Rich, Thanks a bunch, totally understand now and that actually makes total sense. (no need for schematics). This also explains why I used an TA750 to go into a Nortel MICS system, using FXO and no buzz. Totally balanced load from the analog ports on the Nortel across the 5 feet of CAT5 to the FXO on the adtran.
Now I need to get rid of some Adtrans --- Anyone lookin to buy? :) Thanks again.............Greg Gregory P. Scasny Golden Technologies Inc. http://www.golden-tech.com 219-462-7200 -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Rich Adamson Sent: Thursday, April 22, 2004 6:03 PM To: [EMAIL PROTECTED] Cc: [EMAIL PROTECTED] Subject: [SPAM] - Re: [Asterisk-Users] Adtran TA750 Noise - Email found in subject > I have an (actually 2) Adtran TA750s with 8 FXO ports. I get a terrible buzz on every FXO port. If I unplug the Adtran and put an analog phone > on each incoming line, I have no buzz. > > I also have 2 Carrier Access Access Bank Is with 12 FXO ports. When I plug the same analog lines into either one of those, no noise or buzz > whatsoever. > > > > I went so far as to move the TA750 to within 5 feet of the demark and ran a short CAT5 cable from the demark to the TA705 and still lots of buzz. > > > > I have duplicates of every part for the TA750s and swapped every component and cannot get rid of the hum on either unit. I have the power > supply of the Adtran grounded. I am out of ideas L. > > Any assistance would be greatly appreciated. Well, you're the second one in a rather short period of time that has complained about the exact same hum/buzz when the Adtran 750 is used with FXO interfaces and with X100P cards. There was another posting earlier today in which the individual made a comment the 750 FXO interface does not support impedence matching. I thought the statement was rather strange, but since I don't own one of these units, I went to the Adtran site in search of a technical description of the FXO card. I couldn't find one, and for that matter, it appears Adtran has little reference to the 750 being used with any FXO interface. (Its almost like they know there is a problem and removed the 750 FXO options. Selling as FXS only now.) >From what I'm hearing/understanding, its all beginning to make sense (believe or not). If the no-impedence-matching is true (or even if the technical words are slightly/somewhat incorrect), then its beginning to appear the Adtran FXO interface is not presenting a "balanced" interface to the tip & ring pstn line. In other words, one side of the line must have some internal electronics hanging on it that disturbs the balance needed for pstn lines, and that imbalance is causing induced AC power (which is extremely common on most pstn lines) to be heard. This is going to be rather difficult to explain without a drawing, but I'll give it a try. The pstn line (all the way from the CO or fiber mux cabinet) is nothing more then twisted copper pairs, that have a very specific number of twists per unit of length. The twists are actually built into the cables to ensure that whatever outside electrical influence exists (such as AC Power), that outside source influences both tip and ring in "exactly" the same amount. At the end of that cable (whether its in your house or business) if you attached a perfectly balanced piece of equipment, it doesn't make any difference whether that outside influence (in this case, AC power) is ten volts or fifty volts, that influence is cancelled out and not heard. But, its because the attached device (usually an analog phone) presents an equal load to both the tip and ring. (That should be fairly obvious since the typical analog phone doesn't have any real way to create an imbalance since it doesn't have access to ground or AC power. For the real technical types, its the differential voltage between tip and ring that creates the sound.) If one would connect an analog phone to the pstn line that you're having the hum on, and then attach a resister from one side of the line to ground (say, from the tip to ground), you are artifically creating the imbalance that I'm talking about. The analog phone will now have the hum that you're hearing via the Adtran & asterisk because of the imbalanced line. The size of the resister (whether 100 ohms or 1,000,000 ohms) will impact the loudness of the hum; the smaller the value the louder the hum. In the olden days of telephony, we use to install "repeat coils" to isolate the imbalanced equipment (usually customer owned stuff). (Here comes the harder part to describe in words. Really need a visual schematic for this.) Repeat coils were absolutely nothing more then a basic audio transformer with two primary windings and two secondary windings. A couple of 2 ufd capacitors and the repeat coil was all that was needed to isolate the imbalanced piece of equipment from the pstn line, pass the DC component needed for supervision, and elimate the hum. In the '60s and '70's, those repeat coils were almost the size of a beer can, and one use to buy them from Western Electric (and others) mounted on rails that fit a 19" or 23" rack, along with the capacitors on another set of rails. In very unusual cases of imbalance, we use to purchase the same set of repeat coils and capacitors implemented in a box with switches on the front panel. The switches would allow the technician to add resistive values onto one side of the line or the other, in an attempt to "match" the imperfections created by water logged cables, etc. If a cable pair had a problem that indicated 10,000 ohms of resistence to ground on the tip side of the line, this box would attempt to add 10,000 ohms to the ring side of the line to "balance" out the problem and reduce or eliminate the hum. Since a number of the cheap analog phones today have the same problem with the electronics needed to sense ringing, DC power, etc, its a fair bet the old repeat coils exist in even the cheap phones in one form or another. Removing them and testing the theory shouldn't be all that difficult. If anyone with this type of problem has actually read this far, send me an email off list and I'll put together a schematic of what is needed to try this to reduce/eliminate the hum. Keep in mind we're all talking about the problem via email and I could be somewhat off base. Several months ago bkw was rather strongly stating that reversing the tip and ring on an x100p card fixed his echo problem. Assuming that was truly the fact, then the x100p card is also very likely to be presenting an imbalanced interface to the attached pstn line and the repeat coil approach is likely to address some of the x100p problems as well. (off to find out...) Rich [EMAIL PROTECTED] _______________________________________________ Asterisk-Users mailing list [EMAIL PROTECTED] http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users _______________________________________________ Asterisk-Users mailing list [EMAIL PROTECTED] http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
