bypassing old intercom system (45ohm + 150ohm resistors)

Apr 18, 2013
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?
Re: bypassing old intercom system (45ohm + 150ohm resistors)

One of your 200-ohm speaker/resistor combos works out to about 25W on a 70V line. The 45-ohm speaker by itself is around 110W.
Figure out a way to series a few of them and put them all on a basic 70V paging amp with no transformers needed.

Finding a line transformer for a 45-ohm voicecoil won't be easy as I think 45-ohm speakers were designed for direct connection to 25V amplifiers as normally used in school intercom systems (around 15W when on a 25V line)

When we use shielded twisted pair for speaker lines (not very often) I generally ignore the shield
Re: bypassing old intercom system (45ohm + 150ohm resistors)

Same response I posted elsewhere, the "armed intruder drills" implies it being a Mass Notification System or Emergency Communication System and thus needing to comply with all applicable code requirements. There is significant potential liability associated with that so to protect yourself and/or possibly others, as well as to provide due diligence, you probably want to clarify the intended use and make sure that there are no misunderstandings regarding the intended use or what is being provided.