Humming Noise in my amplifier??? - diyAudio
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Old 17th February 2006, 04:44 AM   #1
diyAudio Member
 
Join Date: Jun 2004
Location: Geelong
Default Humming Noise in my amplifier???

Hi everyone.

I have been searching all over the forums to find a solution to my problem but nothin seems to work.

I think its my pre-amp.

I have tried using my sc-480 with a mp3 player and it all works fine but then when I use my pre-amp and have a signal come through there is a hum, and if the ground wire to my pre-amp touches the case it makes a scratchy sound aswell. I am using all sheilded cableing for the audio signal, but there is still a hum.

If you need ANY more information please tell me what you need to know.

ANY help would be much appreciated.

Thank you all very much in advance

Thanks
Stefan
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Old 17th February 2006, 08:10 AM   #2
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Join Date: Jul 2003
Location: Dona paula, Goa
how is ur mp3 to preamp cable made... the details
is the preamp and the amp silent when the audio cable to mp3 is disconnected?
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Old 17th February 2006, 08:26 AM   #3
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Join Date: Jun 2004
Location: Geelong
Hi, thanks for your reply.


The cable from my mp3 is just a normal sheilded cable. and I had no problem with that. but once I hook up my pre-amp with even better quality sheilded cable it makes a buzzing sound.

The pre-amp use to all work when it was out of a case and there was no problem, but now when it is in the case near transformers and stuff it makes the sound.

I still get a audio signal through ( with hardly any bass ) but there is a very loud humm.
Also when I touch the knobs of my pre amp the hum increses but also when I touch the ground of my pre-amp, it goes away abit aswell ( by a fair bit ) but there is always a hum still.

Even attaching the ground to the case makes it worse..

Any ideas??

THANK YOU VERY MUCH

Stefan
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Old 17th February 2006, 08:35 AM   #4
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Join Date: Jun 2004
Location: Geelong
Hello again.

Just to answer your question.

I just unplugged the pre-amps OUTPUT to the amplifier and it still makes a LIGHTER humm, and even still when
I touch the pre-amps knobs it still increases the humm even though it isnt connected via the audio output, soo it must be something to do with the power.
Its somehow interfeering.

Also when I lift the 240V cable to turn the amp off it makes a really scratchy noise like 240VAC cable is also interfeering somehow?????

Also when I connect and dissconect the cable from my mp3 player it doesnt affect it.

All up I have 2 tranformers and 2 power supplies, I am running one power supply with one amp and the other is running another amp INCLUDING a Pre-amp, Do you think the power supply is interfeering??

Any ideas???

THANK YOU very much

Stefan
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Old 17th February 2006, 09:10 AM   #5
diyAudio Member
 
Join Date: Jul 2003
Location: Dona paula, Goa
Disconnecting ur preamp, the power amp still hums, then get rid of this problem first.
U have to come up with better details so that we could help, may be a photo of how things look like.
If u suspect the transformers, what type are u using, are they toroids?
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Old 17th February 2006, 09:37 AM   #6
diyAudio Member
 
Join Date: Jun 2004
Location: Geelong
Hi,

I will go get some pictures now of everything, no the tranny isnt a toroid.

I just dissconnected the pre amp and the power amp still is fine with no hum what so ever.

Thanks
Stefan
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Old 17th February 2006, 10:31 AM   #7
diyAudio Member
 
Join Date: Jul 2003
Location: Dona paula, Goa
tranny=transformer or transistor
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Old 17th February 2006, 10:40 AM   #8
diyAudio Member
 
Join Date: Jul 2003
Location: Dona paula, Goa
Transformer shielding

This is what I saw in a 3in1
A copper plate is wrapped around the winding from outside the core(beware) and magnetic shielding material(may be mu metal) is used around the rectangular core.

try it if u can find the materials.

mu metal is not available, try iron sheet, may be more rounds of it

Gajanan Phadte
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Old 17th February 2006, 10:42 AM   #9
diyAudio Member
 
Join Date: Jun 2004
Location: Geelong
Hello,


Well I uploaded a pic on a site.

I will upload some more pics for you guys

The link is:

http://www.upload2.com/galleryview/2595

please take a look at the pics at present and tell me if u need more specific photos.

Thank you VERY MUCH

Stefan

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Old 17th February 2006, 10:47 AM   #10
sangram is offline sangram  India
diyAudio Member
 
Join Date: Sep 2002
Location: India
It's probably a ground loop in the mains supplies. At least that's what it sounds like to me. Which is why the MP3 player has no problems and the preamp does.

Are the chassis ground connected to the input/output jacks of the preamp and amp? You need to use insulated jacks.

The amp star ground needs to be connected to the amp chassis via a small resistor (10 ohm, 3 watts is a popular value) and the chassis should be connected to mains earth. Sometimes a improper mains earth will also cause this problem. If the amp is correctly grounded but not getting a mains earth, it may hum when other equipment is connected on the same outlet and kept in the vicinity.

There is one more possibility. You say you are using two transformers, and one of them is running an amp and a pre-amp, the second is only running one amp.

There is a ground loop between the two channels.

You need to connect the two power grounds together at the power supply outlets.
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