Question Speaker hum from PC sound card analog outputs ?

CubsWin

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I recently purchased a new Logitech z906 5.1 speaker system to replace my aging X-540 speakers. When I connected everything to my PC using a Sound Blaster AE-5 card (using the 3x3.5mm analog outputs), I got a hum in my speakers that became very audible anytime the volume on the Logitech control pod was turned up past 3. The higher I turn the volume knob up on the control pod, the louder the hum gets. Increasing or decreasing the volume on the source itself (my PC) doesn't change the hum at all and turning the PC volume up usually drowns it out completely. I tried different cables, with the same result.

I connected an optical cable and the sound is perfectly clear with no hum whatsoever. I would prefer not to have to use optical though, so I continued experimenting. I connected another source via RCA audio and there was no hum on that one either, so it is specific to the analog outputs on my PC sound card. At that point, I decided maybe there was a problem with the shielding on the sound card and I ordered a brand new AE-7 card. I installed that, and I'm still experiencing the exact same hum, which seems to rule out the sound card itself. What else could there be? Maybe a ground loop? If so, would it be possible to put a ground loop isolator on each of the 3 analog outputs coming out of my sound card? Is there anything else I could be missing?
 

Paperdoc

Champion
Ambassador
My first suggestion is to maximize the volume output from your PC, and reduce the volume setting on the Logitech control box to compensate. Ideally, of course, you'd want to balance these so that with full output from the PC you get sound a bit too loud. Then you can reduce PC output a bit to get it right, but still have the ability to boost it from your keyboard when you need to. All of this is just to ensure that the Logitech system gets the best possible signal and does not need to amplify it (along with any included noise) hugely.

Then try this IF you can - depends on plug details. SOMEtimes the power connection into your external device (like the Logitech speaker system) is fed from a different source from the PC, and polarity gets reversed and can cause unwanted noise signals like hum. Look closely at the prongs on the speaker system power cord. IF there are three (two flat parallel ones plus a round Ground prong in a triangle) you can't change how that plugs in. IF there are only two flat parallel blades but one is wider than the other, you probably also cannot change this. But IF there are two flat blades of the SAME width, try simply unplugging, turn the plug over, and plug back in. This will change which blade is connected to the wall outlet Hot line, and which to Neutral. Sometimes that can reduce hum.
 
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CubsWin

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My first suggestion is to maximize the volume output from your PC, and reduce the volume setting on the Logitech control box to compensate.
Thank you for the detailed reply. This is basically what I'm doing right now. I'm sitting at the sweet spot where the hum is low enough that I don't really hear it when the room is silent, but I still can get pretty good sound output when I'm playing audio and crank up the volume in Windows. But it still feels like the system should be capable of so much more.

The prongs on the plug are different sizes and cannot be flipped. My PC and speakers are both plugged into the same UPS. Is there anything else I can consider to try to reduce the noise? Ground Loop isolators on the 3.5mm outputs? Replace PC power supply?
 

CubsWin

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Returning to update on my issue:

I tried several things to fix the noise in my speakers, and I believe I have arrived at an acceptable solution. As I mentioned before, I had already replaced the sound card, so I ruled that out. I also tried swapping out the speaker system itself and that also made no difference. I tried plugging into a different electrical outlet, and there was no change. I even ordered a new power supply (which hasn't arrived yet), wondering if it might be my near 10 year old power supply that is the source of the noise being picked up by my sound card. I figured that even if that doesn't fix the problem, it wouldn't hurt to replace that old power supply anyway before it fails on me.

Now on to what actually worked...

https://www.amazon.com/Isolator-Audiophile-Frequency-Without-Distortion/dp/B01N10AQ76

I ordered one of these 3.5mm Ground Loop Isolators and plugged it into one of the outputs on my sound card. I was pleasantly surprised to find out that it eliminated virtually all unwanted noise from that set of speakers. I went ahead and ordered two more of the same item and plugged those into the remaining two analog outputs on my sound card. Now, every speaker in my system is almost entirely free of noise at any reasonable volume. If I crank the control pod volume all the way up to the max, I can still hear a slight hum, but there would be no reason to ever have the volume that high. I can set it to a mid-level on the control pod and then adjust the volume in Windows to the desired level, which still goes louder than I would ever need. As far as I can tell, there is no difference in sound quality when using the Ground Loop Isolators, with the exception of the annoying hum being removed.

So, while I never did pinpoint the source of the noise in my system, I have been able to resolve it with a simple, and cheap ($14.99 x 3) solution. Sometimes the simplest solution is the best one!
 
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InvalidError

Titan
Moderator
This is a typical ground loop problem: the PC's PSU has Y-class capacitors between live and chassis for EMI suppression, which introduces a 60Hz component to chassis ground,
the speakers' PSU should also have a Y-class capacitor between its low-voltage DC output and mains, so any ground voltage potential between the two will cause current to go through the audio cables' ground and introduce a 60/120Hz hum.

Audio isolation transformers are one way to break a ground loop. Another way to greatly reduce ground loop hum would be to have a heavy-gauge ground wire commoning all equipment grounds at the amplifier, though most consumer audio equipment has dropped the equipment grounding lug/screw.
 
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