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Old 9th March 2004, 08:06 AM   #1
Albo is offline Albo  England
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Default Hum, RFI,Whatever.

When I put my ear very close to my speaker I can hear a constant hum from the woofer, and what sounds like a radio between stations from the tweeter.

This is barely audible from a normal listening position, does not increase with volume control and does not seem to spoil the music.

This is the Dec Dun IGC, built exactly as per instructions. With correct earthing arrangement, in an enclosed metal case and using screened input wiring inside and out.

If everyone else has totally quiet amps then something must be wrong.

This is used as a power amp fed by a Linn Genki Cd and using the built in variable output as a preamp.

My TL speakers are not particularly sensitive at 86db.

Comments welcomed

Albo
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Old 9th March 2004, 09:21 AM   #2
Ropie is offline Ropie  United Kingdom
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Hi Albo,

I have built the same amp as you, but in a foil lined wooden case, and don't have any problems with RFI or hum. Make sure that your case to mains earth connection is made via the suggested 0.22uf capacitor across a 100ohm resistor.

It could be that you are in an area that has very high levels of RFI (maybe you are near a transmitter or something like that) or it could be that your interconnects/speaker cables are very long and picking up the RFI. the other possibility is that your Linn is at fault - have you tried other sources?
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Old 9th March 2004, 06:19 PM   #3
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Default Re: Hum, RFI,Whatever.

Quote:
Originally posted by Albo
When I put my ear very close to my speaker I can hear a constant hum from the woofer, and what sounds like a radio between stations from the tweeter.


Albo,

I live VERY close to the local radio transmitter, and had exactly the problem you describe..... If you look through the National datasheet, you will see that some of the schematic have a 220pF cap. across the inputs to the chip... In my desperation to find a solution, I tried this and, Voila, no RFI and no hum either.....

Now, if I put my ear to the speaker with no music, all I can hear is silence ....... and I'm a Dog so it must be quiet !!!
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Old 9th March 2004, 06:25 PM   #4
Albo is offline Albo  England
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Thanks Ropie.

From your suggestions the only thing thats likely is a noisy Linn, or some kind of incompatibility between the two.

I'll have to fit a volume pot in order to try another source, because I don't have a preamp.

However the Linn still seems to work fine with my valve amp which is quiet at the speakers.

Back to the head scratching I'm afraid.

Albo
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Old 9th March 2004, 06:31 PM   #5
Albo is offline Albo  England
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Black Dog.

You posted while I was replying to Ropie.

I am not aware of any transmitters in my area, but for the cost of a couple of caps I'll try it anyway. Do you mean between pins 7 & 8.

Cheers Albo
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Old 9th March 2004, 06:47 PM   #6
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Quote:
Originally posted by Albo
Black Dog.
Do you mean between pins 7 & 8.
I'm not sure what chip you're using, but it's

Pins 7 & 8 on an LM3875
Pins 9 & 10 on an LM3886

Worked for me. I just used a cheapo disc ceramic cap, and placed it as close to the chip itself as I could. ie on the pins.
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Old 9th March 2004, 06:55 PM   #7
Albo is offline Albo  England
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Thanks Blackdog.

Yes, its a 3875.
I'll go and search through my spares box, but I'm not too hopeful.

Al
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