hello, i am in the process of swapping out a bulk audio evac system with 7 class A cicuits. they are run off a 2, 250 watt bulk audio amps. we're installing a FCI E3, with 120watt amps, the question i have is, is there any way of ringing out each cicruit to see how many watts each is drawing (without having to count the number of devices on each, and checking what they are set at). i would really like to have a baseline of each circuit and what its drawing on each amp for future referance. thanks in advance for any help.
determining how many watts a FA speaker circuit is drawing?
- Thread starter neko988
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walkerj
Senior Member
- Location
- Baton Rouge
Find the resistance of the circuit and take the voltage of the source and plug them into ohm's law.
That should give you a close idea.
That should give you a close idea.
peter d
Senior Member
- Location
- New England
Not quite. Assuming he is dealing with speakers, inductive reactance of their coils cannot be simply measured by the DC resistance of the circuit.
- Location
- Massachusetts
I agree with Pete and will add that you would only be reading the primary side of the matching transformers on each speaker.
walkerj
Senior Member
- Location
- Baton Rouge
I stand corrected. Thanks guys
hbiss
EC, Westchester, New York NEC: 2014
- Location
- Hawthorne, New York NEC: 2014
- Occupation
- EC
Not familiar with FA and life safety systems but I have to assume that this is most likely a constant voltage, probably a 70.7 volt system. Even if it isn't, Peter is correct in that DC resistance will tell you nothing. This is an inductive load.
First, you have to determine how this system works then use an audio impedance meter to measure the impedance of each circuit. Then you would use that information and the method of distribution to determine the total power needed.
For instance, if this is a 70 volt system and the total load impedance of all 7 circuits added together is 16 ohms then you will need at least a 300 watt amp.
That said, I would say that if the old system had two 250 watt amps and if what you are replacing it with isn't at least 500 watts total you won't have the proper SPLs and could possibly damage the underpowered amps.
-Hal
First, you have to determine how this system works then use an audio impedance meter to measure the impedance of each circuit. Then you would use that information and the method of distribution to determine the total power needed.
For instance, if this is a 70 volt system and the total load impedance of all 7 circuits added together is 16 ohms then you will need at least a 300 watt amp.
That said, I would say that if the old system had two 250 watt amps and if what you are replacing it with isn't at least 500 watts total you won't have the proper SPLs and could possibly damage the underpowered amps.
-Hal
Last edited:
dbuckley
Senior Member
- Location
- Canterbury, New Zealand
yeah, meters like this one.
Alternatively, when the building is clear of people connect a sine wave oscillator at 1KHz to the amp, and use a couple of DVMs to simultaneously measure the AC voltage and current on the speaker lines. R=E/I. Dont turn the amp up too far just want a few volts on the line.
Alternatively, when the building is clear of people
connect a sine wave oscillator at 1KHz to the amp, and use a couple of DVMs to simultaneously measure the AC voltage and current on the speaker lines. R=E/I. Dont turn the amp up too far just want a few volts on the line.
Speedskater
Senior Member
- Location
- Cleveland, Ohio
- Occupation
- retired broadcast, audio and industrial R&D engineering
Only have one DVM? (but it needs a 2 volt AC range)
Here is one way to measure it.
Use a portable CD player with a test tone CD of 400 or 1000Hz. The DVM doesn't need much accuracy at the test frequency. Let's say that we are testing an 8Ohm system. Remove the speaker wires from the amplifier, connect one end of a 1000 Ohm resistor to the positive speaker terminal, connect one end of a 10 Ohm resistor to the neutral terminal, connect the other ends of both resistors together. Measure the voltage across the 10 Ohm resistor and adjust the signal voltage input until the meter reads 1.0 Volts. This calibrates the meter to read Ohms. 1.0V = 10 Ohm, 0.5V = 5 Ohm, 2.0V = 20 Ohm. Now disconnect the 10 Ohm resistor and connect the speaker wires to the same points and do your measurements.
Here is one way to measure it.
Use a portable CD player with a test tone CD of 400 or 1000Hz. The DVM doesn't need much accuracy at the test frequency. Let's say that we are testing an 8Ohm system. Remove the speaker wires from the amplifier, connect one end of a 1000 Ohm resistor to the positive speaker terminal, connect one end of a 10 Ohm resistor to the neutral terminal, connect the other ends of both resistors together. Measure the voltage across the 10 Ohm resistor and adjust the signal voltage input until the meter reads 1.0 Volts. This calibrates the meter to read Ohms. 1.0V = 10 Ohm, 0.5V = 5 Ohm, 2.0V = 20 Ohm. Now disconnect the 10 Ohm resistor and connect the speaker wires to the same points and do your measurements.
Weaver Road
Member
- Location
- Willington, CT
Audio Circuit Requirements
Audio Circuit Requirements
Neko,
Was this system designed by an engineer from the alarm manufacturer or other similar group such as the electrical engineer for the GC? if not, the AHJ should have asked for that. Then you would have that info. As an AHJ, I always require the relevant calcs before I approve the permit. Depending on the job, it might be battery calcs, audio circuit capacities, voltage drop calcs, etc. Simplex and Seimens are also pretty good about supplying data sheets with some of that info included. I realize I haven't helped you calculate the answer, but I hope I might have pointed out a route to the answer.
Mike
Audio Circuit Requirements
Neko,
Was this system designed by an engineer from the alarm manufacturer or other similar group such as the electrical engineer for the GC? if not, the AHJ should have asked for that. Then you would have that info. As an AHJ, I always require the relevant calcs before I approve the permit. Depending on the job, it might be battery calcs, audio circuit capacities, voltage drop calcs, etc. Simplex and Seimens are also pretty good about supplying data sheets with some of that info included. I realize I haven't helped you calculate the answer, but I hope I might have pointed out a route to the answer.
Mike
- Status