View Full Version : LCD transformer interferes with stereo amp

28-12-2006, 12:42 PM
I have output from my soundcard directed through an older stereo amp and speakers sitting near my pc. I've found the power transformer from my lcd screen causes significant interference (humm) with the stereo amp.

It suits that they stay where they are, is there some relatively simple way I might be able to shield one from the other?

Billy T
28-12-2006, 01:39 PM
Shielding is not easy if it is the magnetic field from the transformer that is the problem. First try moving the audio leads to the amp around and see if that makes it better/worse, you could have a poor earth on the cable shield.

Some questions:

Does it still hum with the volume turned right down? The answer to that will tell us where the noise in entering the audio system.

Is the transformer for your LCD screen a separate item or built in? If separate, use a short extension lead from the power socket and plug the transformer into that, it will let you move it further away from the amp.

What are the "they" that you want to keep where they are? If it is the speakers, they should not be the problem, but it is not totally impossible as in the past I have heard interference injected direct into an amplified speaker.


Billy *<8-{)=

Graham L
28-12-2006, 02:18 PM
First, it's probably not a traditional "transformer", (magnetically) radiating hum.

It will almost certainly be a switchmode power supply, radiating RF which is being rectified by the amplifier.

As a test, try wrapping the power supply in aluminium foil. If that reduces or eliminates the noise, it would be tempting to leave it like that, but the supply might overheat. :(

Another thing to try is to use headphones on the amplifier, with the speaker cables disconnected. Such noise often gets in through the speaker leads: they make very good antennae.

Ferrite suppression sleeves slipped over the speaker leads are indicated if using the headphones improves matters. (These should be as close to the amplifier case as possible.) Jaycar (www.jaycar.co.nz/) LF1258 or LF1260 might do, depending on the cable size. I thought Jaycar had a clipon variety of suppression sleeve, which would let you do the same to the amplifier power cord, but I can't find it on their web site catalogue. A few turns of the power cord wrapped around a ferrite rod (e.g. LF1010)will help. The PSU will probably have a ferrite suppression "lump" on its output lead, but if it doesn't the same treatment would be a good idea.

28-12-2006, 05:51 PM
Thanks Billy T & Graham.

They are the stereo & pc. The former plus speakers are on a table immediately beside the pc desk.

LCD transformer is a separate unit, it is
- AC input 240Vac, 1.5A, 50-60hz
- DC output 19Vdc 3.16A (LPS, Limited Power Source)
has a cylindrical plastic coated lump near screen end of power cable that I presume is ferrite suppression device.

Foil wrapping transformer and distancing via extension cable made no obvious difference.

Hum loudness seems unrelated to pc output volume or stereo volume setting.

Unplugging speakers and using headphones only made no difference. However on reconnecting the hum was only noticeable on the right channel whereas it seemed evenly balanced previously - suggests a cable or plug problem?
Stereo uses 5 pin DIN sockets

At all times, turning the LCD monitor off removes all interference.

Graham L
03-01-2007, 06:34 PM
This starts to look as if it's the screen unit doing the radiating. (The position of the ferrite lump on the cable suggests that the manufacturers know that.:) )

There will be another supply in the screen unit to reduce the 19V from the external supply.

If the volume of the noise doesn't depend on volume settings that's a pretty fair indication that it's being received by the speaker leads (perhaps also by the the headphone lead). Try again with foil wrapping on the speaker leads. If you can find an earth connection on the back of the amplifier it would be nice to connect the foil to that. It will only cost a few dollars to try the ferrite sleeves.

With all this going on, you have probably changed the relative positions of things, which might explain why the noise is now only on one channel. Does rotating the screen change the noise level, and/or bring back the noise on the other channel?

Of course, all this might be wrong.:D It might be an oldfashioned ground loop hum problem. Does the amplifier have a threepin power plug, or is it "double insulated", with no earth pin? The fact that turning off the LCD screen stops the noise noise does seem to rule this mode out.

Has anyone else any ideas?

Billy T
03-01-2007, 10:43 PM
Still sounds like a bad grounding issue. You shouldn't need to add shielding or ferrites, the LCD will be compliant with all normal requirements for radiated emissions, it would not be saleable just about anywhere in the world if it wasn't.

Din plugs are notoriously poor for earthing, I have see very expensive electro-myography units in hospitals picking up all sorts of noise because they use din plugs for the patient leads. They are not much good for stereos either.

Your "older stereo amp" may be the problem so a bit of logical fault tracing and elimination might be in order. Just because the LCD screen is providing the interference source doesn't mean its faulty. Check the signal input leads for the amp, and if all else fails, take the din plugs apart and check that they have their earthing leads intact.

Try them one channel at a time too, that can show up problems. Silly question, but I assume that the audio input lead are shielded coaxial types, not unshielded twin or something silly like that, it's been known to happen!

It is remotely possible that your power outlet lacks an earth as well, so try running it all from an extension lead off another socket.


Billy 8-{)