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Old 12th March 2012, 11:05 PM   #1
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Default Humming due to common ground

Hello Members,

Here is a Humming Problem that i am currently facing in my Class A amplifier i recently built for my self. I am sure some of Expert members have a simple solution for this problem.

Here is the nature of the problem, I built a stereo Class A amp, each channel has dedicated power supply, and individual amplification boards. The Humming only occurs when i make the GROUND common in both the Channel, When the Grounds are not common the sound is crystal clear.

I have attached a diagram of my current amplifier arrangement, this is the arrangement when there is Humming. If i make the Ground Uncommon between both the sides, The Humming stops instantly.

I'll deeply appreciate kind advises from members, Thank you.

Best Regards.
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Last edited by SLAK; 13th March 2012 at 07:37 AM.
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Old 12th March 2012, 11:14 PM   #2
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The grounds need to be connected from the transformer to the filter, then the filter to amplifier. You can sometimes pick up current surges from the power supply input.
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Old 13th March 2012, 11:57 AM   #3
AndrewT is offline AndrewT  Scotland
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Earth the chassis.

Create a Main Audio Ground (MAG) for left channel and another separate one for right channel.

Try connecting one MAG via a Disconnecting Network to the chassis.
Is it quiet?

Try connecting the other MAG via it's own Disconnecting Network to the chassis.
Is it quiet?

The problem you have with your dual mono is that there are two inputs and both are common grounded at their Source end. When these interconnects arrive at the dual mono they get connected again. That creates an enormous ground loop. There are voltage differences around that loop and the two inputs "hear" that AC interference and amplify it to come out of the speakers. You can measure the hum and/or buzz with a DMM set to 200.0mVac attached to the output of the amp.
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Last edited by AndrewT; 13th March 2012 at 12:01 PM.
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Old 13th March 2012, 03:41 PM   #4
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Quote:
Originally Posted by AndrewT View Post
Earth the chassis.

Create a Main Audio Ground (MAG) for left channel and another separate one for right channel.

The problem you have with your dual mono is that there are two inputs and both are common grounded at their Source end. When these interconnects arrive at the dual mono they get connected again. That creates an enormous ground loop. There are voltage differences around that loop and the two inputs "hear" that AC interference and amplify it to come out of the speakers. You can measure the hum and/or buzz with a DMM set to 200.0mVac attached to the output of the amp.
Try connecting one MAG via a Disconnecting Network to the chassis.
Is it quiet?
Yes, It is Quiet

Try connecting the other MAG via it's own Disconnecting Network to the chassis.
Is it quiet?
No, The humming starts when i do this.
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Old 13th March 2012, 03:52 PM   #5
AndrewT is offline AndrewT  Scotland
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What components are in your two Disconnecting Networks?

Disconnect both input interconnects.
Short each Signal Hot to Signal Ground.
Try connecting the two Disconnecting Networks to chassis.
Is it quiet?
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Old 13th March 2012, 03:54 PM   #6
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Quote:
Originally Posted by AndrewT View Post
What components are in your two Disconnecting Networks?

Disconnect both input interconnects.
Try connecting the two Disconnecting Networks to chassis.
Is it quiet?
Disconnect both input interconnects.------> Humming quits
Try connecting the two Disconnecting Networks to chassis.------> Humming starts.
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Old 13th March 2012, 03:55 PM   #7
AndrewT is offline AndrewT  Scotland
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The fault is in the Power Amplifier.
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Old 13th March 2012, 04:13 PM   #8
Bonsai is offline Bonsai  Taiwan
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Are you input RCA connectors completely ISOLATED from the chassis?
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Old 13th March 2012, 04:20 PM   #9
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Are you input RCA connectors completely ISOLATED from the chassis?
100% Isolated.
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Old 13th March 2012, 10:53 PM   #10
Bonsai is offline Bonsai  Taiwan
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Quote:
Originally Posted by SLAK View Post
100% Isolated.
Where and how are you connecting your common ground between the amplifiers?
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