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unable to make a microphone amplifier circuit using LM358

Sashvat

Member
hi guys, I just received 10 LM358 op amps, and started building my circuit. But I am unable to get any sound from my speaker. This is just for a better understanding on op amps which I am doing. My schematic is on paper and its attached below. let me know what's wrong with my circuit. Thanks
 

Attachments

picbits

Well-Known Member
That circuit makes no sense. The LM358 isn't configured as an amplifier and your speaker only has one wire. Where did you get this circuit from ?
 

rjenkinsgb

Well-Known Member
Most Helpful Member
I suspect the original circuit was for an LM386 and the IC number has got confused somewhere.

The single supply with grounded input and driving a speaker make some sense with a 386, not with a general opamp.

eg.
 

Sashvat

Member
I
I suspect the original circuit was for an LM386 and the IC number has got confused somewhere.

The single supply with grounded input and driving a speaker make some sense with a 386, not with a general opamp.

eg.
I did the exact same circuit using a LM386 and there’s lot of noise, Is it because I am using along jumper wires? And what are the values of the resistors and caps in this circuit?
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Which circuit - your original or the example I posted?

In the example circuit I posted:
R1 & C4 are typically 10 ohms and 0.1 uF - they are common values to stabilise a small amp.

C2, 10uF.

VR1, if you use it, 10K.
Or add 10K from the +input to ground.

Connect the mic with it's feed resistor and 10uF coupling cap to either VR1 or +input.

You can add a 0.1uF capacitor from pin 7 to ground.

The cap between pins 1 & 8 increases the amplifier gain. Try it with and without that.
 

Sashvat

Member
Which circuit - your original or the example I posted?

In the example circuit I posted:
R1 & C4 are typically 10 ohms and 0.1 uF - they are common values to stabilise a small amp.

C2, 10uF.

VR1, if you use it, 10K.
Or add 10K from the +input to ground.

Connect the mic with it's feed resistor and 10uF coupling cap to either VR1 or +input.

You can add a 0.1uF capacitor from pin 7 to ground.

The cap between pins 1 & 8 increases the amplifier gain. Try it with and without that.
Yes I asked for the example you gave me.

I don't have 10 ohm resistor, what else can I use?
 

Sashvat

Member
I suspect the original circuit was for an LM386 and the IC number has got confused somewhere.

The single supply with grounded input and driving a speaker make some sense with a 386, not with a general opamp.

eg.
I found this from the internet, do you think this circuit below would work? - 120536
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I found this from the internet, do you think this circuit below would work? - View attachment 120536
Well you said in your original post your purpose was to understand opamps?, the LM386 ISN'T an opamp, it's an audio power amplifer,often misused on the Internet by people with no clue whatsoever about electronics.

There's a 'sticky' on these forums, which will do you more good than an LM386 circuit, and also includes a 'proper' electret mike preamp.

 

AnalogKid

Well-Known Member
Most Helpful Member
I found this from the internet, do you think this circuit below would work?
Yes. There are several application circuits in the LM386 datasheet, and all of them will work as shown - IF - you add strong power supply decoupling. All opamps require power supply decoupling, but as above, the 386 is not a normal opamp and will break into oscillation without it. Add a 0.1 uF ceramic capacitor and 10 uF to 100 uF electrolytic capacitor in parallel across the power pins. they should be as close as possible with the shortest possible leads.

The 386 was designed from the ground up to be a speaker driver. It will work as a microphone preamp (there are many modules available on ebay), but not well. The output will have much more noise than if you used an opamp designed for audio applications. The LM358 also is not a good part to use for audio. The output has much more crossover distortion than an "audio opamp", and the part does not have enough gain-bandwidth for decent fidelity.

ak
 

audioguru

Well-Known Member
Most Helpful Member
He made the LM386 power amplifier circuit and said there was a lot of noise.
But he did not describe the noise. Hum? Hiss? Acoustical feedback howling? All of these noises?

Long wires on the input of any amplifier are antennas that pickup mains hum. Shielded audio cables are used to prevent the hum.
The resistor that powers the microphone must not be connected directly to the positive power supply because the supply might have some hum on it. An RC filter must be used to decouple the power to the mic.

The LM386 is cheap and works best with low gain and fed from a low noise preamp. With the high gain produced by the capacitor between pin 1 and pin 8 then it amplifies its own hiss.

Any sound system produces acoustical feedback howling if the microphone can hear the speaker. Record the mic sound and play it back with the mic turned off.
 

dr pepper

Well-Known Member
Most Helpful Member
Its possible to build a speaker amp from a lm358, but its not a good idea, its much better to use an Lm380.
If you are interested in how you'd use the '358 heres some info:
The second to last schematic is the closest to the Lm380 circuit, but still far inferior performance wise.
 

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