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Author Topic: Little annoyance, need help  (Read 1333 times)

Richard Smith

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Little annoyance, need help
« on: August 22, 2018, 01:13:28 pm »

I have my own custom amp I use to amplify my headphones, and it is 90% perfect for me, except one issue I am failing to totally eliminate.

When I hover my hand near the amp, the noise floor raises quite a few decibels and I can hear hum. The hum is amplified, so at high volumes, it can be quite audible.

The hum used to occur when I touched the chassis, but I when referenced the chassis to the power ground that stopped the hum when I touch the chassis, but now the hum occurs if I hover over the chassis very closely but don't touch it. If I touch it, the hum stops. Really odd, and I am not sure exactly what is going on here. It is a little bit of a problem because of the location it sits, I am always within reach of the volume knob, so I am usually within hover distance of the amp, so the hum starts coming through.

I could probably live with it, but with it being the last issue I have with amp, I would like to correct it. Any ideas what could be causing that and how to fix it?

For more information, the amp is using a 12v power supply, the negative lead is immediately connected to the chassis, then a 1000uf capacitor is connected across positive and negative before the power is fed to the amp. If you need more info, let me know.

The power supply is not referenced to real earth. The real earth in my house is the noisiest earth I have ever seen, so I avoid using it when I can, but I can't help but think it is my noisy house ground that is referencing to me and then capacitively coupling to the chassis and causing the hum. Not sure how to correct that.
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John Roberts {JR}

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Re: Little annoyance, need help
« Reply #1 on: August 22, 2018, 02:04:33 pm »

so with an audio feed connected and cans plugged in, it hums when your hand is near it, otherwise quiet?

There are electrical fields everywhere when AC power is present, but a grounded chassis should shield against that.

Is any of your gear grounded?

JR

PS: You could wear a ground strap, but that is a band aid, and perhaps dangerous if low impedance.
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Listen to what a properly "cleared" drum sounds like.   http://circularscience.com/

Richard Smith

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Re: Little annoyance, need help
« Reply #2 on: August 22, 2018, 02:14:57 pm »

so with an audio feed connected and cans plugged in, it hums when your hand is near it, otherwise quiet?

There are electrical fields everywhere when AC power is present, but a grounded chassis should shield against that.

Is any of your gear grounded?

JR

PS: You could wear a ground strap, but that is a band aid, and perhaps dangerous if low impedance.

No, the amp uses optical, so there is no ground loop issues to speak of. Nothing at all in the audio path that potentially shares ground actually uses real earth ground, but is grounded based on the 0v rail provided by the power supply. All attempts I have made grounding to real earth have lead to some amount of 60hz hum. Perhaps this is the best I can do until I work out the problem with the noisy ground in my house.
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John Roberts {JR}

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Re: Little annoyance, need help
« Reply #3 on: August 22, 2018, 03:14:38 pm »

No, the amp uses optical, so there is no ground loop issues to speak of.

Nothing at all in the audio path that potentially shares ground actually uses real earth ground, but is grounded based on the 0v rail provided by the power supply. All attempts I have made grounding to real earth have lead to some amount of 60hz hum. Perhaps this is the best I can do until I work out the problem with the noisy ground in my house.

There is a great deal written about safety grounds corrupting audio (google pin 1 problem), but that probably isn't your problem.

If your custom amp is inside a metal chassis and that chassis is grounded to 0V (not earth safety ground, just the local 0V) it should not be susceptible to low level external voltage fields (like from an ungrounded human body). The metal (?) chassis of you amp should effectively shield any high impedance circuitry inside. Audio in this case is all relative, and any energy coming from your hand should be harmlessly shunted to local 0V and not contaminate the audio signal.

Touching the chassis bonds your body to the same 0V.

It is not logical that you stop humming when you touch the chassis, but that same chassis can not prevent hum from your hand just being nearby, despite being on the opposite side of a low impedance shield.

I would say hold production and get an engineer to review your complete design (not me).

JR
 
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Listen to what a properly "cleared" drum sounds like.   http://circularscience.com/

Richard Smith

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Re: Little annoyance, need help
« Reply #4 on: August 22, 2018, 03:36:00 pm »

There is a great deal written about safety grounds corrupting audio (google pin 1 problem), but that probably isn't your problem.

If your custom amp is inside a metal chassis and that chassis is grounded to 0V (not earth safety ground, just the local 0V) it should not be susceptible to low level external voltage fields (like from an ungrounded human body). The metal (?) chassis of you amp should effectively shield any high impedance circuitry inside. Audio in this case is all relative, and any energy coming from your hand should be harmlessly shunted to local 0V and not contaminate the audio signal.

Touching the chassis bonds your body to the same 0V.

It is not logical that you stop humming when you touch the chassis, but that same chassis can not prevent hum from your hand just being nearby, despite being on the opposite side of a low impedance shield.

I would say hold production and get an engineer to review your complete design (not me).

JR
Not logical indeed. Thanks.
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Richard Smith

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Re: Little annoyance, need help
« Reply #5 on: August 22, 2018, 03:55:30 pm »

Not logical indeed. Thanks.

Although you gave up, your reassurance that it isn't logical made me question the correct issue. I have an acrylic window on the body that allows the internals to be visible, and as a test I covered it in aluminum tape, and the hum issues 100% resolved, no hum ever.

It appears it is the window that is allowing in the hum. How unfortunate. Perhaps I can use a mesh under the window...
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John Roberts {JR}

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Re: Little annoyance, need help
« Reply #6 on: August 22, 2018, 04:10:07 pm »

Although you gave up, your reassurance that it isn't logical made me question the correct issue. I have an acrylic window on the body that allows the internals to be visible, and as a test I covered it in aluminum tape, and the hum issues 100% resolved, no hum ever.

It appears it is the window that is allowing in the hum. How unfortunate. Perhaps I can use a mesh under the window...
maybe try a window out of a broken microwave oven door...(fine metal screen).   

I never found a compelling reason to look at electronic circuits inside products that aren't broken.

Back in the 70's when working with some prototype BBD ICs, in open top metal cans, the bench work light interfered with their proper operation (it might even cause hum from AC powered lights). 


JR
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Listen to what a properly "cleared" drum sounds like.   http://circularscience.com/

Erik Jerde

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Re: Little annoyance, need help
« Reply #7 on: August 22, 2018, 10:53:00 pm »

Although you gave up, your reassurance that it isn't logical made me question the correct issue. I have an acrylic window on the body that allows the internals to be visible, and as a test I covered it in aluminum tape, and the hum issues 100% resolved, no hum ever.

It appears it is the window that is allowing in the hum. How unfortunate. Perhaps I can use a mesh under the window...

Transparent aluminum.  ;)
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Chris Hindle

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Re: Little annoyance, need help
« Reply #8 on: August 23, 2018, 08:15:55 am »

Transparent aluminum.  ;)
Where's Mr. Scott when you need him......
But I would expect transparent copper to be much more effective.
Chris.
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Frank Koenig

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Re: Little annoyance, need help
« Reply #9 on: August 23, 2018, 11:26:26 am »

Transparent aluminum.  ;)

Indium tin oxide coating  :o
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Yes, it is a giant stereo system!
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