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Author Topic: bypassing old intercom system (45ohm + 150ohm resistors)  (Read 9635 times)

Tom Bourke

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bypassing old intercom system (45ohm + 150ohm resistors)
« on: April 20, 2013, 10:05:06 pm »

I am working on bypassing a 12+ year old simplex intercom system.  The system has had problems since day one and has been dead for several years.  This is an add on to a school and now they NEED paging to work so they can do armed intruder drills and have paging work in all areas of the building.

The signal is daily announcements and class change tones. I spent today tracing wires and toning out the system.  I also found the source of the original problems.  The system uses a 66 punch down for the runs to each room.  The wire was 22 awg shielded.  They punched down the shields and negative wire together in such a way as to create intermittent shorts with the hot wire to each speaker.  My plan is to re-terminate the head end using a better terminal block and parallel each wire to an amp.  The real problem is each speaker is 45 ohm with a 150 ohm resistor in line.  Total impedance per run is between 195 and 217 ohms @1KHz depending on wire run.  17 total runs.  By my calculation that would be 10 to 12 ohms with all runs in parallel.

Short term (week or 2) I plan on brute forcing it with a QSC PLX3402.  Long term I need to offer an appropriate solution.  From a labor and material cost standpoint the least expensive is a low impedance amp in the 500ish watt range (sorry too tired right now to do the math.)  I am thinking the proper solution is upgrading each speaker with a line matching transformer.  Is this really necessary? Also, what to do with the shield wire?  It is not connected at the speaker end.  I normally don't do shielded for speaker runs and any time I have run into them the system had problems.  Should I ground the shield at the amp end or just cut it back and keep it out of contact with other stuff?
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Hal Bissinger/COMSYSTEC

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Re: bypassing old intercom system (45ohm + 150ohm resistors)
« Reply #1 on: April 21, 2013, 02:29:34 am »

The only reason the wiring is shielded is because this is (was) a talkback system. When in intercom mode to a room, the speaker is used as a microphone. So if you are only doing paging you can cut it off.
 
As for your amp and driving all speakers at once, the amount of voltage it can supply is what is going to determine the wattage delivered to each speaker. With that 150 ohm resistor in series with the 45 ohm speaker you have roughly a 3:1 divider so by my math you are going to need about 42 volts to deliver 1 watt to the speakers. So your QSC should do it especially in 8 ohm bridged mono. But as you can see this is a very inefficient setup. With only 17 speakers it shouldn't be too expensive to change the speakers out to ones with 70v line transformers and use a small 70v amp.
 
-Hal
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Brad Weber

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Re: bypassing old intercom system (45ohm + 150ohm resistors)
« Reply #2 on: April 21, 2013, 10:01:05 am »

This is an add on to a school and now they NEED paging to work so they can do armed intruder drills and have paging work in all areas of the building.
Emergency Announce Systems can be quite different than your typical paging or general announcement systems and can have requirements regarding intelligibility, monitoring and so on along with significant associated liability.  If they are really envisioning this as an emergency announcement system then you probably need to start from scratch with an appropriate system.
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Tom Bourke

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Re: bypassing old intercom system (45ohm + 150ohm resistors)
« Reply #3 on: April 21, 2013, 11:01:17 am »

The only reason the wiring is shielded is because this is (was) a talkback system. When in intercom mode to a room, the speaker is used as a microphone. So if you are only doing paging you can cut it off.
I don't think the system was ever used in talk back mode.  There are no controls for such.  This kind of system is in lots of local buildings about the same age.  Usually massive over kill for the intended use.  Also the wiring runs are a bit sloppy.  I think this is a case of an architect rubber stamping in a generic intercom, maybe with a kick back.  The installers just slapped it in with no regard to reliability.
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duane massey

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Re: bypassing old intercom system (45ohm + 150ohm resistors)
« Reply #4 on: April 21, 2013, 12:06:41 pm »

+1 on replacing the speakers and just installing a simple 70v mixer/amp. Atlas or similar speakers, Toa or similar amp, walk away happy and secure. In most installs of this type the labor to install the wire is more costly than the actual gear, and the labor to piece together the existing gear to (kinda) work may prove to be an exercise in futility. Unless they need individual paging or intercom talkback, a simple solution may be the most cost-effective and dependable choice.
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Duane Massey
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Tom Bourke

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Re: bypassing old intercom system (45ohm + 150ohm resistors)
« Reply #5 on: April 21, 2013, 01:29:06 pm »

After doing a bit more calculating I think I have a plan.  The speakers them selves and wires are OK.  A 70v transformer at tapped at 10 watts will put just under 2 watts into a 45 ohm driver.  So re wire with a 10 watt transformer such as http://www.parts-express.com/pe/showdetl.cfm?partnumber=300-040 or http://www.parts-express.com/pe/showdetl.cfm?partnumber=300-039 wired for 15 watt for 3 watt @ 45 ohm. Driven by a 60 watt 70V amp.

Removing the wire nuts and sloppy punch down will go along way towards getting the system reliable.
I would have to do some more measuring to see if the speakers are a standard hole pattern.  They are in one of those clock/speaker combo units.
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Tom Bourke

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Re: bypassing old intercom system (45ohm + 150ohm resistors)
« Reply #6 on: April 21, 2013, 06:43:20 pm »

The remaining concern is the high capacitance of the shielded wire.  I suspect there is several thousand feet of wire all told.  This would be a concern regardless of the amp and speaker combo.
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I have a mild form of Dyslexia that affects my ability to spell.  I do use spell checking to help but it does not always work.  My form of Dyslexia does not affect my reading.  Dyslexics of the world untie! <a href="http://www.cwalv.com" target="_blank">http://www.cwalv.com</a>

Hal Bissinger/COMSYSTEC

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Re: bypassing old intercom system (45ohm + 150ohm resistors)
« Reply #7 on: April 21, 2013, 10:42:13 pm »

I don't think the capacitance of the cable will be a problem. As for hanging transformers on the existing speakers, my feeling is that it's just as easy to do it right and replace the speakers with new ones complete with transformers mounted on them.
 
Emergency Announce Systems can be quite different than your typical paging or general announcement systems and can have requirements regarding intelligibility, monitoring and so on along with significant associated liability.  If they are really envisioning this as an emergency announcement system then you probably need to start from scratch with an appropriate system.

I also think you need to think about what Brad is saying. Is a Life-Safety or emergency announcement system required and they are thinking that the old PA system will suffice?
 
-Hal
 
 
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Tom Bourke

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Re: bypassing old intercom system (45ohm + 150ohm resistors)
« Reply #8 on: April 21, 2013, 10:57:18 pm »

I don't think the capacitance of the cable will be a problem. As for hanging transformers on the existing speakers, my feeling is that it's just as easy to do it right and replace the speakers with new ones complete with transformers mounted on them.
 
I also think you need to think about what Brad is saying. Is a Life-Safety or emergency announcement system required and they are thinking that the old PA system will suffice?
 
-Hal

I have decided to quote it with replacement drivers that include transformers.  From a labor stand point it is less work if the drivers are a standard size. Existing drivers are Valcom 8C5W45 and I plan on using a "standard sized" 8" driver (300-020 from Parts express.)

As for the life safety I will leave that choice to the administrators at the school.  This is separate from the fire alarm system.  It is also just a small subset of the whole building.  The original building has a 30 or 40 year old Bogen intercom system that works very well.  None of the systems are even on a UPS.  A point I intend on bringing to their attention.

The NEED it to work comment was more directed at this system being dead for years and them dragging their feet for months on moving forward with anything.  All of a sudden the school board wants a drill they can't do with out this system working. For several years they have used a runner or phone call to tell this part of the building about such problems. :(
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I have a mild form of Dyslexia that affects my ability to spell.  I do use spell checking to help but it does not always work.  My form of Dyslexia does not affect my reading.  Dyslexics of the world untie! <a href="http://www.cwalv.com" target="_blank">http://www.cwalv.com</a>

Brad Weber

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Re: bypassing old intercom system (45ohm + 150ohm resistors)
« Reply #9 on: April 22, 2013, 05:45:38 am »

As for the life safety I will leave that choice to the administrators at the school.  This is separate from the fire alarm system.  It is also just a small subset of the whole building.  The original building has a 30 or 40 year old Bogen intercom system that works very well.  None of the systems are even on a UPS.  A point I intend on bringing to their attention.

The NEED it to work comment was more directed at this system being dead for years and them dragging their feet for months on moving forward with anything.  All of a sudden the school board wants a drill they can't do with out this system working. For several years they have used a runner or phone call to tell this part of the building about such problems. :(
Nor to belabor the point but they are hiring you as a professional and in that role you have responsibility for due diligence and properly addressing the situation.  Mass Notification Systems (MNS) and/or Emergency Communication Systems (ECS) are addressed in NFPA, which may be part of local building code.  Given that there apparently has been some indication that the intended use could be interpreted as a MNS/ECS, that could imply that not only does the system need to be code compliant for that purpose but also that there is significant potential liability involved.  To protect yourself and possibly others, I suggest either making sure that what you provide is code compliant for that purpose or getting written documentation from your Client stating that the system is not intended to be used as a MNS or ECS.
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Re: bypassing old intercom system (45ohm + 150ohm resistors)
« Reply #9 on: April 22, 2013, 05:45:38 am »


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