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Fix noise in amplifier? Answered

 recently make a lm386 amplifier  test it with battery and my eleminator power supply it is working fine. i also add a filtering capacitor. But when I connect it to a 12v adapter it marking some hum noise .So how can I repair it?



2 years ago

The classic 60Hz hum means that your amplifier doesn't have good power supply rejection ratio. In other words, the ripple voltage from the power supply is working it's way to the audio output. To fix this, you need to add better filtering between the power supply and the amplifier. A really huge capacitor directly across the power supply will help, or better yet, a low pass LC filter. More capacitance means better DC filtering but there is only so much you can do before things start getting ridiculous.

The better solution would be to have another regulator like 78xx regulator between the power supply and the amplifier. This will eliminate a huge chunk of that 60Hz hum. These regulators have an internal voltage references and they have transistors which control the voltage level on the output and always try to force the output voltage to be some multiple of the voltage reference built in. Because of this, the have good load and line regulation. In this case, line regulation is what you need.

Using a different power supply is also an option, such as a regulated switchmode power supply. These generally offer much lower peak to peak ripple, but this ripple is much higher frequency and can be difficult to attenuate. In this case, large high value electrolytic capacitors with high ESL (equivalent series inductance) and ESR (equivalent series resistance) will not be beneficial here, you will need medium value capacitors that have low ESL and ESR, such as surface mount ceramic capacitors or film capacitors, and probably a stage of low pass LC filtering. Lots of small capacitors in parallel is often beneficial here.

However, especially if you plan to use a switch mode power supply (SMPS), conducted noise is not the only way electrical noise can work it's way to the output. the high impedance input of the amplifier may also be picking up EMI from other sources. SMPS's are notorious for creating lots of EMI and electrical noise because of their fundamental working principles. Shielding the power supply from the rest of the circuit as well as separately shielding the sensitive analog circuitry from outside interference is critical to prevent EMI. If the microphone or input is a long cable, make sure that it is coaxial type cable with the shielding connected to ground and the internal wire to the input of the amplifier, and try messing with impedance matching between the microphone output and the amplifier input. All of that should totally annihilate all noise problems if done right.

Skip to 9:15 and 10:20 to see how well this scope is shielded. and to 11:10 to look at the power supply. See all that the output filtering!? That is ideally what you need to do with your circuit. It might be overkill for a simple amplifier, but it will certainly stop any EMI from messing with your circuits!

Sounds like supply noise.

The 12 volt adapter, is it a little black box you just plug into house voltage.

Most of them just have a bridge rectifier some have a capacitor the point is they have a lot of ripple in there voltage.

If you want a noise free 12 volt supply, try this:


The transformer doesn't need to be exact as long as the voltage after the rectifier is between 15 and 35 volts.

The adapter I use it have some capacitor A mini transformer and some smd components .It have a out put of 18-22 volt it is a led driver . I put a 7812 to regulate 12v output So what now to do?

The adapter is a LED driver?

Some LED drivers have built in Pulse Width Modulators for brightness control.

PMWs turn the power on and off repeatedly for bright or dim.

You might be able to compensate for the PMW with a large capacitor between the adapter and the 7812 regulator.

how many uf? and connect it in parallel

between the adapter and the 7812 regulator?

Without putting it on an oscilloscope I can only guess.

Depends on what you have, but a 680 uF 35 volt electrolytic capacitor should do larger like a 2200 uF 35 volt would be better if you still get noise.

Try not to go over 50 volt capacitors just for size.

I add 2 1000uf 25v capacitors in parallel from the 7812 12v out put and gnd But It still have noise So what now ?

Sorry connect the capacitor, the same as the capacitor between the bridge rectifier and the voltage regulator in the schematic, in the link I sent in my first comment.

You may have some noise on the 12V supply lines. Add a big capacitor (like 100.000µF, the more the better) between 12V and GND.

And / or regulate the supply. Put a 7809 in there (still 12V input, but only 9V) or use a 15V power supply and a 7812 to regulate it down to 12V.

If you have an oscilloscope, you can check the supply voltage. With a multimeter, you can measure the ripple if you put it to AC mode.

The adapter I use it have some capacitor A mini transformer and some
smd components .It have a out put of 18-22 volt it is a led driver . I
put a 7812 to regulate 12v output So what now to do?