my layout is still crappy, but its working fine.am thinking of extending it to stereo input + output. just that there is a little noise i'd not want to have.

i've attached the pics bonsai, but the circuit's now working fine. thanks for ur support guys...

also, would a crossover work well with this lm386 setup??? schematic attached
 

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Sorry, Total Error

The LM386 has been out of production for many years, most likely because of oscillation issues. The TDA2003 is a good, cost effective, replacement. I've had many LM386's self-destruct on me, as their overcurrent and overtemp protection systems will not stop oscillation. Many people run these amplifiers above the maximum guaranteed stable gain of V X 10 and oscillation inevitably results. Place an op amp ahead of the LM386 if voltage gain above 10 is needed.

Sorry folks, I got the numbers wrong. My info applies to the LM383 and not the LM386. I've used LM386's in several projects and never had stability issues. My bad on those type numbers.
 
In addition to the small ceramic decoupling cap, place an electrolytic (10 to 100 microF) also. The ceramic should be as close as possible to the chip, with the electro as close as possible to that. The speaker ground should connect directly (as close as possible to) to the ground leg of the electrolytic for truly effective decoupling of speaker current, which is the point of this really.
 
do capacitor and resistor legs protruding out need to b chipped off?
They don't have to be, but it looks better and stops the leads bending over and touching each other.
putting a 0.1 microfarad cap between the V+ and ground worked a charm.
Oh good, I love when stuff fixes itself while I'm asleep. :D
but then there's a lot of distortion.
The LM386 can't give much power into 4 ohms without distorting badly. Small 9V batteries (pp3?) also can't give much power - their voltage drops a lot when you draw too much current. A bigger 9V battery (pp9?) would be better. Headphones aren't a problem due to their higher impedance and lower power requirements.
also, would a crossover work well with this lm386 setup??? schematic attached
That looks like a "blend" circuit for headphones, not a crossover. It should work fine. You could use an opamp or emitter follower on the left and LM386 on the right. Actually, if you make the resistors 10 times bigger and the capacitors 10 times smaller, you don't need anything on the left.