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Topic: Inductor Noise (Read 216 times) previous topic - next topic

adolfhack

Dear All,

I got this situation,
I'm using lm386 amplifier in a circuit to amplify certain sound effects.
the system works on 5 volts.
all worked just fine until i attached 3.7V lithium battery with 5V step-up voltage booster..
now I'm hearing high frequency noise from the speaker,
my guess that the High speed Switching inductor makes some magnetic noise picked up by the lm386 amplifier..
If that's right what can i do, otherwise what could be the problem?!

MarkT

#1
Aug 30, 2019, 10:29 pm Last Edit: Aug 30, 2019, 10:30 pm by MarkT
No your guess is likely wrong.

All switch mode supplies, such as a step-up "boost" converter produce switching noise directly on
the output rail, perhaps at around the 50mV to 100mV level (you pay more for quieter).

As a result they are almost always a mistake for audio.

You use a linear voltage regulator for audio and sensitive analog circuitry normally.

Its quite possible to have the best of both worlds, using a switch-mode converter followed by a low dropout
linear regulator (both efficiency and clean output rail).

Of course inductors do radiate magnetically, and this can be picked up by nearby circuitry, but the direct path for noise from a switch-mode converter is usually dominant.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

gilshultz

Your solution is valid! What you need to do is shield the Analog section, doing the digital as well won't hurt. Then you need to bypass the power supply line coming into and out of your converter unit.   Also add some bulk mode capacitors. Switch-mode converters also reflect noise on there power input which most people forget about. These capacitors need to have good high frequency characteristics. From your description it appears to be conducted noise however the shield will reduce the radiated emissions. If you want you can put in a low pass RL filter (add a small inductor in the power lead)
Good Luck, and Have Fun! Gil
This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!
Gil

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