New high-end amplifier produces decidedly low-end hum.
January 25, 2012 9:18 PM Subscribe
Humming sound through newly acquired amplifier when TV tuner is plugged in (even when tuner input is not active, even when amplifier is in standby mode). How to solve?
I recently purchased a used high end audio amplifier. After swapping it in to my system, I noticed a humming sound. The hum was persistent across all audio inputs, and it continued even when the amplifier was in standby mode.
I isolated the source of the hum to my Kworld HDTV tuner box. The hum happens only when the coaxial cable is connected from my wall outlet to the HDTV tuner, and it hums whether the tuner is powered on or off. As long as the coax is connected to the tuner, the amp hums.
I did not have this problem with my previous amplifier, so I don't believe the tuner to be defective necessarily.
How can I solve this problem?
I recently purchased a used high end audio amplifier. After swapping it in to my system, I noticed a humming sound. The hum was persistent across all audio inputs, and it continued even when the amplifier was in standby mode.
I isolated the source of the hum to my Kworld HDTV tuner box. The hum happens only when the coaxial cable is connected from my wall outlet to the HDTV tuner, and it hums whether the tuner is powered on or off. As long as the coax is connected to the tuner, the amp hums.
I did not have this problem with my previous amplifier, so I don't believe the tuner to be defective necessarily.
How can I solve this problem?
Almost certainly a ground loop. Disconnect the mains ground from the box that provokes the hum. If that fixes it, wire the metal chassis of that box to the metal chassis of your amplifier with yellow/green insulated, mains-rated wire so that you're not relying solely on the audio cable shielding for a safety ground.
posted by flabdablet at 9:35 PM on January 25, 2012
posted by flabdablet at 9:35 PM on January 25, 2012
Response by poster: The TV box has a small AC to DC wall wart and is ungrounded. Also, it doesn't have a metal chassis - it's this one.
And the hum only occurs when the coax cable (connected to Comcast cable in my apartment building) is connected to the tuner.
Could it be a ground issue with the coax cable?
posted by reeddavid at 9:39 PM on January 25, 2012
And the hum only occurs when the coax cable (connected to Comcast cable in my apartment building) is connected to the tuner.
Could it be a ground issue with the coax cable?
posted by reeddavid at 9:39 PM on January 25, 2012
Response by poster: flabdablet-
I realized that the hum occurs only when the shielding of the coax cable touches the coax connector on the tuner. It's not the signal-carrying core that causes the hum.
Could it be that the cable company has not properly grounded their coax cable at some point up the line from my apartment?
posted by reeddavid at 9:50 PM on January 25, 2012
I realized that the hum occurs only when the shielding of the coax cable touches the coax connector on the tuner. It's not the signal-carrying core that causes the hum.
Could it be that the cable company has not properly grounded their coax cable at some point up the line from my apartment?
posted by reeddavid at 9:50 PM on January 25, 2012
Yes, it sounds like the cable connection is the likely source of your issue. That's actually really common. The cable that your house is receiving is coming in either ungrounded or on a separate ground from the rest of your system. They make transformer boxes that are specifically made to address this problem.
I can't personally recommend one but if you google "Ground loop cable box" you will see various accounts of what worked for others. I've seen the Jensen Transformer, Model VRD-1FF as well as a basic radio shack version recommended.
posted by tinamonster at 9:51 PM on January 25, 2012
I can't personally recommend one but if you google "Ground loop cable box" you will see various accounts of what worked for others. I've seen the Jensen Transformer, Model VRD-1FF as well as a basic radio shack version recommended.
posted by tinamonster at 9:51 PM on January 25, 2012
One other suggestion: You can try to have your cable company fix the problem but YMMV on that one.
posted by tinamonster at 9:55 PM on January 25, 2012
posted by tinamonster at 9:55 PM on January 25, 2012
Response by poster: There is a binding post on the splitter that splits the signal to the tuner and my cable modem (I did verify this problem occurs sans-splitter as well). What if I connect a wire from that binding post to the ground in my outlet, or to the grounded chassis of another device similar to flabdablet's earlier suggestion?
If I understand it correctly, the issue is that not all my devices have the same ground connection, and therefore there is a flow of current form one to another.
Is this safe?
posted by reeddavid at 9:58 PM on January 25, 2012
If I understand it correctly, the issue is that not all my devices have the same ground connection, and therefore there is a flow of current form one to another.
Is this safe?
posted by reeddavid at 9:58 PM on January 25, 2012
Response by poster: I'm going to call the cable company, but in the meantime I ordered this TV ground loop isolator. Thank you all.
posted by reeddavid at 10:16 PM on January 25, 2012
posted by reeddavid at 10:16 PM on January 25, 2012
"There is a binding post on the splitter that splits the signal to the tuner and my cable modem (I did verify this problem occurs sans-splitter as well). What if I connect a wire from that binding post to the ground in my outlet, or to the grounded chassis of another device similar to flabdablet's earlier suggestion? "
That's exactly what that binding post is for, in theory. That said …
"Is this safe?"
… I wouldn't do it. The issue is that you don't know how much current may potentially flow in your local ground circuit (i.e. between the cable modem, TV, amp, and anything else you have connected to them). Possibly more importantly, you have no idea how much current will flow in the ground loop on the other side (i.e. between the cable headend, your ground, and the grounds of everybody else connected to the same headend). In theory it should be no more than that which you already have flowing (and causing the hum) - but in practice I've seen for myself that simple changes like that can cause the cable sheath to start cooking due to carrying 10's of amps instead of milliamps.
Grounding issues like that can often be quite complex and beyond the scope of basic electrical theory (and 99.99% of telco / cable company techs). I'd go with an isolation transformer like tinamonster suggests.
posted by Pinback at 10:20 PM on January 25, 2012
That's exactly what that binding post is for, in theory. That said …
"Is this safe?"
… I wouldn't do it. The issue is that you don't know how much current may potentially flow in your local ground circuit (i.e. between the cable modem, TV, amp, and anything else you have connected to them). Possibly more importantly, you have no idea how much current will flow in the ground loop on the other side (i.e. between the cable headend, your ground, and the grounds of everybody else connected to the same headend). In theory it should be no more than that which you already have flowing (and causing the hum) - but in practice I've seen for myself that simple changes like that can cause the cable sheath to start cooking due to carrying 10's of amps instead of milliamps.
Grounding issues like that can often be quite complex and beyond the scope of basic electrical theory (and 99.99% of telco / cable company techs). I'd go with an isolation transformer like tinamonster suggests.
posted by Pinback at 10:20 PM on January 25, 2012
If you install an isolation transformer between the cable outlet and the splitter, that will mean that the only ground connection for your local equipment cluster is the ground wiring in your house; you will no longer have an audio-frequency or DC connection to a cable company "ground" of unknown provenance. So if you still have a little residual hum once that's done, wiring the splitter's binding post back to your amplifier's chassis should be perfectly safe and would be worth trying.
posted by flabdablet at 10:29 PM on January 25, 2012
posted by flabdablet at 10:29 PM on January 25, 2012
Pinback's point about potentially heavy ground current flow via the splitter's binding post should not be an issue once the isolation transformer is in place, by the way; apart from the cable company ground (now isolated) the splitter's only other ground connections will be signal grounds to your cable modem and cable TV tuner, both of which will most likely have a wall-wart supplies and no direct path to mains ground.
posted by flabdablet at 10:35 PM on January 25, 2012
posted by flabdablet at 10:35 PM on January 25, 2012
Response by poster: Thanks for the follow-ups! I will install the isolator, then ground the splitter if any hum persists.
posted by reeddavid at 10:55 PM on January 25, 2012
posted by reeddavid at 10:55 PM on January 25, 2012
Could it be that the cable company has not properly grounded their coax cable at some point up the line from my apartment?
With the best will in the world, a coaxial cable that visits a whole bunch of buildings is going to exhibit noise pickup on the shield relative to the local mains ground.
It's worse in apartments, because most of those will simply not have access to the actual real physical walk-on-it stick-stakes-in-it ground, the electrical wiring will be long and twisty, and capacitive and inductive coupling between conductors within the wiring means that some sort of hum signal is always going to appear between your local mains "ground" and the independently-grounded coax cable shield.
The only way to avoid this would be to connect the cable shield to the local mains ground at every coax outlet, which I expect the cable company would be unwilling to do for the reasons Pinback explained.
Adding an isolation transformer between the cable company coax and the equipment it feeds puts a very high resistance (at audio frequencies) between the cable outlet and the stuff you're feeding from it, and if you also add a ground bypass on the equipment side (by attaching a ground wire to the splitter's binding post) then virtually none of the hum should make it as far as your amplifier.
posted by flabdablet at 11:01 PM on January 25, 2012
With the best will in the world, a coaxial cable that visits a whole bunch of buildings is going to exhibit noise pickup on the shield relative to the local mains ground.
It's worse in apartments, because most of those will simply not have access to the actual real physical walk-on-it stick-stakes-in-it ground, the electrical wiring will be long and twisty, and capacitive and inductive coupling between conductors within the wiring means that some sort of hum signal is always going to appear between your local mains "ground" and the independently-grounded coax cable shield.
The only way to avoid this would be to connect the cable shield to the local mains ground at every coax outlet, which I expect the cable company would be unwilling to do for the reasons Pinback explained.
Adding an isolation transformer between the cable company coax and the equipment it feeds puts a very high resistance (at audio frequencies) between the cable outlet and the stuff you're feeding from it, and if you also add a ground bypass on the equipment side (by attaching a ground wire to the splitter's binding post) then virtually none of the hum should make it as far as your amplifier.
posted by flabdablet at 11:01 PM on January 25, 2012
As usual, flabdablet has filled in the gaps and fleshed things out better than I ever could ;-).
The important thing to remember with ground loops is that they're loops - so, if you've got problems because of one, adding another loop won't actually fix it. It might fix it here, but it almost always just shifts the problem somewhere else (to a place you may or may not care about).
The (usually) easiest and (usually) best way to solve ground loop problems is to break the loop, which is what the isolation transformer / braid breaker does.
"Could it be that the cable company has not properly grounded their coax cable at some point up the line from my apartment?"
Not properly? Unlikely, because bad grounding typically causes them more problems than it does you.
Adequately enough to prevent any ground loop issues? That's a different question, and one which is impossible to solve without everyone living on the same large chunk of superconducting material ;-)
posted by Pinback at 12:59 AM on January 26, 2012
The important thing to remember with ground loops is that they're loops - so, if you've got problems because of one, adding another loop won't actually fix it. It might fix it here, but it almost always just shifts the problem somewhere else (to a place you may or may not care about).
The (usually) easiest and (usually) best way to solve ground loop problems is to break the loop, which is what the isolation transformer / braid breaker does.
"Could it be that the cable company has not properly grounded their coax cable at some point up the line from my apartment?"
Not properly? Unlikely, because bad grounding typically causes them more problems than it does you.
Adequately enough to prevent any ground loop issues? That's a different question, and one which is impossible to solve without everyone living on the same large chunk of superconducting material ;-)
posted by Pinback at 12:59 AM on January 26, 2012
For what it's worth, if you have a pair of these kicking around you can make your own hum eliminator by connecting the two wired ends together.
posted by davey_darling at 5:36 AM on January 26, 2012
posted by davey_darling at 5:36 AM on January 26, 2012
Response by poster: Thanks everyone for your great answers! I installed the isolator just now and the hum is completely gone. And next time I encounter a ground loop, I'll be well-equipped to solve it.
posted by reeddavid at 5:21 PM on January 27, 2012
posted by reeddavid at 5:21 PM on January 27, 2012
Any noticeable difference to the quality of your cable feed and/or data speeds?
posted by flabdablet at 8:45 PM on January 27, 2012
posted by flabdablet at 8:45 PM on January 27, 2012
Response by poster: flabdablet-
The cable feed is unchanged; it's digital and still works. I have noticed anecdotally that data speeds seem faster.
posted by reeddavid at 1:21 PM on January 29, 2012
The cable feed is unchanged; it's digital and still works. I have noticed anecdotally that data speeds seem faster.
posted by reeddavid at 1:21 PM on January 29, 2012
This thread is closed to new comments.
posted by tinamonster at 9:28 PM on January 25, 2012