LM3886 surround amp; problems with 50Hz noise - diyAudio
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Old 15th January 2015, 06:35 PM   #1
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Default LM3886 surround amp; problems with 50Hz noise

Hi! I just finished building my own 5.1 surround amplifier, And I got my new surround receiver yesterday, a Marantz SR5008. I'm using the pre out from the Marantz, into my LM3886 based amplifier. Sounds great on the paper, right? That's what I thought too. When I finally got the Marantz in the mail, I hooked up everything, and there; that awful 50Hz hum.

First, a little bit about the amp I made. I got a 600VA 2x24v AC Toroid transformer. The amplifier is based on 5 separate LM3886's, one for each speaker in the room. The subwoofer got it's own PCB, 3 LM3886's in parallel. All the inputs and outputs are mounted on a laser cut steel back plate, and each connector is isolated from each other with plastic sleeves. The back plate itself is not grounded.

I've been testing a lot, tried to hook up a wire from the chassis on the Marantz and into the ground rail in the amplifier, but no difference. BUT, I discovered that I get rid of the hum if I connect all the speakers, and the signal wires, except for the subwoofer. If I connect the subwoofer signal wire, I get the humming noise back.

I've tried to connect just the center of the subwoofer phono signal wire , but the subwoofer starts creating the humming noise.

How can I get rid of this noise?

I'll attach some pictures so you guys know what we're dealing with
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Old 15th January 2015, 09:46 PM   #2
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It may help if you shorten the PSU-amp leads to the subwoofer amp and connect it to the other amp boards. If there is an improvement we can work on a better layout.

Also try the subwoofer amp without the other amps connected to the receiver.
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Last edited by Mark Whitney; 15th January 2015 at 09:50 PM.
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Old 16th January 2015, 07:39 AM   #3
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There is no hum when only the subwoofer signal wire is connected to the amplifier, and there are no hum when all speakers are connected, except the subwoofer. All the amplifier boards get its power from the same source, so I don't know why this is happening
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Old 16th January 2015, 07:58 AM   #4
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The hum is probably caused by the way you have split the ground into two parts, a 5 channel ground and the subwoofer ground. Replace the subwoofer amp power+GND by connecting the subwoofer amp power+gnd to one of the other amps.
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Old 16th January 2015, 08:31 AM   #5
sangram is online now sangram  India
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So you're running input and output wires in tightly coupled parallel pairs? Bad idea, you will get lots of crosstalk. I would separate them as much as possible.

Also, you're running power wires under the board, near all the sensitive parts of the circuitry? Bad idea number 2. I normally turn the PCBs 90 degrees so that the pads face each other, and use stiff wire to connect them all together (no terminal block). This would also enable to run the speaker wire close to the chassis floor and the signal wire above all of this, closer to the case top. This creates a decent amount of separation between input and output.

Also for such kind of implementation I would use screened wire for the input signals. Also I would use a ground common at the input, and another one at the the output, and join them all together. I have done something like this before and had no hum problems. In addition, in that case I had two different power supplies but making sure all the ground had a common reference helped a lot.

There was another member here who had the exact same problem and solved it using input transformers, That is a good solution, though €xpensive.
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Old 16th January 2015, 12:46 PM   #6
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I've tried to connect the subwoofer amplifier board right on the other LM3886 power terminals; No difference. But I tried to move the internal signal wires away from the transformer, and now there is a little bit less hum.

I also discovered something weird. The hum is completly gone if I unplug the mains, and let the amp bleed out. The rectifier capacitors are quite big, so I still got power for about 5 seconds before they cut. Can this problem be related to the transformer?
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Old 16th January 2015, 01:27 PM   #7
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Is the receiver connected to anything with an earth connection, like a TV with an aerial or a PC with a mains earth.
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Old 16th January 2015, 01:29 PM   #8
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Yes, the receiver is connected to a Chromebox that has mains earth
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Old 16th January 2015, 01:43 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by hansibull View Post
....................

I also discovered something weird. The hum is completly gone if I unplug the mains, and let the amp bleed out. The rectifier capacitors are quite big, so I still got power for about 5 seconds before they cut. Can this problem be related to the transformer?
That is not weird. That is a hum loop.
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Old 16th January 2015, 02:34 PM   #10
infinia is offline infinia  United States
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50Hz hum is always mains and earth ground related, 100 Hz hum is PS filtering and grounding.
also examine neutral connection to audio grounds
Y caps on IEC filters and such, the solution is usually lifting audio grounds with resistors.
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Last edited by infinia; 16th January 2015 at 02:38 PM.
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