atcomsystems.ca/forum Forums Wiring - Telecom Carriers - Trunks & Services Telecom Service & Providers Strange hum problem has me at my wit's end... please help!

I have recently converted my garage into a home office and purchased a "Magic Jack" for my business phone service. The service has worked great with no problems at all on a single-line phone.

I purchased a two-line office phone (ATT 992) so I could receive calls on my home POTS line, as well. With both lines connected to the phone at the same time (via individual jacks), there is a severe hum condition (audible on my end only) that is so bad it prevents Caller-ID and New Call notification from working on both lines.

With just one of the lines connected alone, to either of the input jacks, there is no hum and Caller-ID works fine. This is true for both of the phone lines.

I did an exhaustive internet search for filter devices that might solve my problem, and came across the SNIX. I have been in contact with engineers at SNC Manufacturing -- makers of the SNIX -- and they tell me that their Super SNIX product will solve my problem. And that I will need two, one on each line.

I would be willing to give their suggestion a try if these units did not cost nearly $300 a piece! I am hoping someone out there somewhere might have a more economical suggestion for me. I am an EE and have good electronics skills, so I am not averse to cobbling something together from raw components to solve my problem.

Here are the voltage readings I took for the SNC engineers me:

Open CO circuit, tip-to-ring: 52.5VAC
CO ring-to-ground: 4.5VAC
CO tip-to-ground: 5.2VAC

Open VOIP circuit tip-to-ring: 46.0VAC
VOIP ring-to-ground: 2.0VAC
VOIP tip-to-ground: 45.0VAC

I'd appreciate any help anyone can give me. I am back to using two separate phones, and it doesn't really project the image that I know what I am doing! Thanks!

so your saying that two single line phones work fine but the two line phone doesn't ?


I would try a different two line phone , I m partial to Panasonic for applications like this

(what happens when you reverse the line inputs ?
magic jack to two pots to one )

The two-line phone works great, as long as only one of my two lines is connected. This goes for both input jacks.

When both of my lines are connected, regardless of which input they are connected to, I get the hum and loss of Caller-ID, etc.

Unfortunately, I do not have any other two-line scenario in which to test this phone. But since it works great with a single line connected, I am assuming the problem is not in the phone.

sounds like a poor assumption to me

(you know what they say about that word don't you ? )

Most of these phones have a 4wire jack for two lines or you can use two line cords for the two lines. Try two SINGLE line, line cords, two wire not 4 wire. I'm betting your picking up noise from the computer chassis.

Well, I have a couple of ideas worth looking into:

#1: You should be measuring your line voltages using a DC scale. 45-52 VDC across an open line is normal, ring to ground should be about the same and tip to ground should be minimal. In a perfect world, the tip-ground measurement should be as close to zero as possible. On POTS lines, the tip side is grounded at the CO, so any tip-ground voltages measured will be only minor induced AC influence.

#2: The PC that you are using for your Magic Jack must be grounded. I know, I know, most are. Bear in mind that if it's not, then the voltage from the Magic Jack is "floating", as in no reference potential to ground. I'm no computer whiz, but I'd like to think that all power supply outputs in a computer have a reference point to chassis (earth) ground.

A difference in potential to ground between the two "CO lines" that you are using may be the source of your problem. The common internal ringer circuitry in the telephone set relies upon basic, and VERY basic DC isolation since it's fairly common for both CO lines to have the same reference to ground. This is due to the fact that they typically originate within the same central office and ride over roughly the same length of cable. With two lines that don't originate in this manner, that could be the cause for your issue.

If all else fails, try isolating the computer's chassis ground using a 2/3 prong adapter to isolate the chassis ground by not connecting the grounding terminal on a temporary basis. This will at least help isolate the potential AC interference due to the local ground. Since the Magic Jack is powered by the USB port, there's a good chance that may very well be a floating voltage, causing excessive AC influence between the CO and the computer's power supplies not being bonded together.

Bear in mind that the Magic Jack was not intended to be used in any kind of sophisticated environment. The manufacturer's intention wasn't really much more than to connect a stand alone single-line set to it.

Typical two-line phones aren't sophisticated enough by design to take these issues into account.

lets not forget the wall wart and 9volt battery.

consumer grade , two line phones can be flaky

Occam's razor

"All other things being equal, the simplest solution is the best."

the simplest solution in troubleshooting this would be to swap the phone out


see if the same problem exists on two two line phones from separate manufacturer

I agree with trying another telephone. VTech's quality control has been suspect for years.

AT&T/Lucent branded telephones, manufactured by Philips Electronics and earlier, seem more reliable than the current VTech manufactured models.

Try purchasing another 992 and see if it fares any better.

Wow, thanks for all of the input! I have tried several things as a result:

1. One reply said that the tip side of my Verizon line is supposed to be at ground, but it isn't. So I tried jumping it directly to ground. This killed the hum and related problems when using line one of my two-line phone, but it also rendered the Verizon line unusable.

1a. There's a wire coming out of my NID with a bear end that is just hanging in mid-air. Is this supposed to be a ground wire???

2. I put ground lifters on my computer and all the devices connected to it. This only worsened the hum problem. It did lead me to find a loose ground in my breaker panel, though. My outlet grounds were reading 8 ohms to earth (water pipe). I fixed that, but it didn't reduce the hum.

3. The hum does not change when the wall transformer is disconnected (it powers the speakerphone, clock, and backlight).

4. I tried reversing polarity on each line, with no effect.

I agree that trying another phone or hooking this phone to two other lines would be the next logical step in troubleshooting the problem. I guess I am just cheap. I thought there might be something I could connect to the lines to filter the hum or correct the imbalance.

The SNC engineers have assured me their Super SNIX device will do the trick, but it is almost $300. "Cheap" and $300 don't mix.

I find it curious that no one has mentioned a filter on the line. Is anyone out there familiar with the SNIX? It contains a common-mode choke and a harmonic drainage reactor. I can wind a choke if someone could recommend some specs. But I have no idea what a harmonic drainage reactor is. I am sure I could make one if I did. Any ideas?

Thanks again, all!

You can get a single line one from them a lot cheaper than that, but that should be a last resort.

Per your earlier post it only happens when both lines are hooked up, so I'd say AC power induction is not your problem. You've got more trouble shooting to do.