Hum and Circulating Currents

[Retread]

I keep adding things to my home theater. The latest is a combo DVD/VHS box, which is connected to my TV cable. As soon as I added it, my VTF-3R started humming.

I note that the manual says to use the supplied "cheater plug" to make the hum go away. As an electrical engineer and former member of the Department of Defense Committee on Grounding, Bonding, and Shielding Standards, I view this as not an ideal solution. The reason for 3-wire systems is to reduce electrical shock potential. "Cheater plugs" subvert this protection.

Hum generally comes from a couple of sources.

One is wire reversals at one or more outlets, which results in AC currents circulating on the ground wire. This is a violation of the National Electric Code, is a hazard in itself, and should be corrected if possible.

A second source is grounding differences between the TV cable and the electrical system. The best solution to this problem is to put an isolation transformer in-line with the coax cable. There are several ways to do this. One is to make your own RF isolation transformer with toroids and wires. A second is to make an isolation transformer by putting two 75-Ohm to 300-Ohm transformers back to back. A third, which is my choice, is to buy a professional isolation transformer. An example of such a transformer is the Jensen VR-1FF, available over the Internet for $49 plus shipping.

[tdekany]

this was a very valuable post for me.I moved to a new apartment in July and have the worst humming sound as soon as I connect my dvd/vcr. - I guess video source. The only difference is that I use less expansive monster speaker wire. In my old place there was no humming of any kind. What is interesting is that the humming gets lower when the volume is in the 11:30 position. So I keep it there for Cable tv but with music/dvd it is way too loud. I'll try the product you suggested.
thanks!

[Retread]

[QUOTE]Originally posted by tdekany
[B]this was a very valuable post for me.I moved to a new apartment in July and have the worst humming sound as soon as I connect my dvd/vcr. - I guess video source.

Try this first. Disconnect the TV cable from the electronics altogether, but keep all the electronics connected. If the hum goes away, then it's likely conflict between the electrical and TV cable grounds.

[tdekany]

I have try that just now again, and awile back with Parasound - we spent like 3 hours trying all possible configurations. I called COX too and the technician did the same thing you suggested.
I have no idea what is going on. Do you?
Thanks for your help.

[Retread]

If completely disconnecting the TV cable didn't make the hum go away, then it's probably reversals in the electrical wiring. Sloppy electricians doing the original wiring. This would be virtually impossible to trace down and fix in an apartment complex. I suppose it could be bad electronics, but less likely. You are probably stuck going the cheater route unless you want to try the high-level connections to the sub instead of the single RCA plug connection.

Years ago, The U.S. Military spent huge amounts of money going around to communications sites finding and fixing wiring reversals. Substantial numbers of amps were found flowing where there should have been none.

[tdekany]

Then why didn't I have humming for years? It started in July.

Again, if I just connect a cd player to the preamp, or the tuner, there is no humming sound. Add my dvd player or the vcr and bingo!:confused: :confused: :mad: :eek:

[Retread]

This whole area is black magic. Here's how I would go about isolating the problem:

1. Unplug everything. In the following, do not connect the TV cable to anything.

2. Get a single 3-wire power strip with enough outlets. Also a few cheaters.

3. Isolate all electronics from each other and from any ground.

3. Connect the receiver and the sub, and plug both into the power strip.

4. One at a time, plug other electronics into the power strip and connect to the receiver, see if you have hum, and then disconnect from the receiver and unplug. Record the receiver/electronic device pairs that have hum. If one combo has hum, try cheatering the device, put it back, cheater the receiver, put it back, and cheater the sub. Record results. In this process, I'd avoid bridging myself between the metallic parts of any two devices, or between any one device and ground.

5. If no single combo has hum, then try the receiver/sub with all pairs of electronics. Again, record results and try cheatering in various combinations. If the problem is not associated with the TV cable, I suspect you'll have found the combination causing it by this time. It will be important to make a matrix of combinations ahead of time and carefully check off the combinations, or you'll lose track.

Your problem may have multiple sources. You may have wire reversals in the apartment wiring. You may have conflicts between the apartment ground and the TV ground. You may have a problem with some particular piece of electronics. Or you may have all three.

When the simple approach fails, there's always brute-force. They make AC power isolation transformers. One of those plus a TV cable isolation transformer should remedy all the issues except possibly bad electronics.