lm386 oscillation

[noobman92]

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.

[godfrey]

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

[leadbelly]

Could be wiper lift. I would try a high value resistor from pin 3 to ground.

[godfrey]

Originally Posted by leadbelly

Could be wiper lift. I would try a high value resistor from pin 3 to ground.

The chip's got one built-in (50K) - it's pretty stupid-proof re biasing.

[macboy]

Do you have a power supply decoupling cap between V+ and GND? You need one, but it isn't shown in the schematic.

[noobman92]

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.

[noobman92]

hi everyone
putting a 0.1 microfarad cap between the V+ and ground worked a charm.but then there's a lot of distortion.maybe my speaker is not able to take the gain well (4ohms 5 watt).what say guys?am i right? my amp gain is 20.
thanx

[AndrewT]

Hi,
distortion due to overloading the amp with too much signal and too low an output impedance.

Look at the datasheet. This amp is specified as typical 700mW into 8r0 @ 10% distortion.

[Bonsai]

noobman92, I have some questions
1. How do you know its oscillating? can you tell us what you here as you rotate the volule pot from min to max
2. can you post a photograph of your wiring and contruction?

[imfree707]

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.