PA system input

[ammklq143]

I have a local school that is looking at adding several PA system speakers. I've done some of this many years ago, but not much. I just wanted to ask people that do a lot of it for some input so I use the right materials, etc. since it's somewhat out of my realm of expertise. Below is a message to the school official from the guy that used to do their PA work.

"Some locations like stairways would probably do best with a horn. Large noisy rooms should probably have a horn too. I think you will see that in the shop and gyms. If there is a T-Bar ceiling I use an 8 inch speaker with baffle, tile bridge and back box. For some locations a wall mount 8 inch speaker in a wood box is the thing to use. Bogen makes those. These are all 70 volt and get tapped to what the location requires for power/volume. I use whatever brand is available in a timely manner and at a good price. Atlas, Bogen and JBL are some of the brands that make 70 Volt speakers. I just installed a new 240 Watt amplifier on the system so there should be ample reserve power to add on some more speakers.They should be hooked into whatever zone makes the most sense for the location."

Would these be 2 wire speakers with a volume tap on them? It seems like I recall some speakers that were 4 wire.

What cable should a person run for these and what are the length limitations on different sizes? Some will likely be a couple hundred feet long. Some will be 50 feet long.

I'm going to stop and take a look at the equipment sometime this week to see what's existing, but thought I'd ask and see if anyone had any input upfront or anything I should look for when I go there to see what they have now.

Thank you for any help!

 

[tom baker]

Since it’s a 70 volt system VD and impedance matching is not a concern. I Used 18 gage wire, riser or plenum rated.
If you have a plenum ceiling then it requires special boxes or use wall boxes.
I used a metal plate that went across the t bar to support the speakers. The speakers I used had volume controls… but in the lunchroom, those speakers always got turned down, so that’s a consideration in some areas not to have volume controls

 

[brantmacga]

I just wanted to ask people that do a lot of it for some input so I use the right materials, etc. since it's somewhat out of my realm of expertise.

I can give you a list of reasons to sub this out. But the primary one is that you aren't familiar enough with the systems to effectively train them how to use it and troubleshoot the issues that will arise.

 

[ammklq143]

I can give you a list of reasons to sub this out. But the primary one is that you aren't familiar enough with the systems to effectively train them how to use it and troubleshoot the issues that will arise.

The system is already installed and they are just adding to it. They have an IT company that takes care of it and just need an electrical contractor to run the cables and do the connections.

 

[hbiss]

They have an IT company that takes care of it and just need an electrical contractor to run the cables and do the connections.

If they have an IT company (GASP!) that will supply the speakers and the brain power and all you have to do is run wire that should be an easy job for you... if you know where to run it from. These speakers are two wire, usually 18/2 twisted. You can tap off an existing speaker within the same location. If you are called upon to do any more than that such as installing the speakers and determining the power taps on their transformers it gets slightly more complicated.

-Hal

 

[Speedskater]

On 70 Volt sound systems:

Constant-Voltage Audio Distribution Systems
Dennis Bohn, Rane
RaneNote 136 written 1997; last revised 3/07

25, 70.7 & 100 Volts
U.S. Standards
Just What is "Constant" Anyway?
Voltage Variations -- Make Up Your Mind
Calculating Losses -- Chasing Your Tail

Constant-Voltage Audio Distribution Systems: 25, 70.7 & 100 Volts

Use a transformer to step-up the power amplifier's output, drive long lines, and use another transformer to step-down the voltage at each loudspeaker.

www.ranecommercial.com

 

[PaulMmn]

"Would these be 2 wire speakers with a volume tap on them? It seems like I recall some speakers that were 4 wire."
.
These speakers will have a transformer inside.

 

[hbiss]

The 4 wire speakers are by Valcom and Viking. Those speakers have built-in amplifiers that require a power pair in addition to the audio. This is not what you are talking about here. Your speakers will have a transformer, either mounted to the back of the speaker or inside. With an external transformer such as on a ceiling speaker, you adjust the wattage (volume) by connecting to a common (black) and a tap for which information will be printed on the transformer. Other enclosed speakers will simply have a switch that gets set to the desired wattage.

That Rane white paper is an excellent reference.

-Hal

 

[MarkDMN]

Having worked for an "IT" company it would surprise me if they know much about 70.7 volt audio.

This comment thread is old enough that perhaps the OP has either solved the problem or walked away from it. But in the interest of helping out:

It is very possible to combine 2 wire 70.7 V and 4 wire self amplified speaker systems. This can be very useful to add speakers where the amplifier is near fully loaded (more on this in a minute), or to extend long cable runs (as the 4 wire speakers can take audio into 1000+ foot cable distances easily). As mentioned by hbiss, these speakers do require separate power (usually 24 volts DC) BUT that power can be provided nearer to the the speaker (and on long cable runs this is desirable to reduce voltage drop anyway). Additionally, they can utilize existing telephone cabling (house wiring) within a building that may now be idle due to a conversion to IP phones (common in schools these days), thus potentially saving much cabling work. Keep in mind though that the audio pair of a 4 wire speaker is 600 Ohm and must not be connected directly to the existing 70.7V speaker connection, nor to any 2,4,8,16 ohm connection! Many commercial amplifiers have provisions for 600 ohm output and there are matching transformers available to adapt systems that do not already have 600 Ohm audio outputs.

Now a major challenge when doing new work expanding an existing 70.7V system is that you MUST uncover/discover details about the installed speakers and amplifier(s). If you do not do this you run the risk of overloading the amplifier(s) because the original system was designed tightly (no room for expansion). This means tracing and documenting every speaker and also identifying how each speaker transformer is tapped. Yes, on a 70.7 V system, each speaker has a transformer with 6 to 8 "taps" providing a range of audio wattage for that speaker. This tap would have been selected by the original designer/installer in order to balance the audio level. The total tap settings for all speakers in the system must NOT exceed 80% of the amplifier power. I have seen many installed systems where speakers were added after the initial installation (sometimes over many years) without knowing or considering what audio power was already being used. In these situations eventually the amplifier will not be happy. I have also found that since these systems are often not well understood by the people supporting them (IT for example), rogue speakers that are not 70.7V type get installed. This is a bad thing and will usually cause an customer (end user) complaint quickly.

Finally, schools often use "zone paging" via a complex control panel. You must determine where to tap the audio for the new speakers in order to fit into the customers existing zone scheme. Sometimes this is easy sometimes not. There has also been much new paging equipment that is IP connected and/or controlled. These may use Cat6 wiring and be very technical.

 

[ammklq143]

Having worked for an "IT" company it would surprise me if they know much about 70.7 volt audio.

This comment thread is old enough that perhaps the OP has either solved the problem or walked away from it. But in the interest of helping out:

It is very possible to combine 2 wire 70.7 V and 4 wire self amplified speaker systems. This can be very useful to add speakers where the amplifier is near fully loaded (more on this in a minute), or to extend long cable runs (as the 4 wire speakers can take audio into 1000+ foot cable distances easily). As mentioned by hbiss, these speakers do require separate power (usually 24 volts DC) BUT that power can be provided nearer to the the speaker (and on long cable runs this is desirable to reduce voltage drop anyway). Additionally, they can utilize existing telephone cabling (house wiring) within a building that may now be idle due to a conversion to IP phones (common in schools these days), thus potentially saving much cabling work. Keep in mind though that the audio pair of a 4 wire speaker is 600 Ohm and must not be connected directly to the existing 70.7V speaker connection, nor to any 2,4,8,16 ohm connection! Many commercial amplifiers have provisions for 600 ohm output and there are matching transformers available to adapt systems that do not already have 600 Ohm audio outputs.

Now a major challenge when doing new work expanding an existing 70.7V system is that you MUST uncover/discover details about the installed speakers and amplifier(s). If you do not do this you run the risk of overloading the amplifier(s) because the original system was designed tightly (no room for expansion). This means tracing and documenting every speaker and also identifying how each speaker transformer is tapped. Yes, on a 70.7 V system, each speaker has a transformer with 6 to 8 "taps" providing a range of audio wattage for that speaker. This tap would have been selected by the original designer/installer in order to balance the audio level. The total tap settings for all speakers in the system must NOT exceed 80% of the amplifier power. I have seen many installed systems where speakers were added after the initial installation (sometimes over many years) without knowing or considering what audio power was already being used. In these situations eventually the amplifier will not be happy. I have also found that since these systems are often not well understood by the people supporting them (IT for example), rogue speakers that are not 70.7V type get installed. This is a bad thing and will usually cause an customer (end user) complaint quickly.

Finally, schools often use "zone paging" via a complex control panel. You must determine where to tap the audio for the new speakers in order to fit into the customers existing zone scheme. Sometimes this is easy sometimes not. There has also been much new paging equipment that is IP connected and/or controlled. These may use Cat6 wiring and be very technical.

Thanks for the information Mark. One thing you mentioned was something I was wondering about the other day because they've asked that the new PA speakers have volume controls. The speakers have taps for different wattages along with the volume control knob. Do I need to tap these speakers at the lowest wattage that still allows for adequate sound levels or can I just put them on the highest wattage and adjust the volume down to the desired level? Basically, if I use the 4 watt tap but turn the volume down to 1/4, will the power consumption from the amp only be 1 watt for that speaker because the volume control is at 25% or will it be the full 4 watts because it's tapped there?

 

[MarkDMN]

You would want to tap any new speaker at the wattage that is only as loud as you ever would need it to be (after determining that the amplifier has sufficient power available) then the volume control will allow you to set from 0 to maximum (needed) volume. Install the speaker, pick a tap and then test it. Re adjust as necessary and lower if possible. Generally you don't want to use any extra watts and you don't want the customer (or a prankster) to be able to ever set it too loud. The power actually consumed would never be more than the tap you have select on the transformer and will be less as you set the volume control lower. On some speakers the transformer has a rotary switch that moves the contact to different taps. Keep in mind that the amplifier volume settings will have an overall effect as well but usually you don't change that in an existing environment (because someone else balanced it). Again, set the speaker tap only as high as you'll ever need for volume in the area around the new speaker(s) not more. I should mention that there are 70.7V remote volume controls that can be installed in the wiring run to the speaker or even a string of speakers. When using this kind of volume control you still set the speaker tap(s) no higher than what is necessary and you have to supply a volume control that can carry the total wattage of all the speakers on that string. Between you and me, I advise customers to put in LESS volume controls not more. This may be counterintuitive but if the system is designed right and balanced (tuned) correctly very little adjustment should ever be needed. Extra knobs are just a temptation for fiddling and future complaints. That said,fiddling is less of a problem with the small almost hidden volume controls on some ceiling speakers

Horn style speakers will make much more volume per watt than cone (paper or other) style speakers. I have set speakers in quiet office areas as low as 1/4 watt with perfect results. 4 watts would maybe be right in a noisy area. Naturally this is all dependent on the speaker(s) you are using and the speaker spacing. Speaker efficiencies vary widely and it also depends a little on what the audio program is going to be. Music can be different than voice paging as far as the wattage requirements. You have to test them out to know for sure.

Also, horn speakers can make painful sound levels with as little as 5 watts. Use caution when testing them!