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Pesky 120Hz Buzz / Hum From Speakers
Pesky 120Hz Buzz / Hum From Speakers
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Old 29th January 2019, 05:15 PM   #21
Bonsai is offline Bonsai  Europe
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Are you doing it as on page 37 on the lower part of the slide? This is the recommended way of doing it.

Normally if you just put a resistor in series with the shield (like its shown in the top half of the slide) it makes the problem worse because the loop current across the resistor causes a noise voltage that is in series with the signal. If you do insert a resistor in series with the shield and it makes no difference, then likely you have no cross-channel ground loop problem - not unheard of if your internal layout is good and the input screens are bonded where they enter the amplifier.


Without seeing your layout, the other explanation might be that any internal cross channel loop is not routing through the shield, but through some other internal path.


The main thing with all these techniques, is to keep loop currents away from the shield.
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Old 29th January 2019, 05:27 PM   #22
scottjoplin is offline scottjoplin  Wales
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I know the right way to do it I had a hum when I connected to preamp, since doing it the right way required major disassembly and cutting PCB tracks I haven't got around to it yet. I put a 5 ohm resistor in the shield of just one channel, if this makes a difference, and the hum is gone. I appreciate tracking down loops can be complex and it may not be a cross channel ground loop
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Old 29th January 2019, 06:39 PM   #23
Bonsai is offline Bonsai  Europe
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cool
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Old 29th January 2019, 06:44 PM   #24
Evenharmonics is offline Evenharmonics  United States
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Quote:
Originally Posted by Bonsai View Post
Ok. It’s very important to note that you cannot simply put a HBR resistor in series with the shield connection. You will make it worse by doing that, because the noise loop current flowing through the resistor is in series with the signal.

See slide 37 on how to do it correctly. When it’s done the right way it will attenuate the noise signal and reduce the noise loop current. A 20 dB improvement is usually what you can achieve.

Note that when I mentioned input jack bonding, it just means ‘connect the two signal grounds together where they enter the housing - do not run any separate wire to the 0V on the PSU!

The input cable routing to get rid of the hum is critical. See slides 61 to 63. If you just run the cables directly from the input jacks to the amp modules (shortest route) you are probably creating a huge loop area inside the amp. Run the two cables together to the fist amp module, and continue the second input cable to the second module. You may well find that if you run it back to the PSU alongside the 0V return, and from the PSU alongside the 0V to the second amp module, you get the best result - you will need to experiment a bit to get the best location though. This is where I recommend the ‘headphone trick’ - See slide 65 and 66.

Don’t forget to rotate you transformer +- 60 degrees to find the noise null point as well.
If it helps to point out, I found the schematic and board layout of this amp on this forum.
Attached Files
File Type: pdf L20SE Schematic.pdf (53.2 KB, 6 views)
File Type: pdf L20SE Layout.pdf (137.9 KB, 5 views)
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Old 29th January 2019, 06:49 PM   #25
Evenharmonics is offline Evenharmonics  United States
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Here are JPG version of those.
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File Type: jpg L20SE Schematic.jpg (328.1 KB, 62 views)
File Type: jpg L20SE board layout.jpg (251.0 KB, 59 views)
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Old 29th January 2019, 07:50 PM   #26
tmuikku is offline tmuikku  Finland
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Input shield should go directly to chassis at the connector, don't bring it to the amp board. its for shielding and carries error current. Signal should be in the tip and ring / xlr pins 2 and 3. Google "pin 1 problem".
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Old 29th January 2019, 07:52 PM   #27
scottjoplin is offline scottjoplin  Wales
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That is correct for balanced connections, however the discussion here relates to unbalanced
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Old 29th January 2019, 08:01 PM   #28
Bonsai is offline Bonsai  Europe
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I looked at the amp overlay - there's not much you can do about the power supply connections being on opposite sides of the PCB at this stage - just something you have to keep in mind for the future.

Attached is how to add an HBR to the amp.
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File Type: pdf LJM HBR149.pdf (484.6 KB, 17 views)
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Old 29th January 2019, 08:06 PM   #29
tmuikku is offline tmuikku  Finland
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yep, yet there is connectors having all three pins connected and shielded twisted pair cable used. Why not take advantage of the shield since its there? how is your cabling from the device before this amp? check your connections connect shield (pin 1) to chassis at the connector and nowhere else including the preceding device. don't connect pin3 to the shield (pin1). Now your "chassis" is extended to protect input wires from the preceding device all the way to the pcb screw terminal.

"... I put a 5 ohm resistor in the shield of just one channel, if this makes a difference..."
HBR should not go to shield but the pin 3 which is the "ground" of unbalanced connection. Again, shield is for shielding keep it separate from audio.

Sorry if I appear to be rude, not my point Ive had my share on connecting diy, prosumer and pro gear my tip might or might not cure your problem, but please confirm your cabling first. Might get you on the track solving the problemo

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Old 29th January 2019, 08:10 PM   #30
Mark Whitney is offline Mark Whitney  Netherlands
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Inserted HBR in the schematic.

There could be a ground loop through the mains PE. Does the PC have a three-pronged plug? Try another source.
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File Type: jpg L20SE Schematic.jpg (223.5 KB, 55 views)
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