Author Topic: Audio HUM fixed  (Read 371 times)

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Offline Zfrenchy

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Audio HUM fixed
« on: May 16, 2021, 07:38:16 pm »
Not sure it is at the right forum, but ...

I had a very bad ground loop hum on my audio system, after I tried a lot of things, I finished by totally eliminated the hum with this circuit.


JUST TO BE SAFE, can a EE tell me if this will put my AC circuit or my equipment in danger ?

Online bob91343

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Re: Audio HUM fixed
« Reply #1 on: May 16, 2021, 11:24:19 pm »
Looks reasonable but the font is so small I can't read it.

Offline Vovk_Z

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Re: Audio HUM fixed
« Reply #2 on: May 17, 2021, 08:31:44 am »
Yes, looks fine. But if somebody (not EE) asks this question then I can't be sure. :)

Online richnormand

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Re: Audio HUM fixed
« Reply #3 on: May 17, 2021, 10:59:16 pm »
You show two-prong AC plugs. On my old audio system with several components pre-dating the three prong usage I used to locate a good copper pipe and run a ground wire to it or use a three prong plug with a known real ground pin to test the setup. Use a DMM on AC between the ground and each device chassis. Do not have anything else plugged to it or between them. If there is a sizable reading reverse the (two prong) AC plug to minimize the reading. Do this for each unit. Once they are all OK I would run a ground between all of them to the real ground and the hum would disappear.

You can get cheap testers at the hardware store to see if the neutral and hot are reversed and if the safety ground is OK at the wall. Several old houses I lived in had only two prong sockets and found many were reversed (hot/neutral) with respect to a proper ground. There was a period where a two prong plug had a wider blade on one side to have proper polarisation but they were pretty useless in that scenario as they would fit both way in the old style socket.

From the equipment you mentioned I would assume the two-prog plugs are indeed polarised (wider blade on one side) and should be OK.
On the subwoofer is there a ground lug or some screws directly on the chassis available instead of using a clip on the RCA? Would be OK if the RCA outer shield is at chassis ground but not if it is isolated/floating to prevent ground loops...
« Last Edit: May 19, 2021, 10:51:34 pm by richnormand »
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Online Brumby

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Re: Audio HUM fixed
« Reply #4 on: May 18, 2021, 12:57:36 pm »
Single point earthing of each piece of equipment is a good first step and isolating signal paths (as shown) is the simplest and most effective solution - as long as the transformer performance is up to spec. to give you suitable audio quality.

Looks good.   :-+

Offline BrianHG

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Re: Audio HUM fixed
« Reply #5 on: May 19, 2021, 09:30:18 pm »
Yes your theory and concept is correct.  However, do be warned that an audio ground loop isolator transformer usually do not respond/pass through flat to low frequencies, IE Subsonic ones, unless they are absurdly expensive.  For the normal ones, the lower end of the frequencies will be at a much weaker volume than the audio frequencies higher up at 40-80Hz.  IE the 15-25Hz, the LFE range may be at 1/4 volume or less while a 50Hz tone will go through the transformer at full volume.

Verify the quality of your RCA cable's GND coax.  Make sure it is a really thick good quality braided copper.  The frame ground connection you have may also help.  Unless you are willing to pay for an ground isolation transformer which operates flat below 20Hz, it may be advantageous to verify if you can fix your hum issue without the transformer.

Also, I've see some isolation transformers incorrectly internally wired which invert the amplitude phase of the audio going through.  For a sub-woofer which you want firing outward on a (for example) explosion in a movie you are watching, such an incorrectly wired transformer will make the sub-woofer fire inward instead of outward.  I would not expect this issue with an expensive ground isolator which was designed for very low frequencies.
« Last Edit: May 19, 2021, 09:35:03 pm by BrianHG »

Offline duckduck

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Re: Audio HUM fixed
« Reply #6 on: May 20, 2021, 02:02:56 am »
This PDF gives a good overview of removing hum from interconnected audio devices. Yes, the author's transformers are USD600 each. Ouch. I'm sure they are nice for professional use.
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