I recently moved and connected my new Cox Contour DVR to an audio receiver and TV. (A Blu-ray player is also connected to the receiver and TV; this m ay or may not have any relevance to my issues.) There was a pretty bad hum so I inserted an isolation transformer between the cable feed and the DVR, which eliminated the hum. I measured the voltage between the outside of t he cable connector and safety ground with a cheap VOM and found 0.2VAC and
1-9 mVDC. I have a number of questions that I hope someone can answer for me.
Is this voltage difference the cause of the hum?
Was my measurement approach appropriate?
Do the measured voltages indicate that the cable is improperly grounded?
If the cable were to be bonded to the house neutral with 3/0AWG, would this likely eliminate the need for the isolation transformer?
Whether it would eliminate the hum or not, should I have it done?
Does the cable company have any obligation to do it in order to comply with regulations?
r and TV. (A Blu-ray player is also connected to the receiver and TV; this may or may not have any relevance to my issues.) There was a pretty bad h um so I inserted an isolation transformer between the cable feed and the DV R, which eliminated the hum. I measured the voltage between the outside of the cable connector and safety ground with a cheap VOM and found 0.2VAC an d 1-9 mVDC. I have a number of questions that I hope someone can answer fo r me.
is likely eliminate the need for the isolation transformer?
th regulations?
Since the iso transformer got rid of the hum it's a real good bet that the
200mV is your problem. I had a similar problem with tuners in the computers and antenna / cable feeds and hum. A 'galvanic isolator' from these folks cleaned everything up.
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The Voltage in my system is similar to yours so safety really isn't an issu e. The isolator solved the problem and as far as I'm concerned, it's a perm anent solution.
r and TV. (A Blu-ray player is also connected to the receiver and TV; this may or may not have any relevance to my issues.) There was a pretty bad h um so I inserted an isolation transformer between the cable feed and the DV R, which eliminated the hum. I measured the voltage between the outside of the cable connector and safety ground ..
Good; measurement is the right thing to do.
Probably.
Yes!
Not necessarily; the grounding of the cable is intended to protect you from lightning, and isn't supposed to pass a 'hum' test.
is likely eliminate the need for the isolation transformer?
Hopefully, you mean bonded to the house GROUND, since the only neutral bonding allowed by code is inside the circuit breaker box. No, confusing the matter with multiple straps won't help. Any cable with multiple connections to ground is a ground loop, and those CREATE hum.
If your audio path can be replaced with digital or TOSlink, that would be another solution (and would simplify the wiring instead of adding to it). RCA-plug stereo connection is common, and commonly causes problems, in part because (unless one is clever) it creates ground loops.
It seems entirely possible and likely. It could easily result in 200 millivolts of AC ripple appearing on the audio signal. Since audio these days is usually 2 volts or so, peak-to-peak, you'd end up with a serious hum.
Common problem with cable installs, and the solution you've used is a common one.
What I'd suggest doing, is disconnect the DVR from all of your other A/V equipment, and reconnect it directly to the cable. Then, measure the AC voltage between its A/V ground (e.g. the shell of the RCA connectors for the audio-out) and the corresponding AC ground on something else in your A/V setup.
I strongly suspect you'll get roughly the same voltage rating.
DO NOT BOND IT TO NEUTRAL. Neutral, by definition, is a current-carrying wire, and it can be pulled several volts away from ground by voltage-drop in the wiring. The only place this isn't the case is back at the service panel, where neutral and ground are bonded together.
The only place you should bond grounds to, is other grounds (and ideally do so at a single point).
You don't need 3/0 to comply with NEC - 14/0 or heavier is apparently adequate.
There's no harm to doing so, it's a common solution, and since it fixed your problem, Be Happy!
Grounding the coax at the point of entry (per NEC) would provide you a bit of additional protection against something like a near-strike by lightning onto the cable wiring.
My recollection is that in most jurisdictions, cable-TV drops are supposed to have their braids bonded to building ground at the point of entry. A lot of installers skip this step, as there's often no "good" ground at the point of entry, and they don't want to run a heavy ground wire back to the panel and do a proper job of bonding at both ends.
National Electric Code, article 820, seems to be the relevant one. I don't have the full text here (my old salvaged copy of the NEC is at home) but the summary I see on-line does indicate that this is necessary:
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Sidebar: 820 Tips
Determine point of entrance.
Ground the incoming cable as close as practicable to the point of entrance.
If you run cables above a suspended ceiling, route and support them to allow access via ceiling panel removal.
If you use a separate grounding electrode, bond it to the power grounding system.
Use the correct cable type and raceway for application the general, plenum, or riser.
Unless you are using an indoor antenna, somewhere the cable from wherever i s grounded. It is obviously not grounded at the same place as the electrica l ground.
While newer electrical installs will have a ground rod stuck ten feet in th e ground, I think in most areas cable and phone installers can still use a cold water pipe because those are more for lightning than ground faults in appliances, like if an internal hot wire gets shaved and touches the metal body.
The cable or antenna ground is so that if lightning strikes it does not arc across your house and kill you from ten feet away. It doesn't really prote ct the equipment much either.
There are ground gradients, I had a similar problem with an electician's ho use. He has a large house, so large that he decided to use two ground rids. We sold himm a video projectot which at the time was the only piece of his system that had a three prog grounded plug. He comes back and says that as soon as he plugs in the cable he gets a bar running up the screen. This is called a hum bar and had the exact same cause as your audio hum, but when the video gets the hum you get a bar. Our solution was to find an antenna i solator from an old hot chassis type TV and adapt it to be F to F rather th an F to modified RCA. Finding such an isolator these days might not be so e asy since TVs all when to SMPS type supplies and thus need no isolation.
You can actually build one, in case you would rather have that isolation tr ansformer for other uses. If so, I'll look into the best/easiest way to do it, but if you are happy now and this is all academic you can just leave i t as it is. However, your equipment is no longer grounded, which should not be a problem.
In cases like this, chances are that the cable's nearest ground is either out at a street-side service pole, or a curb-side vault, or a neighbor's house. There's all kinds of opportunity for current loops between the cable ground, house ground, and neutral.
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shows a nice way to rebuild a standard cheap 75-to-300-ohm balun, to turn it into an isolated 75-to-75-ohm unun. The result is probably rather similar to what you'd find in a commercial 75-ohm coaxial groundbreaker.
(This very problem was the source of a discussion on the FMTuners mailing list last week, and that's where I cribbed the reference to this particular DIY project.)
In a really perfect world, every ground should go back to the single point ground.
The first part is to have Telco, electrical and cable tie together at only one place.
The telco protector is a potential source of problems, but it's on the telc o side of the NID and connected to the ground rod. Hum usually shows up in the land line.
Another issue is the cable. I should also be home run if from many locatio ns. A splitter here and a splitter there isn't very good because of potent ial loops.
Each 120 V branch circuit is already a loop, because it has multiple outlet grounds in parallel separated by some distance. If each outlet went all t he way back to the panel, the potential loop would not be there.
Some device along a circuit can potentially put noise on ground from say a switching power supply filter that's grounded. A fault can raise the poten tial of some device say near the middle of the loop from a lightning strike or ground at the end of the loop.
Ground is supposed to be low impedance and in theory nothing should happen.
Everything works really well when you have one phone, one device connected to the TV or the devices are clustered and you have a single point ground.
USB also uses ground as a common to the power supply. Switching power suppl lied that are grounded also put some amount of noise on the ground. The RC A phono jack is the consumer (single-ended) version of what's done commerci ally using differential signals and DIN connectors. there is a in phase a nd out of phase signal relative to ground. The wires are twisted to reduce EMI and shielded to reduce RFI.
Shields should actually only be connected at one end, but sometimes that is n't possible.
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