how to fix noisy laptop power supply?

[gezzer]

No, though you got a good answer the wrong way.

A single totem-pole amp driving 8 Ohms looks like ~~48 Ohms to the power supply.

Your amps are bridged, so you have two t-p outputs each seeing 4 Ohms. So two 24 Ohm loads makes 12 Ohms.

Two such channels makes 6 Ohms. So in-sight of your "8 Ohm" number, but you can't just take the speaker impedance as the amp impedance without working through the current paths and tossing some pi at it.

BUT: this is at FULL output which you will never reach. And playing FULL you will not hear your cicadas. The problem is apparently at low/zero volume. When DC current is very low. For class D, maybe even 1%. So the amp draws 0.03 Amps and acts-like maybe 600 Ohms.

Well, as an engineer(not an electrical but a mechanical engineer) i just made a crude assumption, later i calculated different bode plots to see if the filter is okay at different loads from 4 to 400 ohms, so it is okay i guess🙂

A hard-core coil will saturate. 100mH is a bit large for air-core with low DCR. However when the current is high the music is loud and the chirp is masked. The main thing is to have high I at your lower currents, perhaps up to 0.3 Amps.

I later learned that, thats why i decided to leave a small gap between the cores of my inductor to decrease the relative permeability while increasing the saturation current and decreasing inductance. Now i winded the inductor and built the filter, the new inductor is 6mH with 4.8A saturation current(calculated). The calculated saturation current was around a useless 0.18A when the inductor was around 150mH. So i reselected the second cap as 10mF according to new lower inductance, the bode plots said i'll still get a good attenuation around 5khz.

Also the "chirp" is still present in moderate volumes, especially in tracks with dominant smooth bass. One example is "Slippery - Migos".

As the coil is a costly mystery object, and caps are cheap and good, I would cut the coil to like 100mH (which should be available pre-made) and use 10uFd-100uFd capacitance.

While i have a pretty large number of electronic parts sellers in my area(more than 50, istanbul is a packed city), i couldn't find a single seller who sells big pre-made inductors. So i wound the inductor myself around a big core from a broken SMPS that was laying around.

So i made the filter circuit and will test tonight(in 3-4 hours).

SMPS often switch to burst mode in case of very low load. This is what you probably hear when it is whining. Increasing DC-load sufficiently change the mode to continous - i.e. no more whining

Well i didn't know that. If thats the case i would either get a more suitable power supply, or i'll have to properly filter that noise. Because i'll use my amplifiers and speakers from "roomie sleeping next room with door open" volumes to "house party with dj performance" volumes.

Btw - it would be helfpul for diagnosis if you could upload a sound sample of that noise.

I can record and upload that tomorrow, but i gave a spectogram of that sound in the first post which i thought to be a lot more meaningful.

 

[MAAC0]

What happens when You disconnect the PSU AC ground wire ?

 

[gezzer]

What happens when You disconnect the PSU AC ground wire ?

PSU continues to supply power for about 3 seconds, if the music is playing it continues until all the remaining power is depleted. During that time the sound output is very clear without noticable background noise.


I've finally put the C-L-C filter i made between the power supply and amplifier. But unfortunately, it had only a small effect on the noise. I'm not sure why, maybe my calculations were wrong or the capacitor didn't perform good at higher frequencies.

However, after trying C-L-C filter, i've put a volume pot before the line input of amplifiers. The first pot i tried was a B100K, when it is on by about 5-10%, it eliminated almost all the noise! But when it's between 20% and 80% positions, i got a great amount of 60Hz humming.

So then i tried another A10K stereo pot laying around, the result is almost perfect! No annoying background noise is present at normal listening levels...

After all that discussion, the fix turns about to be loading the line signal with a potentiometer... My problem was mostly related with line noise after all...

Thanks to everyone that joined the discussion, i still learned a lot about power supply ripple filters, and learned much more about designing inductors from my researches during this discussion.

The same cicada like noise is still present when i turn the volume pot all the way up, but i won't use it that position anyway. Maybe i'll go onto that again sometime.

 

[gezzer]

What happens when You disconnect the PSU AC ground wire ?

Sorry i misread your question, i didn't try to remove the ac ground wire and it doesn't seem easy without cutting up ac cables.

 

[PRR]

> i didn't try to remove the ac ground wire and it doesn't seem easy without cutting up ac cables.

In the US, there is a common adapter to put a 3-pin (with ground) plug into a 2-pin (no ground) outlet. Works just as well 3-to-3 except it breaks the ground. Normally you do not want to break ground. Diagnosing a hi-fi, you might.

I do not know what plugs you use. Are you sure there is no equivalent adapter?

 

[MAAC0]

For Djing we usually lift the ground on the laptops in order not to hear the HDD spinning on the speakers. That's why I asked to try. I've seen modern laptops coming with non-grounded PSUs.


Try something like this Electrical Plugs, Sockets & Adapters in France