Audio Amplifier from Scratch

[quantum1]

I plan to build my own audio amplifier circuit using a LM386 chip. However, I am in need of assistance. Here is the datasheet for the LM386 chip.

http://www.national.com/ds/LM/LM386.pdf

First off, I would just like to amplify the output signal of a speaker. If it is possible, I would like to amplify the input signal of, say a microphone.

Leads 1 & 8 are the gain pins, what would I do with those pins?
Lead 2 & 3 are the input pins, so would I the oringinal speaker wires?
Lead 4 is the ground pin, what would I do with that lead? Should I just attach it to the negative terminal end of a battery?
Lead 6, I'm assuming,is the voltage source (Vs). If so, is it the positive terminal of the voltage source? If so, which is the lead to the negative terminal of the voltage source.
Lead 5, I'm assuming, is the voltage output pin, for what would that be.
And finally, lead 7 is the bypass, for what would that be for.

 

[audioguru2]

Hi Quantum,
I guess that you know that the LM386 doesn't have much output power, so it won't be very loud before it distorts.
The schematics in the applications section of the datasheet show exactly what to do with its pins. The positive terminal of the battery connects to pin 6, and the negative terminal to pin 4. The ground symbol on the schematics show that the signal source, pin 2, the 10 ohm resistor and the speaker must also connect to pin 4.

You want to amplify "the ouput signal of a speaker"? A speaker has an output signal only when it is being used as a microphone. But a speaker as a microphone has a lousy sound quality and a very low output signal.
You also want to amplify "the input signal of a microphone"? A microphone doesn't have an input, but its output signal is probably too low (like using a speaker as a microphone), even if the LM386 has its gain boosted to 200 with the 10uF capacitor between pins 1 and 8. Therefore a pre-amplifier circuit is probably needed so that the microphone's (or speaker's) output signal is high enough for the LM386.
Try connecting a speaker or microphone to the input of the LM386 and see (hear?) if there is enough gain without a pre-amplifier. You probably must speak loudly, very close to the microphone.

 

[quantum1]

I see your point Audioguru, I accidently switched the terms around. What I would like to do is just amplify a speaker. Which is amplifying the input of the speaker signals, because speakers don't give off much output if not at all.(Correct)? I also added that I wanted to amplify a (electret) microphone. Which now I know is to preamplify the output signal of the microphone instead of the other way around.

Are you suppose to add grounding points of leads 3,4, and the two of lead 5 together and link them to the negative terminal of the battery of the schematic: amplifier with gain=20.

 

[audioguru2]

Hi Quantum,
Don't connect pin 3 of the LM386 to ground, since it is the LM386's input pin. It is shown connected to a "10k" volume control, and your signal source connects to its "Vin" terminal, and the other wire of your signal source connects to ground.
Also, don't connect pin 5 to ground. It connects only to the two capacitors as shown.
As I said before and the schematics show, the negative terminal of the battery connects to the signal source, pin 2, pin 4, the 10 ohm resistor and the speaker. It also connects to the ground terminal of the volume control, if you use one.

When using a speaker as a microphone, or an electret microphone, you are certainly going to need the added gain from the 10uF capacitor across pins 1 and 8.

If you use a 2-wire electret micrphone, then its positive terminal needs a resistor (10K ohms) to the positive supply, and a coupling capacitor (0.22uF) to the volume control. Its negative terminal connects to ground.

 

[quantum1]

Hello Audioguru,
You say to connect the negative terminal of the battery to the signal source.
Is that the input source?

 

[audioguru2]

That's right, Quantum,
The signal source and the input source are the same thing, and have 2 wires. Connect it using shielded (RCA) cable and the "hot" terminal of the signal/input source connects to the inner wire of the shielded cable and to the amp's volume control or input. The ground terminal of the signal/input source connects to the shield (outer) wire of the shielded cable and to the amp's ground (battery negative, etc.).

 

[quantum1]

Audioguru, I have trumendous news. I finally built the amplifier circuit since I found out all my answers. And my confindience of it working was 1 to 10 (least confideance), but I tryed. And what would you know, it work. I amplifyed an answering machine. The potentiometer work extremly well. However it wasen't the loudest thing in the world, but it sure made the sound louder than it was oringinally!!!!!!!!!!!! ;D This circuit I made was the lowest amplifying circuit of all of them, but it was sure loud enough to be satisfing whenever the phone answering man sayed you have no mail. ;D

 

[MP1]

congratulations quantum!! ;D

It is good to hear of your success.

MP

 

[audioguru2]

Hi Quantum,
That's great! Your amplifier works well.
What will be your next project?
How about making a Knightrider LED circuit? Our project is here:
http://www.electronics-lab.com/projects/motor_light/035/index.html

It makes 6 LEDs light in a back and forth action like this:

View attachment 35569

 

[quantum1]

Thanks guys!
I took Audioguru's advice and added a 10uF capacitor to to pins 1 and 8. That worked at pretty well. I also tried a 100uF capacitor, but it was just too loud/annoying. ;D

I have thought of the Nightrider circuit, and it seems pretty neat and catchy. Although I hope Radio Shack has the ICs, because thats what they lack is variety of ICs. I may just order offline.