Suble hum in 2.1 system, not ground loops - diyAudio
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Old 29th August 2012, 02:33 PM   #1
ZLyzen is offline ZLyzen  United States
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Location: Ann Arbor
Default Suble hum in 2.1 system, not ground loops

I have a set of self powered 2.1 speakers that I have just finished building and before I mount everything to the sub's back panel I'm testing it out and notice this.

When the 2 Ic's aren't muted but volume is all the way down there is a slight hum.

I have a preamp board, a power amp board, and a power supply board.

The power amp and preamp are separate except 6 wires:
- Sub + & - (because I will be driving the sub in bridged)(its just a low pass of the left and right but one is inverted)
- Sat L & R (essentially the High pass equivalent, but neither inverted)
-Both wires that connect to the mute pins of each ic (lm4780 for sub, lm4766 for satellite speakers)

Besides that the ground of the pre amp goes to the power supply board and the ground of the power amp board goes to the power supply board which is the large trace that the + and - rail capacitors share.
-- and from this "star" safety earth is connected by a large solid copper wire.

The hum is constant volume no matter what the volume pots are set at and its not because they pots aren't grounded, they are. (i've tried it ungrounded too, no dice) Also it goes away if the speakers are muted but power to the whole device remains, and it's in the satellites and the sub. I was louder but some of it was seriously from my soldering iron, the second I turned that off it went away, but there is still parasitic noise and I've unplugged everything else/tried other rooms/outlets

Now, the psu on each rail is:
-18VAC > .1uF > 4700uF > 4700uF > .1uF >and then ~26Vdc goes to either the power amp, or to regulators (317, 337) for the preamp 17Vdc

do I need more filtering on my board? maybe some 10uF or 220pF?

when I switch off the mains "master switch" but ground is still connected etc the noise disappears and the music plays for a couple seconds before the caps discharge so I'm guessing that narrows it down the the psu

Any insight into my AC>DC conversion?
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Old 29th August 2012, 04:16 PM   #2
DF96 is offline DF96  England
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Probably grounding. Where does the transformer secondary CT connect to the PSU?
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Old 29th August 2012, 06:31 PM   #3
ZLyzen is offline ZLyzen  United States
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Location: Ann Arbor
the transformers has 2 secondaries and 2 primaries. The primaries are in parallel (115VAC) and the 2 secondaries are in series, where the positive of one secondary and the negative of another are connected to one end of the psu board. At the other end ( after full wave rectifiers, and caps) are the connects for amp, pre, and earth.
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Old 29th August 2012, 06:37 PM   #4
DF96 is offline DF96  England
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OK, that sounds correct. I asked because many people get this wrong.
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Old 29th August 2012, 06:45 PM   #5
ZLyzen is offline ZLyzen  United States
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Join Date: Dec 2011
Location: Ann Arbor
Thanks perhaps when it's all mounted on the panel and sealed up in the sub some interference will be canceled and it will be subdued slightly, but it isn't very loud right now.

And yeah I researched how to hook everything up and when I finally noticed the small hum I tried a few other wiring routes and they were all louder and had different types of hum.
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Old 1st September 2012, 09:38 AM   #6
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I am also facing same problem of hum. When i turn my volume control to its lowest position then hum increases, but when i position it to the middle it decreases also when i remove ground connection from the volume control then it disappers completely and there is no hum from the woofer.I tried to increase the power supply capacitors but no change. I am using split unregulated 16-0-16 power supply and my amplifier is running in bridge mode using TDA2030A ic(Ckt is given in datasheet). Please help me.
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