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About power supply noise

Hi,

I built a small LM386 amplifier and attached an antenna and an envelope detector to it. For a few days i was able to listen to some local radio through it but when i tried to power the circuit from my ATX psu(5V), all i heard was a loud noise. It's fine while powering from a 9V battery but when i switch to a power supply, this problem arises. When the input is my PC's sound card, i can listen to anything without noise, and also when i disconnect the antenna(long wire) from the detector, the noise disappears. I have tried this with a linear regulated supply, too. The result is the same noise.

What is causing this noise? The first think that comes to my mind is that it is the noise from the AC line but then why it disappears when i disconnect the antenna or when i use it as a normal AF amplifier? I am totally confused.

What i tried;

Put an active decoupler between the circuit and psu, used a 7805 with 100n caps on each side, tried a linearly regulated power supply, put a 10 uF bypassing cap between pin 7 and ground, put two(470 uF and 100 nF) bypassing cap between the V+ pin and ground, moved the psu away from the circuit, put a small 3.3 mH filtering choke between the psu and the circuit and other countless attempts that i don't remember now. None of them actually eliminated this loud noise.

I am waiting for your advices, thanks in advance...

Reply to
phlnxsc
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Computer power suppliescan be very noisy, use an external power supply, preferably a linear one

martin

Reply to
martin griffith

I have tried using one as i said in my previous message. It didn't help. Also i am not talking about high frequency switching noise, it's not at a specific frequency, it's just roaring. :) I hope you understand me.

Thanks..

Reply to
phlnxsc

If the noise is coming in over the antenna then you have to consider that it's not the output of the supply but the RFI it generates just by being on. Have you tried powering from the 9V battery and then turning on the supply independently to see if the noise starts?

--
Ben Jackson AD7GD

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Reply to
Ben Jackson

Yes i have tried it, i ran the circuit from 9V battery and than turned on the adjacent power supply, it didn't practically have any effect. There were no noise. It is only present when i run it from power supply.

Reply to
phlnxsc

you need an RF choke filter on the rails of your device.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
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Reply to
Jamie

The test may not be valid if there is no load on the powersupply. Connect it to a resistor that draws about the same current as your circuit, then test with the battery powering the 386.

Even if the supply is not generating RF, it could be conducting into your circuit, so the post about adding filters may be the answer.

Ed

Reply to
ehsjr

I also tried it with a load the result is the same. Thanks anyway..

Should connecting two high inductance (higher is better, right?) chokes on both negative and positive rails help? Or do i need something more?

Thanks..

Reply to
phlnxsc

I don't know if higher is better or not. You described the noise as "roaring", so it beats me what is in that noise. You can try inductors in series with the power supply leads, and a capacitor across them.

Can you back the gain off on the 386?

Ed

Reply to
ehsjr

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