| rs1026 |
| 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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| Nuuk |
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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| chipco3434 |
| Read that. Twice. Pay attention to grounding suggestions. |
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| rs1026 |
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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| Nuuk |
| 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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| rs1026 |
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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| Nuuk |
| So you don't have a connection from PCB to case? |
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| peranders |
| ... and what about earthing the case in the mains? Do you have a mains inlet with filter? |
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| rs1026 |
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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| peranders |
| 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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| rs1026 |
| No difference its exactly same.Only thing is I have put everyhting into a metal box now before it was just lying on a piece of wooden board. |
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| peranders |
I'm not totally convinced that this is all the truth. Can't you take picture?
Is it "soft" hum 50 Hz even with input shorted? Is it "hard" 100 Hz hum with lot's of harmonics.
50 Hz, problem with ground loops and the input.
100 Hz some trouble with your power supply and grounding technique.
Is it a stereo amp (yes), one transformer(yes) , same hum in both channels?
Is the transformer more near your Lm3886 now than before? (picking up magnetic field) |
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| rs1026 |
>>I'm not totally convinced that this is all the truth. Can't you take picture?
I dont have a Digicam thats the problem.
>>Is it "soft" hum 50 Hz even with input shorted? Is it "hard" 100 Hz hum with lot's of harmonics.
Its a soft hum not very loud.But it is noticeable.
>>Is it a stereo amp (yes), one transformer(yes) , same hum in both channels?
yes,It is a stereo amp.Single transformer.Yes the hum is same in both the channels.
>>Is the transformer more near your Lm3886 now than before? (picking up magnetic field)
Yes,Now its is very near one of the lm3886 chips.before everything was spread out.So could it be magnetic field?
Can I connect the PCB grounds,mains earth to a single point on a chasis?? |
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| Nuuk |
| quote: | | Can I connect the PCB grounds,mains earth to a single point on a chasis?? |
This is explained on the FAQ page! That's why I asked if you had read it. ;) |
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| peranders |
| quote: | Originally posted by rs1026
>>Is it "soft" hum 50 Hz even with input shorted? Is it "hard" 100 Hz hum with lot's of harmonics.
Its a soft hum not very loud.But it is noticeable.
>>Is the transformer more near your Lm3886 now than before? (picking up magnetic field)
Yes,Now its is very near one of the lm3886 chips.before everything was spread out.So could it be magnetic field?
Can I connect the PCB grounds,mains earth to a single point on a chasis?? |
Try to move the transformer, less hum?
A toroid can be placed very near the electronics but a EI core has much more magnetic leakage. |
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| gmphadte |
If u cannot move the transformer away from the electronics, put one iron sheet covering over the transformer(s). Need not be air tight as ventilation is necessary. This helps a lot.
Gajanan Phadte |
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