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 Post subject: Small hum issue
PostPosted: 19 Jan 2016, 12:55 
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Joined: 26 Sep 2015, 13:59
Posts: 60
My home built preamp was working well. I had acheived hum free running and all was good. So I put the lid on the enclosure - and it started to hum! Lid off again, silence. Lid on - HUM!! It's only a small amount of hum and not intrusive but I hate ANY hum, especially when that's what I get with the lid off.

What's going on? I'd have thought a lid, at worst, should make no difference and, at best, should improve things with the added shielding.

Anyone had a similar experience and managed to fix it?

Thanks.


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 Post subject: Re: Small hum issue
PostPosted: 19 Jan 2016, 13:38 
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Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
You created a ground loop with the case. Insulate one side of the lid with plastic washers and nylon screws. Some commercial units (like Mitsubishi) do that.

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* Ratings are for transistors - tubes have guidelines*
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 Post subject: Re: Small hum issue
PostPosted: 20 Jan 2016, 16:24 
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Joined: 26 Sep 2015, 13:59
Posts: 60
Thanks for this.

I am always very careful about ground loops and I generally follow the rules on here to avoid them. I have never hear of a case lid causing a ground loop, how does this happen?

Mark


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 Post subject: Re: Small hum issue
PostPosted: 21 Jan 2016, 02:24 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
It creates an electrical short-circuit around the amplifier, reflecting all stray fiends and currents back inside as well as any currents (however miniscule) flowing through the chassis creates its own EMF.

Good for RF shielding, bad for HiFi ;)

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* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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 Post subject: Re: Small hum issue
PostPosted: 21 Jan 2016, 04:41 
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Joined: 26 Sep 2015, 13:59
Posts: 60
Ahh, I see. Thanks for that. Very helpful indeed.

Mark


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 Post subject: Re: Small hum issue
PostPosted: 21 Jan 2016, 13:58 
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Joined: 04 Jun 2008, 20:59
Posts: 4097
Location: Arizona, USA
Hi, An alternate possibility is that the lid isn't making electrical contact and acting as an antenna. I have had both situations happen. Usually it is because the top isn't connected electrically though (about 90% time in my stuff). This most often happens with powder coated and painted chassis.

Good listening
Bruce

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