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Author Topic: Audio over telephone dry-pair  (Read 10978 times)

Mac Kerr

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Re: Audio over telephone dry-pair
« Reply #10 on: January 19, 2014, 10:15:05 am »

Commonly available passive Di boxes will work well for this application.

Have the low Z "facing" the copper line and the high Z to the equipment.

In addition to the problem Lee mentioned, many passive DIs reduce the level on the balanced side to match XLR mic inputs on consoles. You want to maintain the high level signal through the unshielded twisted pair. If the source equipment is a good balanced line driver there is no need for a transformer on the output, but the destination end should have a good input transformer on it. This will maximize the CMRR and provide electrical isolation. Transformers by their nature are perfectly balanced, while a non-transformer balanced input may not be, reducing the CMRR.

There are specific transformers designed to be input transformers, and ones designed as output transformers. For the best results they are not interchangeable.

For a one time gig you will probably be fine with any good quality 1:1 isolation transformer, if this is a circumstance that will happen often it may be worth buying the right transformer.

Mac
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Chris Clark

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Re: Audio over telephone dry-pair
« Reply #11 on: January 19, 2014, 11:49:35 am »

The outputs of the MX32 are listed as servo-balanced. (Is this the same thing as "electronic balancing"?) The existing amp (currently running off a local CD player, which is what we are looking to eliminate) is an older ComTech (honestly I don't remember if it is a x00 or x10 series, and won't have a chance to look for a few weeks but I don't think it makes a huge difference) using the basic terminal block PIP which if memory serves correctly is simply electronically balanced.

So if I were to get an Input transformer it would go closer to the amplifier, and if it was an output transformer it would go closer to the digitool, am I understanding that correctly?

I'd have to double check inventories, but I may have a Biamp CMA kicking around, these have a switch on input 1 for "600 ohm telephone" - would this serve the purpose I'm looking for with transformer isolation? If not I'll keep researching for isolation transformers.
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Ivan Beaver

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Re: Audio over telephone dry-pair
« Reply #12 on: January 19, 2014, 01:57:56 pm »

Just to add to what others have said.

The freq response of a typical telephone connection is limited to 300-3Khz.

This is NOT because of the wire-but because of the equipment on either end.  This was done to keep the bandwidth within the parameters of the mic and ear pieces and works fine.

So as long as you dealing with just wire and no telco gear you will be fine.

Use line level-NOT normal instrument level- Di boxes. and I would make sure they are 1:1 so you get the maximum voltage transfer.  Of course most line level boxes are 1:1-since they are used for isolation and not so concerned with impedance loading.
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Tim Perry

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Re: Audio over telephone dry-pair
« Reply #13 on: January 19, 2014, 02:06:17 pm »

The outputs of the MX32 are listed as servo-balanced. (Is this the same thing as "electronic balancing"?) The existing amp (currently running off a local CD player, which is what we are looking to eliminate) is an older ComTech (honestly I don't remember if it is a x00 or x10 series, and won't have a chance to look for a few weeks but I don't think it makes a huge difference) using the basic terminal block PIP which if memory serves correctly is simply electronically balanced.

Servo balanced is a method of keeping levels the same when the negative side is grounded (when using a 2 conductor plug). 

A transformer at each end will give you your best chance for  hum free and RFI free operation.

Quote

So if I were to get an Input transformer it would go closer to the amplifier, and if it was an output transformer it would go closer to the digitool, am I understanding that correctly?

This is nit-picking. you will have a difficult time sourcing anything marketed as an "input transformer"  for this application. 


Quote

I'd have to double check inventories, but I may have a Biamp CMA kicking around, these have a switch on input 1 for "600 ohm telephone" - would this serve the purpose I'm looking for with transformer isolation? If not I'll keep researching for isolation transformers.

When using 600 ohm systems terminating it with a 600 ohm load  helps with impedance matching. see: http://en.wikipedia.org/wiki/Impedance_matching.  In this case you are bridging a very low Z output... probably 50 or 100 ohms (whatever series resistor is used for protection in the unit)

Personally I'd probably use Barix ehernet solution http://www.barix.com/ or Studiohub http://www.studiohub.com/matchjackDAs.php

Contact me if you want WE11C repeat coils. I have some I can spare.

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Lee Douglas

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Re: Audio over telephone dry-pair
« Reply #14 on: January 19, 2014, 02:12:23 pm »

It keeps nagging at me every time I come across this thread, so I gotta ask; I keep thinking in my mind that this is older wire, although the OP says nothing about it.  If this were old 100 pr cat 3 or less, with a lot less of a twist to each pair compared to new cat 5/6, wouldn't that make a huge difference in isolation and cross talk via induction given that this would be a shared with an existing operating telco line?
« Last Edit: January 19, 2014, 04:22:01 pm by Lee Douglas »
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Mac Kerr

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Re: Audio over telephone dry-pair
« Reply #15 on: January 19, 2014, 02:56:24 pm »

A transformer at each end will give you your best chance for  hum free and RFI free operation.

The CMRR noise rejection happens at the input. Servo balanced, impedance balanced or transformer balanced at the output will all have the same CMRR with a transformer input

Quote
This is nit-picking. you will have a difficult time sourcing anything marketed as an "input transformer"  for this application.

HEREHEREHERE, and HERE for a start.

Quote
When using 600 ohm systems terminating it with a 600 ohm load  helps with impedance matching. see: http://en.wikipedia.org/wiki/Impedance_matching.  In this case you are bridging a very low Z output... probably 50 or 100 ohms (whatever series resistor is used for protection in the unit)

Most modern audio gear is low output impedance and relatively high input impedance (bridging), there is no real need to match impedance.

Mac
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Jerome Malsack

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Re: Audio over telephone dry-pair
« Reply #16 on: January 19, 2014, 04:20:13 pm »

Another question in the problem is the wiring direct building to building, or to the telco exchange? 

We had to special request to telco to make pairs building to building in the Past. 

If it goes through an exchange would the frequency response be limited to Phone requirements,
say 300 to 12K ???
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Chris Clark

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Re: Audio over telephone dry-pair
« Reply #17 on: January 19, 2014, 05:20:54 pm »

No we are not requesting a dry-pair from the local exchange. The infrastructure this would be on is all internal, a pair cross-connected from the source to destination and not patched into the PBX and, as far as the local telco is concerned, basically non-existant. The concern I had was what equipment would be necessary to reliably drive the ~1500ft twisted pair (yes, it is probably CAT3).

Because I'm rather limited on where I can order from (setting up a purchasing account sometimes takes an act of congress around here...) one of the companies we have an account with has a Rane LT22 available, seems to be one of the few isolation units I can find through any of them. Would a pair of these (one at each end) be sufficient, do you think?
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Ivan Beaver

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Re: Audio over telephone dry-pair
« Reply #18 on: January 19, 2014, 05:21:35 pm »

Another question in the problem is the wiring direct building to building, or to the telco exchange? 

We had to special request to telco to make pairs building to building in the Past. 

If it goes through an exchange would the frequency response be limited to Phone requirements,
say 300 to 12K ???
Back when I was doing telephone work-the response was limited to 300-3K.  Maybe it has changed since then.  That was 30+ years ago.
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Ivan Beaver
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Ivan Beaver

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Re: Audio over telephone dry-pair
« Reply #19 on: January 19, 2014, 05:24:48 pm »

. The concern I had was what equipment would be necessary to reliably drive the ~1500ft twisted pair (yes, it is probably CAT3).


1500 ft is nothing.  Until you get past 4000 feet, I would not worry about it.

You will not have enough loss to worry about-especially at the impedances of modern gear.

Speaker lines would be a different issue.
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Ivan Beaver
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