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Old 27th August 2007, 03:11 PM   #1
ghisdal is offline ghisdal  Norway
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Default Hum on BrianGT kits.....

I have finished my kit, and after having som hum problems I found a grounding picture from carlosfm. I have changed the ground according to that picture.

I have included a picture which shows where the ground goes in my amp. Can anyone help me get rid of the hum please?

Can you see any problems?

I do not have any hum when there are no RCA cables connected (i.e. no source)
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Old 27th August 2007, 03:36 PM   #2
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The picture not clear and details enough, did you check your whole system step by step?

If you sure there is no problem of the amp and any other equipments/cables/wiring, have you try discounnt the amp's ground from mains, only grounded to chassis?
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Old 27th August 2007, 03:42 PM   #3
ghisdal is offline ghisdal  Norway
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It doesn't affect the hum if I disconnect it from mains ground.

It does go away if I remove the rca cables. Could that have something to do with it?
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Old 27th August 2007, 04:09 PM   #4
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I am wondering the hum maybe from other equipment(s), did you connect it to a preamp? Is your preamp also DIY? Do you make sure there is nothing wrong with the preamp?

Also make sure did the cables have nothing wrong?
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Old 29th August 2007, 03:02 PM   #5
chrish is online now chrish  Australia
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Posted this response to another hum problem above. I had a four channel amp with hum. Due to ground loop. I think there is a loop that goes from one channel, through the signal ground, to your source, then back through the signal ground of the other channel. Try tying the two signal input grounds together at the RCA jacks and then linking this to the chassis at that point. You can then tie this chassis earth to the safety earth that should be very close to your ac power input. If there is a loop, it should loop back before getting to your amp stage.

Tube amps are very susceptible to hum, so it is worth searching for tube amp grounding to give you some tips.

Cheers,

Chris
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Old 30th August 2007, 08:48 AM   #6
Dxvideo is offline Dxvideo  Turkey
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Thats a problem which I had before lots of time.
I gues your problem is;
- You have one PSU and two seperated amp board.
- So you have one GND from signal line on each amp board.
- One GND from PSU on each board.
- However theyre going to the same place sooner or later.
That means you have a gorund loop. My solution; If you will make a stereo amp then;
Make a single amp PCB and put a star gorund point to it, JUST ONE GND.
Or make a dual mono amp, seperated trafos, seperated PSUs and seperated amp boards.

You dont have any hum while you have no signal cable on inputz, because WITHOUT any signal source, a signal cable works as an antenna even its shilelded. Because if you dont carry anything on shield then its also an antenna.
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Old 30th August 2007, 09:13 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
It's the LOOP that acts as an antenna for the electromagnetic fields around all electrical equipment and cabling.
If you eliminate the LOOP, you get rid of the hum caused by the loop. There may still be residual hum from other sources that are modulating the quiet/clean audio ground.
Alternatively if you minimise the AREA of the LOOP you attenuate the hum.
Finally, I rarely see it, but Dr Cherry drew up a typical PCB layout where he addressed specifically interference effects. LOOP AREAS on the PCB were minimised and/or of opposite phase to cancel each other. He also used a triple trace along the middle of this demo PCB. The power ran +ve, ground, -ve immediately adjacent to each other and all the tappings for the amp ran to either side of this triple strip. Again this is fairly unusual, since most PCB designers maximise the PSU LOOP AREA by running +ve and -ve on opposite sides of the PCB and keep ground at one end, running long traces to other locations that must increase inductance.
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Old 30th August 2007, 09:16 AM   #8
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Andrew, do you have a reference for that Cherry paper?
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Old 30th August 2007, 10:04 AM   #9
AndrewT is offline AndrewT  Scotland
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Hi Pinky,
it was an article in a mag,
either WW or ETI.
probably early to mid 90s.
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Old 30th August 2007, 01:07 PM   #10
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Thanks Andrew.

Ah well, no luck with general googling or searching on their sites for it. If you come across it again, I would appreciate a better reference, (or a copy ). The more you learn, the more there is to learn...
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