Questions about LM386 Audio Amplifier.

[Lerior]

Hello again,

I'm trying to build a simple audio amplifier using LM386. I'm trying to understand few things so I came up with few questions. Referring to the circuit diagram below:

1- Why do we use 10Ω and 10kΩ? Why these values in particular?

2- What determines the value of the capacitors? For instance, why 0.1µ and 10µ? Is there a way to know how to select these values (equations?)

3- Would using 3Ω speaker work with the same values?

Also, would adding a preamplifier (Like a single stage BJT transistor amplifier) work to enhance the sound quality by increasing the voltage gain? (More than 200?).
Edit: I attached another circuit with a BJT amplifier.

I apologize if these questions seem very obvious, I'm new to electronics and want to learn the theory behind LM386 amplifier.

Thank you.

 

[kellys_eye]

As with any electronic device, getting hold of the datasheet will reveal a whole plethora of details - from the basic specification of the device to application suggestions and, in between, the technical details for why certain components are chosen to support its function.

I suggest the exercise you have been given is to see if you can find said datasheet and extract the information from it - an essential part (skill) of circuit design.

The only 'answers' you will therefore receive on this question will be:

http://pdf1.alldatasheet.com/datasheet-pdf/view/413781/UTC/LM386.html

One last thing - different manufacturers give different details (in terms of 'amount' rather than 'accuracy') so don't just rely on the first data sheet you find - check out other versions to see if they offer more insight on pin functions.

 

[BobK]

This does not sound like a homework question. If it is, what is the actual assignment?

Bob

 

[Audioguru]

The schematic from Circuit Basics was drawn by a little kid who never saw the datasheet:
1) C6 is shorted so it does nothing.
2) C7 duplicates C5.
3) R2 ruins the filtering done by C8. The datasheet does not show R2.

The schematic from Eleccircuit was also drawn by a little kid who knows nothing about electronics:
1) The value of C3 is so high that it cuts all frequencies above 320Hz or less, depending on the volume control position. Our hearing reaches 20kHz.
2) Why does it have a transistor circuit with a gain of 10 when adding a single capacitor to the LM386 produces an increased gain of 10? The LM386 produces hiss when its gain is increased to 200 so maybe there is less hiss with the transistor circuit. But C3 cuts all high hiss frequencies anyway.

The 0.1uFcapacitor at the output with a series resistor to ground is a "zobel network". Look it up. An applications note for the similar LM380 amplifier explains that it prevents oscillation at a high frequency.

The datasheet for the LM386 shows that it works best with an 8 ohm speaker. They show that a 4 ohm speaker does not increase the output power, instead it increases the IC heating. It might burn out driving a 3 ohm speaker.

Why do you think increasing the gain enhances the sound quality? It would probably cause the output of the amplifier to try to go higher than it can with severe clipping distortion.

 

[Lerior]

This does not sound like a homework question. If it is, what is the actual assignment?

Bob

It's not exactly a homework question, it's related to a project that I'm working on. So, I did not know where would that go.

 

[Lerior]

The schematic from Circuit Basics was drawn by a little kid who never saw the datasheet:
1) C6 is shorted so it does nothing.
2) C7 duplicates C5.
3) R2 ruins the filtering done by C8. The datasheet does not show R2.

The schematic from Eleccircuit was also drawn by a little kid who knows nothing about electronics:
1) The value of C3 is so high that it cuts all frequencies above 320Hz or less, depending on the volume control position. Our hearing reaches 20kHz.
2) Why does it have a transistor circuit with a gain of 10 when adding a single capacitor to the LM386 produces an increased gain of 10? The LM386 produces hiss when its gain is increased to 200 so maybe there is less hiss with the transistor circuit. But C3 cuts all high hiss frequencies anyway.

The 0.1uFcapacitor at the output with a series resistor to ground is a "zobel network". Look it up. An applications note for the similar LM380 amplifier explains that it prevents oscillation at a high frequency.

The datasheet for the LM386 shows that it works best with an 8 ohm speaker. They show that a 4 ohm speaker does not increase the output power, instead it increases the IC heating. It might burn out driving a 3 ohm speaker.

Why do you think increasing the gain enhances the sound quality? It would probably cause the output of the amplifier to try to go higher than it can with severe clipping distortion.

Thanks, I will look up the datasheet. I assumed increasing the gain will just make it louder. I shouldn't have said enhance the sound quality.

 

[BobK]

It's not exactly a homework question, it's related to a project that I'm working on. So, I did not know where would that go.

Audio comes to mind as the correct category.

Homework help is a special forum in which we do not give direct answers, since that would be cheating on your homework.

Bob

 

[Audioguru]

Increasing the gain makes it louder only if the sound level is low. The maximum loudness of an LM386 before it reaches clipping distortion is only 0.45W into 8 ohms when powered from 9V which is the loudness of a cheap clock radio. Many modern little Bluetooth amplifiers and radios produce 3W into each stereo speaker.

 

[davenn]

thread moved to audio section

and I agree with AG, that first schematic is dreadful