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Discussion Starter · #1 ·
Hope this is a good forum for this post. I am getting an intermittent hum from my HT system.


I use a PC as the source. The PC, the projector, the amp for my butt-shakers, the cable-tv box, a mic/turntable mixer and a small tv monitor are all powered off one outlet. A set of powered multimedia speakers connected to the PC is powered off another outlet (2 prong). There is overhead halogen track lighting in the room. And there is a stairwell off the room, with its own light.


When the speakers are switched to standby, I always get a loud 60 cycle hum. This is not a huge issue for me, as I leave them either on with the volume turned all the way down or unplug them when not in use. But I suspect there is a clue in there somewhere, which is why I mention it.


The thing that is unacceptable and has me stumped is that occasionally, for no reason I can identify, I suddenly get a loud hum from the speakers when they are on. They just start humming.


The solution is to turn on the overhead halogen track lights. The hum stops. And even more weird, I can turn the lights back off and the hum does not resume. The track lighting is on a 3-way switch.


The same thing happens with the butt shakers. I was playing a video game the other night, and they just start into this 60 cycle hum. Aha, I thought. I turned on the track lights. Nothing. The particular switch I was standing at shares a wallplate with another 3-way switch that controls the stairwell light. I flipped the stairway light switch, and the hum from the butt-shakers ceased. Turned it back off, the hum does not resume. All the while, the speakers are quiet.


A third scenario: I was watching a movie last night, and the butt shakers start humming again. I forgot and flipped the light for the overhead track lights. The speakers start humming! I turned on the stairwell light and both the speakers and the butt-shakers go quiet. Flipping either of the switches again does not cause it to come back to either audio device.


What the heck is going on!? I can't even guess how to start troubleshooting this. The track lighting is new, so I am going to re-check it this weekend. Does this sound like something obvious and stupid I have done? Is there anything that might kick on elsewhere in the house that could cause something like this? Why does it go away and not come back?


Please help!
 

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Does the track light have a dimmer switch?


Dimmer switchs will destroy audio. They work by pulsing the voltage and can introduce DC and various levels of HF noise. Really tough to block it, even with good power conditioners.
 

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You probably have a neutral that's shared between more than one circuit with a few non-linear loads in series like fluorescent lights, computer power supplies, switching amps, light dimmers, microwave ovens or anything that dosen't use power in phase with the ac waveform. The noise each non-linear load generates is additive, enough non-linear loads in series on the neutral..., well you get the picture.
 

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Discussion Starter · #4 ·
So, if this were the case, short of completely rewiring the branch circuit in question (not an option really, nor is adding a dedicated line), is there anything I can do about it? Would an isolation transformer help?


I am really puzzled by the intermittent nature of the problem, plus the fact that flipping the switch makes it go away until it decides on its own to start up again. Almost like something is charging capacitavely and then flipping the switch discharges it.


Bill: There is not a dimmer on the track lighting, but we do have some dimmers elsewhere in the house.


Thanks,

Gerald
 

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Actually, you may very well have two problems. What type of dwelling do you live in?


First, disconnect the cable from the wall and see if much of the problem doesn't go away. I suspect you have a ground loop.


We'll talk more about the halogens after we see if disconnecting the cable helps. FYI- halogens are banned from my house!
 

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Discussion Starter · #6 ·
I live in an approx 15yo house.


I watched a couple of movies over the weekend and so far, the problem has not reappeared, even though I have not done anything to solve it. I am sure it will come back right in the middle of something really gripping...


What about the halogens?
 

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Halogens, like dimmer switches, use transformers which are EMI machines.


If it comes back disconnect the cable from the wall and see if the problem goes away. Also, do you have a multimeter and/or an outlet checker?
 

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Discussion Starter · #8 ·
Halogens and transformers... yeah, I shoulda remembered that.


If it comes back, I will remove the cable (you mean CATV, right?) to see if it stops. I actually run the cable from a digital box to the TV capture card via SVGA, if that makes any difference.


Watched another movie last night though, and all was well. Frustrating...


I have a multimeter and an outlet checker.
 

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Be sure to check the outlet. Check and be sure it is properly grounded and that hot-neutral and hot-ground voltage is about 120volts.


You can also disconnect the cable from the cable box and measure voltage between the shield of the cable and the ground of the outlet.
 

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Try plugging all your equipment and grounding your cable TV, telephone modem, network connection or any other external com lines into something like the Panamax Max 8 with allpath modules if you have any of those connections. Ground loop currents also flow through com lines. Grounding everything at a single point will eliminate ground loops.


Good luck
 

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Discussion Starter · #11 ·
Well, it came back in the transducers the other night. I decided I needed some light to see what I was doing when I went to disconnect the CATV. DUH! Flipping the light made it stop again.


Luckily (?!) the next time I turned on my PC it was back, and very loud. Messing with various switches, CATV, etc. did not help. I finally discovered that a bad jack/plug connection in a mono extension cable was not quite up to snuff. Jiggling the cable made the hum go away, etc. So I replaced that connection.


Everything seemed OK until I was watching a movie last night, when the transducers started sort of a very subtle throbbing. It didn't really make any sense for it to do this at the part of the movie that I noticed it, but it was The Exorcist: The Version You Have Never Seen, so who knows.


I turned the transducer amp off at that point because I didn't feel like dealing with it at the time.


I am sure that the loose connection needed fixing, but I am still confused about some things:


1) why would flipping a light switch have any impact?

2) why did the speaker system become involved during one instance?


All the outlets check out OK, but I will check the voltages as well.


Educate me about how power strips fit into the "plug everything into one wall outlet" concept. Would the length of the power strip cord have any signifigant impact to ground loops? How about extension cords? UPS?


Thanks for bearing with me so far.


Gerald
 

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What transducers are you talking about?


And did the 60 hz hum you always get when the unit is on standby go away when you disconnected the CATV?


The purpose of a power strip is just to provide more outlets on the same circuit.


A ground loop exists when your electronics have more than one path to ground. By effectively plugging all of the parts into the same outlet you can be certain all of the ground potentials are equal and (hopefully) eliminate ground loops. The other grounded systems in a house are typically the telephone and CATV. Often the CATV is not properly grounded in the house and the nearest ground will be somwhere in the CATV distribution system which will have a dramatically different potential from your main house ground and current will flow and you will get hum. I've never heard of a ground loop involving the telephone line affecting a HT system, but it wouldn't surprise me if it occurred.


It is looking more and more like the problem is in your AC power. Matt outlined the reasons nicely. Perhaps it is noise from your halogens, perhaps not. Heck- it could even be from your neighbors appliances.


If you can't have a dedicated line, perhaps you could go to Best Buy and pick up one of those Monster power conditioners. Maybe the 2500. Set it up and give it a shot. You always have 30 days to return it if it doesn't help!


Isolation transformers are better, but to buy them new with the VA requirements you will need will be quite expensive (that's the next step).
 

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Noise causing currents cancel each other out if they're common mode (on both the hot & neutral) and of equal magnitude and opposite polarity. The line noise introduced by turning the lights on is canceling out existing noise. The length of the power strip cord or extensions cords connecting equipment to the power strip is irrelevant. as long as everything is plugged into the same strip you're ok with ground loop noise. Noise caused by non linear loads in series is another issue.
 

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Matt-


What you say makes perfect sense, but I'm having a hard time believing that somebody would be lucky(?) enough to get common mode noise of equal magnitude but opposite polarity. Can you describe a situation where you think that might occur? I guess it would not need to cancel exactly, but only to the point of being below the noise floor of the system.


My thought was that turning on/off the lights changed the loading of the circuit and alterted the impedence of the circuit. I guess it's also possible that the power supply of the lights has some H-G caps to filter RFI and they could dump some of the HF noise (but this would not be a 60 hz hum).
 

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Bill,

If the neutral is shared between circuits powered by both phases in a typical 2 phase 240/120 US residence then the noise causing currents generated by loads on either phase would be out of phase with each other and cancel. I believe the NEC ( I can't state chapter and verse) mandates that neutrals can only be shared among circuits powered by separate phases. Shared neutrals are as common as dirt.
 

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Discussion Starter · #16 ·
Bill: The hum present during standby does not disappear when the CATV cable is disconnected from the digital cable box. I actually have two additional devices elsewhere in the house that are connected to CATV. Should I disconnect these also?


The transducers I refer to are Aura Interactors that I have mounted to my sofa. I run these off one of the cheap amps that came with them. Signal is from the .1 output of my soundcard. I referred to them as "butt shakers" earlier.


I am going to re-visit the scene where these started acting up last night. If it repeats in the same spot I am going to write it off as just being a weird fluke in the soundtrack.


If it does not repeat, I will plan on spending some time rearranging cables to ensure that everything is off the same wall outlet in the near future.


Any opinion about the Trip Lite (sp?) series of isolation transformers? These seem to be reasonably priced.


Matt: can you explain what "noise caused by non linear loads in series" means?


Thanks guys!
 

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Discussion Starter · #17 ·
I checked the voltages between hot/common and hot/ground and got around 115v or so in each case. I checked for voltage between the ground of the outlet and the shield on the CATV cable and found none.


Now to summon the Exorcist...
 

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Tripplite is a reputable brand, but this is probably not as cheap as you think.


You need to add up the maximum wattage of your equipment and multiply by roughly 2 to determine the VA rating of your isolation transformer. Generally two transformers are used, one for power amps and one for digital components. The power amp is where the dollars are going to go, you are looking at pretty high power requriements. You can get cheaper used ones on ebay.


I'm not even convinced you will fix the problem with one.


Tell me a little more detail about your situation. Do you live in a house or apartment and what part of the world? How old is the construction?


I would consider two things. First, beg, borrow, or steal a quality power conditioner and give it a test run. Second, disconnect the halogen lights temporarily and see if the problem still exists.
 

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Gerald,

outlets on the same circuit are setup like Christmas tree lights, one after the other or in series. If there is a non-linear load like halogen lights plugged into an outlet on the same circuit as your HTPC then the halogen is said to be series with your HTPC.

Bill,

I may have got it wrong about how non-linear loads introduce noise-making currents on to the line. I believe now its only differential (on the neutral only.) So common mode noise rejection even on circuits with a shared neutral is not possible - sorry - Your theory about the halogens filtering may be right. - thanks
 

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Quote:
Originally posted by Bill Lummus Second, disconnect the halogen lights temporarily and see if the problem still exists. [/b]
Assuming Gerald has a shared neutral he may have lighting on other circuits contributing noise. So he may need to disconnect all Halogens in his house/apt.
 
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