Low Voltage Stereo Amplifier

[thedarkness3300]

Hey guys, I'm currently doing a diploma lab project ( Low Voltage Stereo Amplifier ) in the figure above. When point 1 and 8 of the IC LM386N is connected together, I have found out that there is a lot of noise when songs are playing through speaker. Can you all give me some suggestion how to reduce the noise but the sound is still amplified? Thanks alot !!

 

[Hero999]

There will be lots of noise because the gain is too high.

Reduce the gain by leaving pin 1 and 8 open and use a low noise op-amp based pre-amplifier to increase the gain.

This isn't a stereo amplifier, it's mono, you need to build two, if you need stereo.

 

[thedarkness3300]

Hey hero999, thanks alot for replying ! But lecturer have ask us to put a switch between pin 1 and 8, therefore it can turn on to amplify or deamplify it. which type of low noise op-amp based pre-amplifier should i get and where should i connect to the circuit? sorry i'm a newbie, i just starting to learn. . Yeah, you're right, i'm building two circuits for stereo. thanks man !

 

[Hero999]

Then you have no other option than to include an explanation for the noise in your write up. There's no way round it, the LM386 is noisy when the gain is high because there isn't enough negative feedback to cancel the noise.

The NE5532 or TL072 are both low noise op-amps which can be configured as an amplifier with a gain of 10. The idea is to move some of the gain from the noisy LM386 to a low noise op-amp.

Ask your lecturer for his input.

 

[thedarkness3300]

oh i see, so you mean that if i have to use the NE5532 or TL072, means I have to replace the LM386 IC ? Thanks.

 

[thedarkness3300]

Hey sorry for the wrong reply. . I really want to know how to i connect this NE5532 or TL072 to the LM386 IC? Please tell me . Thanks alot !

 

[Hero999]

oh i see, so you mean that if i have to use the NE5532 or TL072, means I have to replace the LM386 IC ? Thanks.

No, you use it as well as the LM386.

Hey sorry for the wrong reply. . I really want to know how to i connect this NE5532 or TL072 to the LM386 IC? Please tell me . Thanks alot !

I already told you, configure the op-amp as an amplifier (makes no difference whether it's inverting or non-inverting) with a gain of 10.

There are two op-amps on the IC (see the datasheets for the pin out) so you only need one IC.

Look at the datasheet for the LM386, note how connecting a bypass capacitor between pins 1 and 8 changes the gain and hopefully you'll see why you need to add an amplifier with a gain of 10 to keep the gain the same as when pins 1 and 8 are connected.

 

[thedarkness3300]

hey, i have take a look to the datasheet for the LM386, for the above circuit like this, when connected pin1 to pin 8 (there's a capacitor of 10microFarad between). It will has a gain of 200. when connecting bypass capacitor to pin7 then to ground, it will become a amplifier with bass boost. The problems that I am facing now is how to i configure the op-amp as an amplifier with a gain of 10? and how do i connect this TL072 to LM386. Thanks alot for replying .. I'm sorry i'm really a newbie to this.. =(

 

[Hero999]

Sorry but I'm not going to spoon feed you

The problems that I am facing now is how to i configure the op-amp as an amplifier with a gain of 10? and how do i connect this TL072 to LM386.

Didn't your lecturer teach you about operational amplifiers?

If not try doing some Googling, ask your lecturer, then if you don't get anywhere ask again here.

 

[thedarkness3300]

I have to hand in the project by next week . . I have learnt about the op-amp.. The problem im facing now is i don't really know how to connect this op-amp tl072 to this lm386 circuit. . Thanks for the help !!! thanks alot !

 

[Hero999]

Then it should be obvious how to connect it to the LM386, you just need to give it a bit more thought.

If you're really worried about getting it wrong, post a schematic, showing how'd do it and I'll tell you if it's right.

 

[audioguru]

Connecting a capacitor from pin7 to ground on an LM386 power amplifier does not provide bass boost. It simply reduces its output hum when the supply has lots of hum.
The bass boost circuit is shown in the datasheet as providing more negative feedback from the output to pin1 at the higher frequencies to reduce gain then using a resistor and capacitor to provide less negative feedback at lower frequencies for more gain at lower frequencies.

Maybe the "noise" you hear is actually severe clipping distortion when the gain of the amplifier is much too high. Does the noise" beco,me less when the volume control is turned down?

 

[thedarkness3300]

hey audioguru, its less when the volume control is turned down... what should i do? i'm trying to figure out how to connect tl072 to lm386

 

[thedarkness3300]

**broken link removed**
is it connected like this? or do I have to add some more components like capacitor or resistor. .Sorry guys im really very poor at this. Thanks for helping.

 

[thedarkness3300]

Hello guys, this is the schematic diagram. . I really dont know whether i connected correctly or not. . i'm really poor at this . . Please help me . Thanks alot !

 

[thedarkness3300]

**broken link removed** . guys, i cant upload the photo at here. the photo is at this link

 

[audioguru]

Your TL072 has no power supply voltages so it won't do anything.

Its output is shorted to the output of the LM386 power amplifier so the higher power from the LM386 will win and might burn out the output of the TL072.
You must never connect ouputs together!

The (-) input of the TL072 is connected to ground so it will be turned off. The datasheet for the TL072 shows that the minimum input voltage is about +3V and the output goes high when the input voltage is less. The (-) input also has no negative feedback so if the opamp is biased properly then its low frequency gain will be from 200,000 to 1 million instead of 10.
The (+) input is connected to the unknown input DC voltage so it also might not work.

Your LM386 power amplifiers still have C2 and C9 so their gain is still 200 instead of 20. Then they will still be noisy.

 

[thedarkness3300]

Thanks for replying.. Should i connect the (-) and (+) like this? i will put a switch between pin 1 and 8 so i can switch it on and off. . But where should i connect the output of the TL072 to?

 

[thedarkness3300]

What should i do to the C2 and C9? But i need to produce a loud and clear output ..

 

[audioguru]

Don't you know anything about this extremely simple opamp preamp circuit?
1) It is still not powered so it won't do anything.
2) Its output is supposed to drive the input of the LM386 power amplifier. The coupling capacitor is still needed but the volume control should be at the input of the preamp.
3) The (-) input is still connected to ground (0V) so it still will not work. The opamp still has no negative feedback to reduce its gain to 10.
4) Its (+) input is now connected to the positive supply so it also will not work. There is no input signal.
5) It is a very low power amplifier so it will be extremely distorted if its volume control or if its gain is too high. It will not be loud.
6) Do not use the switch to increase the gain and noise. Then C2 and C9 are not needed.
When its volume control or gain is turned down so the output has no distortion then it will sound very clear.