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Old 15th April 2004, 01:53 AM   #1
oog is offline oog  United States
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Default Telephone Hybrid help..

Anyone know where I can get some info about designing a telephone hybrid interface? I would like to use National Semiconductor's TP3071 to deal with elimination of crosstalk on the line, and I think it could handle gain as well...

Here's a link for info on that chip: http://www.national.com/ds/TP/TP3071.pdf

Anyway, I really don't have a clue about designing circuits when it comes to anything much more complex than powering a light bulb. I can build something from a schematic, but designing one is beyond me. I know there are units like this available to buy, but there is something to be said about building one yourself, not to mention the undoubtedly large savings of cash spent!

Thanks for any help you can provide...

oh, also where would I be able to buy those chips? Looking at the National website it looks like I could only get 1000 or more from them, that would be serious overkill!
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Old 15th April 2004, 04:13 PM   #2
hermanv is offline hermanv  United States
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Telephone hybrid design is pretty speciallized. The problems include high voltage isolation (lightning strikes on the phone wires) immunity to ringing voltage noramlly 90 VRMS at 20 Hz. Conducting holding current (usually 20 mA and up) and driving voice signals into and out of a single pair of wires.

You must also match telephone company AC impedance and DC current rules in order not to upset their automatic line quality test equipment.

Given those issues, two comon techniques are in use.

1. Use a SLIC (Subscriber Line Interface Circuit) These are ICs made by people such as TI, Siemens, Agere, Intersil etc. See their data sheets for application circuits. Be aware that all these will need 24V or 48 VDC power.

2. Use a transformer. The line side will need to be DC blocked - traditionally with a 2.15 uF capacitor. The other side should be center tapped to allow for the hybrid function. Complex networks will be needed to terminate the transformer and to provide the hybrid loss function.

Its a big deal. I've made a living building these and can help if you are determined to proceed. Probably the forum will not realy care about details but if you e-mail me we can continue the discussion.
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Old 15th April 2004, 04:15 PM   #3
oog is offline oog  United States
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I appreciate the reply.. perhaps at some point down the road I would like to persue this further, but I just stumbled on a REALLY good deal on a commercial unit, so for the time being that should suffice.
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Old 15th April 2004, 09:36 PM   #4
oog is offline oog  United States
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Well.. then again, maybe not.. some bid sniper booted me out of the dutch auction at the last minute.. right before I got home too.. that sucks
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Old 15th April 2004, 10:59 PM   #5
hermanv is offline hermanv  United States
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Too bad.

I remember that HAM radio operators use a device called a phone patch. This usually has audio input and output jacks. If you know of a HAM site see if you could pick up a used one.

All Modems have this kind of circuit. Maybe you can find an external modem - it will have a box, a phone jack, a power supply and 90% of what you would need.

If all else fails it can be built. I have designed probably 30 telephony interfaces as a EE, if I wanted this function I would spend considerable energy trying to find a used one 'cause it's a significant task if you want to do it right.

If you search on Google with "telephone SLIC" you will be able to get to some data sheets to help map the size of problem.

I usually check my e-mails every third day or so so if you send to me be a little patient expecting a reply.
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Old 16th April 2004, 04:51 PM   #6
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I posted some links and thoughts about hybrids in this thread:

http://www.diyaudio.com/forums/showt...711#post348711
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