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Author Topic: Rf into my sons stereo speakers  (Read 685 times)
N2JXN
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Posts: 189




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« on: January 02, 2008, 01:37:36 PM »

Hello again. And thanks for taking the time to read and reply. My son got a new stereo for chrismas and now when I am on the radio with the amp on. With anything more then a few hundred watts he hears me on the speakers on his stereo. So before I open the speakers up any ideas as to what the best methods to cut down or stop theis from happening.

This only happend on his stereo and not my daughters.
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KB9CRY
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« Reply #1 on: January 02, 2008, 01:41:59 PM »

Tell him to study his textbooks more.  Classic RFI overload; look at the speaker wires first then the unit's power cord.  Do a search on the RFI forum.
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N2JXN
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Posts: 189




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« Reply #2 on: January 02, 2008, 01:49:17 PM »

Thanks will do
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WQ3T
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« Reply #3 on: January 02, 2008, 01:52:40 PM »

Get your son interested in ham radio.
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N2JXN
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Posts: 189




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« Reply #4 on: January 02, 2008, 01:58:33 PM »

He is interested. He loves to listen when I am on the radio and hang around when I have a few ham friends around.
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AD5X
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« Reply #5 on: January 02, 2008, 02:01:01 PM »

Put snap-on ferrites on the speaker leads right at the output of the amplifier.  Wrap several turns of the speaker leads through the snap-ons.  I've found that the chaep snap-ons available from www.allelectronics.com work very well.

Phil - AD5X
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W3LK
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« Reply #6 on: January 02, 2008, 03:05:40 PM »

Don't forget to check your antenna system for common mode currents on your feed line(s). This is one of the common (no pun intended) causes of RF getting into consumer electronics.

73,

Lon - W3LK
Naugatuck, Connecticut
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N3OX
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« Reply #7 on: January 02, 2008, 03:09:49 PM »

"Tell him to study his textbooks more"

Here's a nice, free 60 page RFI textbook by K9YC:

http://www.audiosystemsgroup.com/RFI-Ham.pdf

73,
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W5FYI
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Posts: 1045




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« Reply #8 on: January 02, 2008, 05:57:13 PM »

It's a good chance that the speaker wires are acting as an antenna, so lengthening or shortening them, effectively changing their resonant frequency, might cure the problem. Since the power cord and wall wiring might also be a resonant length, try lengthening or shortening them as well. Ferrite chokes on the lines could help. If the stereo volume control has little effect on the RFI, of if the stereo works fine through a headphone without the speaker wires attached, it probably is the speaker wiring.

The FCC web site has an information page on consumer interference issues, including information about how to get the stereo manufacturer to come up with a fix for the problem.
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WA3SKN
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« Reply #9 on: January 03, 2008, 03:51:37 AM »

RFI can be very educational!
If you take a closer look, the speaker wires act as an antenna, you have a PN junction at one end, and a audio transducer on the other... a regular crystal set!
They no longer recommend just putting .01 caps across the speakers, there can be a problem with that!  The standard answers of ferrites and caps still apply.
I have found that simply replacing the standard wiring with some RG58 or RG59 coax can do wonders!
If you have some spare, try it!
73s

-Mike.
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W4VR
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« Reply #10 on: January 03, 2008, 08:43:48 AM »

You failed to mention the length of the speaker leads.  I once had a neighbor who ran speaker wire from the HiFi amp in one room to the speakers in a different room.  I installed an 80 mH choke in each lead at the speaker terminals and that cured the problem.  Nowadays you can do the same thing with ferrite beads.
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K7PEH
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« Reply #11 on: January 03, 2008, 05:59:29 PM »

I would like to clarify...

Where is the best place to put some ferrite RF chokes?  Closest to the speakers or closest to the amp?  Also, has anyone used twin coaxial cable -- that is, twin conductors wrapped by a single braided conductor as speaker wire?  And, if so, is this an improvement if the outer braid is grounded on each end.
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WA3SKN
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« Reply #12 on: January 04, 2008, 05:22:23 AM »

Put any ferrites at the stereo end of the wires.
What you describe is not twin coax, it is "shielded twisted pair", and you want to ground the shield at one end only!  Yes, it also can work wonders!

-Mike.
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W3LK
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« Reply #13 on: January 04, 2008, 06:51:17 AM »

There is a coax with two center conductors inside a shield. It is called Twinax and was used by IBM for mainframe interconnections. If memory serves, it had an impedance of 110 ohms.

73,

Lon - W3LK
Naugatuck, Connecticut
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N2JXN
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Posts: 189




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« Reply #14 on: January 05, 2008, 02:47:25 AM »

Thanks for all the replys. The speakers are close to the reciever. Basiclly one on each side of the reciever. Will look into the coax with the 2 wires in it. I bought from amidon some mix 31 beads. An put them on the leads reduced the the rf but still there. Next will be to open the speakers and install the caps on the leads.
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