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LM386 Amplifier Module help

Owensims

Aug 18, 2020
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Hello Forum

Amateur and wholly inexpert user here...

I am making a TARDIS (long story!) and I have a key fob which generates the sound of the TARDIS materialising. Basically, a circuit board makes a sound come out of a tiny little speaker.

I've taken the thing apart and connected the two speaker wires to a slightly bigger speaker (3W, 4ohm) and it makes a quiet noise. I've tried to connect the same speaker wires to the audio input of the LM386 module, and the bigger speaker to its output. Now I get no sound.

I know the power is working to the LM386 module because the little D1 LED is lit. What am I doing wrong?

Thanks much.
 

Harald Kapp

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The keyfob may be designed to drive a (more or less) specific load. The input of the amplifier is probably grossly mismatched to this design target. Two things to try:
  1. Does the amplifier module (with speak) work if you connect another audio input? If so the module is o.k. If not, the module may be defect.
  2. Connect the original small speaker to the key fob, then connect the input of the amplifier in parallel to the small speaker. Does that give you sound at the output?
Addendum: The key fob may have a DC offset at the output which brings the amplifier into saturation. putting a capacitor between the key fob and the amplifier (in the signal path, not ground) may help, too.
 
Last edited:

bertus

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Hello,

I assume the key fob is working on a button cell of 3 Volts.

Wich LM386 module are you using?

What is the supply voltage of the LM386 module?
It needs at least 5 Volts to work.
The max voltage for the LM386 is 12 Volts.

Bertus
 

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WHONOES

May 20, 2017
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In your diagram, you have pin 1 of the LM386 connected directly to ground presumably to set the gain to 200. According to it's datasheet, if you want a gain of 200, pin 1 should be connected to ground through a 10uf capacitor. Doing as you have will probably cause a large offset on the output.
 

bertus

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Hello,

@PETERDECO , I also am missing the zobel network on output:

LM386_typical_applications_annotated.png
The capacitor and resistor are there for stability and to avoid oscillations.

Bertus
 

PETERDECO

Dec 19, 2019
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Thanks WHO. I just checked the circuit. Pin 1 is going to ground. If there's any offset, the speaker's not chewing up battery power with the output cap. When I turn up the volume, there is room filling sound with no distortion. It works fine for us without any oscillations on the scope. The parts are on a PCB with very short traces. Maybe that's why.
 

bertus

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Hello,

The LM386 can not work when pin 1 is grounded.
You are basicaly disabeling half of the differential amplifier:

LM386_internal_schematic.png

Bertus
 

PETERDECO

Dec 19, 2019
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My apologies to everyone. I just got to work and opened a product with the LM358 amplifier. Pin 1 has NO connection to it. The rest of the diagram is OK.
 

bertus

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Hello,

@PETERDECO , Then I still don't get your schematic.
The LM358 is a low power dual opamp:

LM358_pinout.png

Pin 6 would be the input of the second opamp.

Bertus
 

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Peter's schematic of his LM386 power amplifier is also missing an important supply bypass capacitor but it does not matter since the ground pin 4 is not connected to the supply ground.
 
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