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Old 13th December 2004, 05:31 PM   #1
rs1026 is offline rs1026  India
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Default Hum

Its almost 3 months since i built a lm3886 based gainclone .till now I had kept all the components on a wooden board and the GC was dead silent on when powered.I put all the components in a metal box including the transformer.Now the GC gives a hum it is quite audible.Have I done anything wrong over here??Please suggest some ideas to eliminate this problem.
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Old 13th December 2004, 07:11 PM   #2
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Have you read this ?

And if you read the recent posts on this same subject , you will get a lot of clues!
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Old 13th December 2004, 07:16 PM   #3
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Read that. Twice. Pay attention to grounding suggestions.
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Old 14th December 2004, 01:50 AM   #4
rs1026 is offline rs1026  India
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Default Preplexed

Nuuk,
I have read your website may be "n+1" number of times Infact it was a source of inspiration to start the work. I have taken all ground loop problems into account.The PCB is also designed properly.What I am not able to understand is there was no hum when everything was mounted on a wooden board but once everything is transfered into a metal chasis there is hum. Infact the bottom of the box is isolated properly. really strange!!

One more thing that I noticed was the bass response has come down!! I know its nothing to do with the box but I have added two speaker connecting terminals now ,I presume that is acting as a sort of HPF! Am I correct?
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Old 14th December 2004, 08:29 AM   #5
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Quote:
I have added two speaker connecting terminals now ,I presume that is acting as a sort of HPF! Am I correct?
No way unless you have added a resistor/cap somewhere as well!.

To help further, we really need to see exactly what you have in that box!
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Old 14th December 2004, 09:59 AM   #6
rs1026 is offline rs1026  India
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Default Contents

I dont have a Digicam so I will put the list here.I dont know if this will help to solve the problem but still.

Contents of the metal box:

1.One monster 220VA EI transformer[it may weigh around 800 grams ]
2.One 4A slow blow fuse with holder
3.One bridge rectifier nest to the transformer with 6800uf Cap per rail
4.2 3886 based pcb along with the heat sink towards the edge of the box
5.One stereo to RCA cable that goes as input signal to the amps
6.Two speaker connectors mounted at the back of the chasis.

Thats pretty much it.Its a simple 2 channel GC.Before putting these components into the box there was no hum and sound was very good. Now there is slight hum also bass response seems a bit low I need to increase the gain by controlling the vol control from the PC to get more bass.
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Old 14th December 2004, 11:44 AM   #7
Nuuk is offline Nuuk  United Kingdom
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So you don't have a connection from PCB to case?
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Old 14th December 2004, 12:00 PM   #8
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... and what about earthing the case in the mains? Do you have a mains inlet with filter?
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Old 14th December 2004, 01:49 PM   #9
rs1026 is offline rs1026  India
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Default Nope

No I havent connected PCB to chasis nor the mains earth to the chasis.

Is this the problem then?I thought ground loop problem existed at PCB level.

So shall I connect the mains earth,PCB ground to a single point on the chasis using a nut.Is this OK?
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Old 14th December 2004, 01:52 PM   #10
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What is the _exact_ difference between the amp when it was on wood and now? I was thinking of the grounding. Was it mono block then and now stereo with one transformer?
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