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Author Topic: I found my RFI issue! What next??  (Read 6159 times)
KD2CJJ
Member

Posts: 369




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« on: May 04, 2013, 12:48:20 PM »

I have narrowed down my RFI issue to my amplified subwoofer!!!  I am shocked that it was that... I went through the house turning off circuts, etc. and can clearly see this is what is making my 15 meter band absolutely unusable!!!!

Now, I do NOT want to replace this sub!  I absolutely love it! 

What can I do to get rid of this RFI without having to shut the sub down? 
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73

Mike
KD2CJJ
AA4HA
Member

Posts: 1378




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« Reply #1 on: May 04, 2013, 07:20:57 PM »

Ferrite beads are a good start. That will significantly attenuate higher (outside of the audio band) frequencies that may be using your wiring as an antenna.

Check out the power supply for the amplified sub, you may find that it is a switching supply. These may be "wall wart" supplies. Ferrites there may help. If it does not you may need to try shielding, grounding (putting the supply in a metal box with input and output filtering).

Sometimes capacitors work on the audio leads but you really need to be careful with that. Some amps do not care for it at all and you also may cause problems with your audio signal.
« Last Edit: May 04, 2013, 07:24:00 PM by AA4HA » Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
KD2CJJ
Member

Posts: 369




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« Reply #2 on: May 05, 2013, 04:48:37 AM »

It's the reverse... 

The sub is causing rfi on 15 meters.  The sub is on another floor of my house thus I ssuspect the rfi is being transmitted via the electrical wireing in my house.  I tried tutting ferries on the power cord but it did not help... I may need more of them as I could only get two wounds....




Ferrite beads are a good start. That will significantly attenuate higher (outside of the audio band) frequencies that may be using your wiring as an antenna.

Check out the power supply for the amplified sub, you may find that it is a switching supply. These may be "wall wart" supplies. Ferrites there may help. If it does not you may need to try shielding, grounding (putting the supply in a metal box with input and output filtering).

Sometimes capacitors work on the audio leads but you really need to be careful with that. Some amps do not care for it at all and you also may cause problems with your audio signal.
Logged

73

Mike
KD2CJJ
K1CJS
Member

Posts: 5879




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« Reply #3 on: May 05, 2013, 08:53:19 AM »

If you believe that the RFI is going through the house wiring, the first thing you should look to do is to actually find out if that is the case.  It will be easy if the subwoofer is powered by a wall wart.  Simply substitute another wall wart of the same voltage output (making sure that the replacement wall wart supplies AC or DC as the original one does) and the same polarity if it's a DC wall wart.

Check the other wall warts you may have in your house before you buy one--it may save you a few bucks.

If the subwoofer is powered by 110 volts, you'll have to take it apart to see what kind of power supply is in it--then do the same thing--that is, try substituting another power source.  If you're uncomfortable doing such work, have a friend (fellow ham) do it, or you'll simply have to try other things such as ferrites or capacitors.

There just isn't many other ways around it--and those other ways require test equipment you may not have.

BTW, if the subwoofer is fairly new, you may be able to exchange it.  Just a final thought.  73.

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KD2CJJ
Member

Posts: 369




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« Reply #4 on: May 07, 2013, 02:24:13 PM »

This is a high end sub.. the power supply is internal to the sub.  I know its the sub because when I turn it off the RFI goes away...  Unfortunately its not new and the company is no longer in business...  One thing I can note is that the power plug does NOT have a ground...  This is very typical on high end subs to avoid ground loop feedback... I suspect your heading down the direction that my power supply is starting to go out of phase and resonating on a frequency other than 60hz?  Unfortunately I am NOT comfortable to open up this speaker... For now I will just shut it off as its not used all that often and when I do need it, its easy for me to turn it on. 

In the mean while I will try putting a power strip that has filters on it or a UPS systems to see if that helps...



If you believe that the RFI is going through the house wiring, the first thing you should look to do is to actually find out if that is the case.  It will be easy if the subwoofer is powered by a wall wart.  Simply substitute another wall wart of the same voltage output (making sure that the replacement wall wart supplies AC or DC as the original one does) and the same polarity if it's a DC wall wart.

Check the other wall warts you may have in your house before you buy one--it may save you a few bucks.

If the subwoofer is powered by 110 volts, you'll have to take it apart to see what kind of power supply is in it--then do the same thing--that is, try substituting another power source.  If you're uncomfortable doing such work, have a friend (fellow ham) do it, or you'll simply have to try other things such as ferrites or capacitors.

There just isn't many other ways around it--and those other ways require test equipment you may not have.

BTW, if the subwoofer is fairly new, you may be able to exchange it.  Just a final thought.  73.


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73

Mike
KD2CJJ
KD0REQ
Member

Posts: 857




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« Reply #5 on: May 07, 2013, 03:23:14 PM »

subwoofers run 200-400 watts AF power, you aren't going to have a wall wart for that.  it's a switcher.  only the base (mounting) plate is likely at ground level, and if it isn't, verify with a voltmeter you can ground it, and then do.

ferrites on all wires going to and from the switching PS, put a wrap-around on the AC line, and you might just slip some over the speaker connections in and out of the sub as well.

some switchers are just slapdash-engineered, and born noisy.  you might have to find out the DC voltages to the sub's power rails, get a transformer, and build a linear supply.  but you can try contacting the manufacturer cc the FCC with your noise report and see if they will make it right for you.

one note... I had to redo the primary side of a 16-channel logic analyzer in college because it was not reliable.  there were series toroids to the power line in that one, a brute-force filter with X-caps, and they had varnish into the solder holes on the board.  but if you don't have that brute-force filter, hang a Corcom job of 3 to 5 amps, and see if that works for you.  I have not seen big filter toroids on the board in a long, long time.
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KH2G
Member

Posts: 239




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« Reply #6 on: May 08, 2013, 03:05:46 PM »

You might also try plugging it into a different wall circuit. ( Make sure it is controlled by a separate C/B)
Regards,
Dick
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QRP4U2
Member

Posts: 108




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« Reply #7 on: May 08, 2013, 04:14:32 PM »

It could also be that the subwoofer's amplifier circuitry is a Class D (switch mode)topology whose output is not well filtered or it's filtering has been compromsed.
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K1ZJH
Member

Posts: 901




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« Reply #8 on: May 10, 2013, 09:36:09 AM »

Sounds like the internal switching power supply. I suspect you will need to install AC line rated bypass caps inside of the amp to kill the noise. It sounds like common-mode radiation, where all three legs of the power line are being used as a "single wire antenna."  Ferrites will help with common mode radiation, but it takes a lot of inductance in some cases.
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WX7G
Member

Posts: 5920




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« Reply #9 on: May 14, 2013, 12:17:42 PM »

Unplug the sub woofer when you are on the air.
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KD2CJJ
Member

Posts: 369




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« Reply #10 on: May 14, 2013, 06:20:32 PM »

That's exactly my plan... Grin


Unplug the sub woofer when you are on the air.
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73

Mike
KD2CJJ
KG6YV
Member

Posts: 506




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« Reply #11 on: May 16, 2013, 04:39:11 PM »

IF the sub is putting garbage on the house wiring putting the sub on an isolation transformer may eliminate or reduce the noise. 

Gud luck
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