Telephone Line Amplifier

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brandonl
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Telephone Line Amplifier

Post by brandonl » Tue May 19, 2009 2:34 pm

Hi All,

I am not familiar with telephone circuits so maybe someone here can give me some advice or suggest some reading but here is my problem.
We have phone cards that we install inside elevator cars, when we have multiple cars these cards connect to a call director, which shares the incoming phone line (ma bell). When someone pushes the button in the car the call director detects the phone card going off hook and dials the programmed phone number, and then acts as a pass through (audio and dtmf). This works well... most of the time. Sometimes the audio is so low that you can not hear the person on the other end very well or they cant hear you, and i have seen the card not able to respond to dtmf commands as well. This usually involves trying different configurations of grounding the shielded cables, (sometimes helping) and usually it's chalked up as a noisy or bad line.

What i'd like to be able to try is designing a amplifier to put between the phone line and call director. The amplifier would ideally be adjustable gain and possibly have some bandpass stages to amplify voice and dtmf. It should also be able to handle the ring as it is possible and sometimes required to call the call director.

Don't know if this is possible but any input would be appreciated.

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Externet
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Re: Telephone Line Amplifier

Post by Externet » Tue May 19, 2009 3:19 pm

Hi.
Not familiar with your specific circuitry, and unsure of what you mean by "handling the ring" but try:

Code: Select all

L1-------------------------------------------------------------L1
L2--------------------------uuu--------------------------------L2
                           nnnnnn
uuu = 8 ohm 'secondary' of any audio output transformer (in series to one wire)
nnnnnn = 'primary' of such transformer
L1 and L2 are your telephone wires

Connect the amplifier input to the nnnnnn secondary; which can be a LM386 IC; its data sheet gives you how to wire it.
Connect two counterparallel 1N4007 diodes across the nnnnn winding too.

It will pass ring signals, will not load the line, will isolate the power from your amplification circuit and the line power; will keep floating signals as they are.

Come back if hurdles show up.
Miguel

Edited---> added : If the intention is not for monitoring but for increasing the audio level in the telephone line; try replacing the microphones in the cars first, they may be weak/bad.
- Abolish the deciBel ! -

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haklesup
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Re: Telephone Line Amplifier

Post by haklesup » Tue May 19, 2009 4:20 pm

If it is caused by a noisy line, adding an amplifier could just amplify the noise as well.

From your description, it sounds (pardon the pun) that it works some times. More likely you have variable attenuation due to an inconsistant interconnect. You never mentioned wireless so I assume there is some sort of conductor that goes from the elevator car to the call director. Unless this is a long cable that follows the car and is soldered firmly at both ends then there must be some sort of sliding contact. This would be a good place to look for extra resistance in the line. My experience is that a phone can accept a significant amount of noise but is quite sensitive to poor resistive connections.

Bottom line is, you may be better off doing more diagnostics than just building an amplifier. Normally when we think of a phone amplifier, we think of the ear piece on the reciever end but you want to boost the signal in the POTS (plain old telephone service) line and thats a little different. Off the top of my head I don't have a recommendation but when I do, I'll repost.

There are many books detailing how telephones work. Try a Google book search for something in the technical range you need and can understand. Google gives a substantial preview of the actual book so you may find what you need to know without actually buying the book.

Ultimately the solution may be higher quality cable and soldered instead of crimped connections. Not knowing the details of the wiring and connects, I could only guess though.

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Re: Telephone Line Amplifier

Post by brandonl » Tue May 19, 2009 5:59 pm

thanks for the quick replays.

externet - what I was looking for was to amplify the signals, not monitor them. I don't believe the problem is with the microphone for two reasons:

1 - we have tried on occasion swapping out the board in the car as well as swapping the call director board with no change.
2 - The two way communication is quiet both ways.

What i meant by "handling the ring" was the amplifier was too boost the audio and dtmf but not the ring. (never had a problem with the system detecting the ring)

haklsup -
Sorry let me explain the wiring a little better. The call director is normally mounted in the elevator machine room. The incoming
phone line connects to this. There are 8 output ports on the call director and a local service port( a regular touch tone can be hooked up here for programming) a shielded twisted pair is run from each output port (1 per elevator) to the elevator controller(same room) and connected to a terminal strip (waco connector). A twisted shielded pair from the elevator travelling cable is connected to the same terminal. At the car end the wires are connected to the phone board screw terminal. The shield is grounded at the call director and carried through but not grounded at the car. This is what the manufacturer of the call director suggests. The quality of the cable i believe is good, and is not really an option. Once the phone system is hooked up and we find we have a problem, it is not cost effective to replace the whole travelling cable. ( All the wiring for the car is contained in the travelling cables).

The noisy line is what the engineer from the manufacturer said was the problem, I personally don't buy it. Besides these things are marketed towards elevators, which can be a noisy environment, and it should still function.

I was hoping i could boost the audio and dtmf somehow.

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Lenp
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Re: Telephone Line Amplifier

Post by Lenp » Wed May 20, 2009 5:56 am

Brandoni
What you are referring to, in telephone parlance, is a repeater. The telco line is bidirectional in that the audio travels both ways on the same cable. An amplifier is directional, input to output. A repeater separates the audio into two channels. Some older telephone engineering manuals may refer to this as East and West, or four wire. After amplifying, attenuating, or other conditioning each signal it combines them back again to 2 wires. In this manner, the gain and quality of each direction signal can be adjusted. The ringing signal, a low frequency AC at about 90 volts is applied across the line and is either routed around the repeater or regenerated and applied to the 'called' side of the line while the calling side gets an audible ring tone. All the repeaters I have seen are designed for central office installations and not suitable, or readily available, for outside plant use.

Is the shielded cable recommended by the manufacturer? Twisted pair is the norm for telco use but shielded is rare. Do the basic tests to see what the central office line looks like. There may be low or high voltage, leakage, high loop current or noise that affects the system. There is usually a local telco number that will transmit a 1Khz tone at the 0dbm level (0dbm=600mw/600Ohm or .775V/600 Ohm) as a reference. If you know or can get that number, make a test with and without the call director in the circuit to see if it causing any losses. If you can't get the number, set up a tone generator on another line, outside of the same exchange, and us it as a reference. If the outside cable system is old, conditions can change with weather and temperature so you may want to do several tests. If you do a lot of this work consider getting a 'Sidekick'. It's a specialized telephone instrument that simplifies much of the testing. They are generally available on ebay.

Because the telco line is a balanced pair, adding anything to one side of the line can cause an imbalance and noise is the result. While a transformer in series with one wire will indeed couple the audio off to an external amplifier for monitoring, two identical transformers, one in each line with the secondaries properly connected is a better approach to maintain line balance.

There also is a special line sense relay that is available. It is a reed relay with two coils that are placed in series with the telco line. When a station goes off-hook, the relay operates. Again, a balanced arrangement.

Here's a place to go! http://www.sandman.com/ There is a wealth of information on troubleshooting problems as well as tools and accessories.

Hope this helps..

Len
Len

“To invent, you need a good imagination and a big pile of junk.” (T. Edison)
"I must be on the way to success since I already have the junk". (Me)

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haklesup
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Re: Telephone Line Amplifier

Post by haklesup » Wed May 20, 2009 8:51 am

I might suggest this experiment. Connect an ohmmeter to the cable near the output of the call director, short the other end of the twisted pair at the phone card and note the resistance. Move on up the cable to the next interconnection point and short that and measure again until you have numbers for each wire segment and interconnect. Repeat this for each elevator car. If it is a resistance problem, you will see variance between cars that is more than a few ohms. Watching the ohmmeter when you manipulate the interconnects will also show where bad connections are.

You couls also try connecting a phone card close to the director with a short wire and proove that it is something in between.

It may be possible to modify the phone cards to transmit a stronger signal but making the director more sensitive is probably going to introduce more problems. Likewise the call director may also be able to be boosted.

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Dave Dixon
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Re: Telephone Line Amplifier

Post by Dave Dixon » Thu May 21, 2009 4:53 am

I just checked out the sandman site! That link at the bottom of the page for the "kids taseler" is something else :grin: Thanks for the laugh!

brandonl
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Re: Telephone Line Amplifier

Post by brandonl » Fri May 22, 2009 9:44 am

Thanks for all the replies. It may be a while before
I'm back at a sight with phone problems
but I'll have a lot more things to check, and I'll
Update the post with my findings.

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frhrwa
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Re: Telephone Line Amplifier

Post by frhrwa » Wed May 27, 2009 2:01 pm

the old days, elevator folks used the old R-G-Y-Bk.. no twist.. bad for everything.. make sure that's not what you have.. there are some good shielded cables available now, and you might even find something in the CAT 5 category.. that would be really good.. Never had problems with elevators I had to install phones in, other than the old 4 wire crap.. and we just replaced it with CAT5 at that time.. took care of all noise interference from the elevator motors..
JESUS”…… don’t leave EARTH without HIM!

jillmason7

Re: Telephone Line Amplifier

Post by jillmason7 » Fri Oct 22, 2010 11:41 am

haklesup-
Voip Phone SystemvoipTop Voip Provider
thanks for your response that solved a similar issue I was having.

-Jim

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