Tuesday, September 4, 2012

The noisy PC power supply

Some time ago I was experimenting with indoor HF antennas. My goal was to receive some digital communications. But as soon as I started to experiment, I noticed a big S9+ noise in the receiver, so the combination of indoor antenna and S9+ noise makes the experiments useless.

Some time before, I noticed the noise comes from my computer. It was a really broadband noise covering all HF bands, with some 100 Hz reminiscences and some peaks along the spectrum, separated about 80 kHz between them. This was the definitive clue: 80 kHz peaks and 100 Hz noise pointed directly to the PC power supply. That power supply was working fine, so what could be the cause for that big noise?

After some time, I decided to open the computer for clean it ( Fans can accumulate a lot of dust ) and I checked the power supply also. Once I removed the supply cover, I just spent about one millisecond to figure out what was the problem with it:

The power supply does not was filtered at all!!! All the filter components were removed from the PCB (better saying, never put on it) so the noise generated could reach the mains and get radiated all along the house.

I remembered I had some faulty PC power supplies, so I opened all of them and I compared the PCBs inside. All of them seems to be made by the same manufacturer, with small modifications between them and same serigraph on all. So I decided to compare the input for all power supplies, just to see how the filter looks like. The noisy unfiltered power supply input stage looks in this way:

And one of the filtered faulty power supplies input stage looks in this another way:

As you can see, there were notable differences, In fact, there was these differences:

A toroidal coil, a 100nF capacitor and a 680Kohm resistor. So, I soldered these components in the working noisy power supply and tested it:

The computer was working fine, so I tested one of the indoor antennas. Results where really incredible. Noise level was zero. I had a really clean HF band, as you can see with this image taken with an indoor loaded dipole located at only one meter from the computer:

Why the power supply does not had the EMI filter components?? The answer is clear: Those cheap Asiatic importers. A capacitor can cost under a cent. But for thousand of power supplies build every day means a nice buck of dollars in the pocket. Add many non-essential components and you will get many bucks of dollars. Of course, these power supplies does not conform with any EMI standard (but they are marked as compliant!!!).

You are lucky if YOUR power supply is noisy, because you can replace or fix it, but... what if the noise comes from your neighbour's power supply?? You will loose your HF bands... This is a real and serious problem for today's ham radio.


  1. I wish there was a simple solution for the sealed laptop power supplies that share the noise proplem!

  2. Could this same solution be applied in the AC terminal strip the Laptop power supply uses?

  3. It's worth a try. Maybe a good EMI filter as close as possible to the power supply can mitigate the interference.

  4. I have the same problem with a Lenovo laptop power adapter, model PA-1650-16I. Input 110-240vAC output 20V DC at 3.25amps power supply 20V, 65W. Noise level on two brand new units test at S9+10db on 80 and 10 meters each! DO NOT BUY THIS POWER SUPPLY! It also gets too warm for my personal likes. I'm going to replace it with a 90 watt ANTEC generic laptop adapter. We sell tons of them and they work well....I just need to test it to make sure it is not noisy either!!!

    What we as hams could really use is a web site that documents ALL desktop and laptop power supplies. Since I work in a computer shop and see just about all of them, I might have to start photo documenting all the desktop units that come across my bench that have to be opened for repair. (blown caps), and photo document all which have missing RF filtering. Then post them toa web site as units to avoid or at least info on how to modify/restore them.


    Lyndel, N7LT
    Kalispell, MT USA

  5. HEllo! Thank you for these tips! I want to share with you one more reason of noisy power supply. Recently I faced to the problem of very very noisy power supply. I couldn't imagine what to do with it (btw I have this model and the problem was in dust in fan. After cleaning everything works as a clock!

  6. This is why I salvage old PC PSU's for useful components, especially these EMI filter components. Cheap PC PSU's no longer contain them, most of which are from China. They are also useful in places where mains are especially noisy. Tested on linear power supplies and works wonders for filtering noise that can be heard as hum on my radio.

  7. Nice article, thanks for sharing.

  8. A nice example of a clean switching PSU is the Motorola F2369, which uses a DUAL SECTION line filter, which effectively blocks incoming RFI and outgoing QRM. It is an enlightening example of well done engineering.

    Jose, CO2JA