Attenuation Pad Cross Talk Mixing Board/Phone Install

[jeffwell]

Ok im kinda new to all of this so bare with me.

I am trying to hook up a mixing boards caller input and output up to a telephone system.

I decieced to try and make my life easy by using the option Headset output to get my Input and output from the phone system.

I made a pad to drop the mic 40 db to get it down to the right level for the phone.

---- R1 ------
| |
R2 R2
| |
------R1-------

I cant rember the resitors i used but when i test the output on my meters it looked right to be at -40db just what the phone wanted.

The problem is that when i hook up the mic line, i get really bad cross talk.
We tried puting Transformers on the speaker input (Caller In) and the mic input (Caller Out), with the padding, and still this really bad cross talk, an dyou could barely hear the mike it would kinda cut in and out.

However when i left the Mic off the speaker (Caller In) hooked up, and the origial mic from the phone handset being used, it works just fine.

We are going to try to hack right into the phone next but they are new and i want to try and save the warnty and such on them, so i was wondering if anyone had any ideas on how to fix this at all.

Could it be a problem using the headset jack should I try to splice the wires right into the phone .
Aslo how can i drop the level from 15khz down to the 3-5khz that phone would like, since i think this may be causing the cross talk.

jeff

[Da5id4Vz]

You’re having a problem with side-tone, the amount of the microphone that it is allowed to leak into the earpiece on a telephone. This is done to keep talkers from yelling while they talk on the phone.

In order to get better performance there are two things you should consider. Both involve giving up on your current telephone.

The first thing to try would be doing what you are doing, except use a speakerphone instead. These are typically designed to have much lower side-tone than does a normal telephone.

The transformers are an excellent idea. I know of one US network television program that did daily telephone calls for almost 10 years on a modified Sears speakerphone.

Don’t even think about trying this with a PBX. You’ll just give yourself more grief than its worth. Use a POTS line. If you need multilines a 1A2 key system can be had cheap on EBay or new even from Graybar. If you’ve got some dough, but a combination hybrid/key system.

Here is the second, ugly but practical option. Purchase a telephone hybrid made by Getner or Telos. They are both great companies with decades of experience in this specialty. Both were started by avid DIY hounds too. (Russ Getner and Steve Church) The boxes these guys make deliver excellent trans hybrid null performance. I bet they can be found on EBay kind of cheap too.

I think you have the mix minus thing worked out. This is the process of using an Aux buss on the mixer to feed the program audio back to the person on the other end of the phone line. The phone line is fed everything MINUS the audio coming from the telephone. Hence the name.

Being in Niagara Falls you might want to try contacting Bob Lovejoy at Unistage in buffalo. Id be surprised if he didn’t have a good unit in his rental stock. I expect that for the process of a sail, rental or even just good will he’d spend some time with you explaining and doing a hands on demonstration of the best way to set it up and what has and hasn’t worked for him. He's a good guy that way. Most of the buffalo broadcasters depend on him for such sage advice. Systems wireless in Maryland also stocks rental units, but then your paying for shipping too.

Oh yah one more thing, it sounds like your level differences are from putting the phone line into mike level inputs. 40-60 dB will be about the right range for attenuation to do this, but its much easier if your using a mixer with line level in and out.

Your aux buss out should ideally be capable of +4dB at 0 Vu to hit the phone line too.

Clear as Mud?
I doubt it.
I think I'm typing too fast.
I'll check back here and in Email later to see if I can answer any questions for better explanation.

-Dave

http://www.gentner.com
http://www.telos-systems.com
http://www.unistage.com

[Da5id4Vz]

FYI:

http://cgi.ebay.com/ws/eBayISAPI.dll...&category=3278

http://cgi.ebay.com/ws/eBayISAPI.dll...category=51279

[jeffwell]

Thanks for all the information dave your awesome.

Most of it is pretty clear we have a few things were going to try when we go back out to the site next and i am going to look at all the adive you gave too and hopefully get a new contact in buffalo if my employer does not already have him.

We have come across one thing that may also be causing the cross talk. The dam electrician seeemd to have crossed the local ground and normal ground together.
Since he mouteded a grounded stell outlet to the metal racks with all the equimpent in it, it is cause the two ground lines to cross.
we are hoping that this is causing the problem right now.

I will have to see..

I will let you know about my results once things are said and done so hopefully others wont come across the same problem.


Jeff

[Da5id4Vz]

You can avoid some of the grounding issues from the electrician by specifying an isolated ground receptacle. But that can cost 2-3x as much, mostly because they don’t have the parts on hand to do it.

I'll be a little surprised if that cures your headaches, but stranger things have happened.

Try theses Google searches:

"Mix Minus"
and
"Telephone Hybrid"


This is probably the best article I've seen on the subject:

http://www.telos-systems.com/?/techtalk/default.htm

-Dave
(Former broadcaster, gone over to the dark side… The Phone Company)