Audio Amplifier problem

[Sylvester]

Hi
I'm working on a project right now related to sonar. So in the hardware I have a signal of around 2KHz given to a filter and then to a LM386 audio amp circuit.

Now at the output I've connected an 8 ohm speaker. Now when I connect the setup, the speaker gives a tone. But to check if all's good I check the output with an oscilloscope and I see a lot of distortion and no proper sinewave. It's almost as if the wave is clipped but its peak to peak isnt greater than the amp supply voltage..so the amp is not saturated. So I disconnect the speaker and check again and then I find a perfect sinewave at the output of the LM386.

So I thought something is wrong with the speaker. So next I connect a signal generator generating a 2kHz sinewave and give to the speaker directly. Then I measure the waveform again at the input of speaker...Now again I get a perfect sinewave on the oscilloscope.

So I'm thinking that whatever distortion I'm getting is coz of some mismatch between speaker and amp output impedance or sort of. Now I have several stages of speaker each having its associated filter and amp. So a few stages a functioning perfectly even after connection. The only difference between the erroneous and functional stages is the IC. The functional IC is LM386N nd the defective one is LM386N-1. Both are of national semiconductor...So I try to replace all ICs with a different make...but i cant get my hands on another make..so I have to make do with this itself. Now since the amps are functional without connecting speaker, so I'm thinking theres some deeper issue. Can you give me some solution to this? Thanks

I've attached an image of the distorted waveform after connecting the speaker.

The amplifier circuit is also attached.

 

[KrisBlueNZ]

Hi there and welcome to Electronics Point.

You have some nasty oscillation happening there. Here are a few things to try.

• The components in the Zobel network aka Boucherot cell, R1 and C3, must be good quality. C3 should be a film capacitor, not ceramic.
• The decoupling capacitor, C1, must be connected as directly as possible between pins 6 and 4 of the LM386. Ideally it should be soldered on the underside directly between the pins. It should be a ceramic capacitor with a 50V rating.
• You can also try adding a 100 µF, 25V low-impedance electrolytic in parallel with C1.
• C2 must also be positioned close to the LM386 and if the circuit is constructed on stripboard, the strips should be cut immediately after C2.
• The ground point should use a star configuration, with a single point that is connected through separate wires or tracks to the negative supply, pin 4 of the LM386, C3, the speaker return, and the 0V rail of the circuit that drives the LM386.
• You may have reject LM386 devices - relabelled and counterfeit components are not uncommon in India.
• Try the circuit at a lower supply voltage, e.g. 12V.

If none of these suggestions fixes the problem, upload a photo of your construction - both sides of the board.

 

[Sylvester]

Hi there and welcome to Electronics Point.

You have some nasty oscillation happening there. Here are a few things to try.

• The components in the Zobel network aka Boucherot cell, R1 and C3, must be good quality. C3 should be a film capacitor, not ceramic.
• The decoupling capacitor, C1, must be connected as directly as possible between pins 6 and 4 of the LM386. Ideally it should be soldered on the underside directly between the pins. It should be a ceramic capacitor with a 50V rating.
• You can also try adding a 100 µF, 25V low-impedance electrolytic in parallel with C1.
• C2 must also be positioned close to the LM386 and if the circuit is constructed on stripboard, the strips should be cut immediately after C2.
• The ground point should use a star configuration, with a single point that is connected through separate wires or tracks to the negative supply, pin 4 of the LM386, C3, the speaker return, and the 0V rail of the circuit that drives the LM386.
• You may have reject LM386 devices - relabelled and counterfeit components are not uncommon in India.
• Try the circuit at a lower supply voltage, e.g. 12V.

If none of these suggestions fixes the problem, upload a photo of your construction - both sides of the board.

Thanks. I forgot to add that I'm using a 5volt supply. But I'll try these solutions.

 

[Arouse1973]

Thanks. I forgot to add that I'm using a 5volt supply. But I'll try these solutions.

Hi Sylvester. When you reply to Kris can you add the following information please.

• What type of speaker, mid range, tweeter, bass or full range are you using and what wattage rating does it have.
• What speaker cable and length are you using.
• What power source are you using. Battery?, Wall wart? or other.
• What is the length of the power cables from the wall wart or battery or other are you using.
• What is the amplitude of the input signal?

Thanks
Adam

 

[CommanderLake]

Using electrolytics in parallel can create oscillations at several harmonics, I discovered this when making a headphone amplifier on a breadboard and testing with RIAA.

 

[Sylvester]

Hey,
I soldered a 100uF electrolytic capacitor between pin 6 (+5 volt) and pin 4 (ground) and it did the trick. Thanks for the help. Can you explain the theory behind this? How does this decoupling work?

 

[KrisBlueNZ]

I could, but it's already been done...

 

[Arouse1973]

I could, but it's already been done...

LOL how did you do that? Brilliant Kris!
Adam

 

[KrisBlueNZ]

LOL how did you do that? Brilliant Kris!

You mean lmgtfy? It's been around for quite a while. I learnt about it from (*steve*). Good eh!

 

[Arouse1973]

You mean lmgtfy? It's been around for quite a while. I learnt about it from (*steve*). Good eh!

Yeah brilliant.

 

[BobK]

Adam,

Just visit lmgtfy.com and you can create a link for any search you want. Used in forums all around the world.

Bob

 

[Arouse1973]

Adam,

Just visit lmgtfy.com and you can create a link for any search you want. Used in forums all around the world.

Bob

Cheers Bob, I think that is so cool. I'll have loads of fun at work with that. I work with a priest and I am a strict non believer.
Adam

 

[CommanderLake]

Decoupling is effectively a low pass RC filter on the power supply (R being the resistance of the supply and C being the decoupling capacitor) to absorb the high frequency changes in voltage as the load changes.