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Installs paging system

Discussion in 'Sound, Music, and Intercom' started by NHStech, Jan 5, 2010.

  1. NHStech

    NHStech Member

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    We have a paging system that was installed so there is a set of two standard ceiling speakers that were installed in a single hallway adjacent to the stage. I would like to take one and reinstall it in the choir room adjacent to it (normally used as a dressing room), take another and send it to the other room we tend to use as a dressing room, which is across another hallway, and chain a third speaker in the series so that there will be one speaker still in the hallway.
    The PA system has a 60-watt amp. I asked the previous SM about hooking up a third, and he said it wouldn't work hooking up a third. I forget why, but I have heard others say it would probably be okay. Like I said - these are your average round ceiling speakers.
    So, my question is A) can I chain a third speaker, and B) If I move one of the hallway speakers to across the hallway (about 50' or so of wire), would that affect the volume much? I wouldn't think, but it is a small amp. Thanks!
  2. cutlunch

    cutlunch Member

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    Hi.

    The first thing you have to establish is whether the amplifier and speakers are working on a 100V system, which is common for paging applications. Or is the amplifier used just like a normal stage amplifier where you connect speakers to an output that is measured in Ohms , 8 being typical. If you know the make / model of the amplifier you can look it up.

    If it uses a 100V system ( sometimes a 70V system) it should show this marked on the back. If it is a 100V system then moving and adding another speaker is not to hard. On a 100V system each speaker normaly has a transformer on the back. And the speaker with transformer is given a rating in Watts. So the speaker may a 5 Watt unit. So for a 100V system all you do is add up the Wattage of all speakers used on that line and make sure the Wattage is not larger then the wattage. Normally you don't run to the full Wattage to allow for losses. The speakers are normally wired in parallel.

    If it not a 100V system it gets trickier. You can add more speakers but you have to make sure the impedance for the combined speaker load is within a range the amplifier can handle. You need to know how the current speakers are wired. If they are in parallel then the impedance the amplifier is half the impedance of the impedance of one speaker. (typically both speakers will have the same impedance). If you then added a third speaker in parallel the impedance will drop again allowing more curent to flow. This may damage the amplfier and if the wire if it has not been rated high enough for current flow.

    In either system so long as you use the right gauge of wire adding 50' won't drop the signal to much.

    So back to the beginning you need to definitely find out how the amplifier is wired, 100V system or ordinary impedance based system. You have to find out what type the current speakers how they are wired.

    Then you can design the new circuit then you have to get the school's permission / money to update. They might not let students crawl around in roof spaces laying cable.

    To make adding speakers to a Paging system is why the 100V system was invented. All you have to do is add the wattage of the speakers you want to use.
  3. FMEng

    FMEng Well-Known Member

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    70 Volt and 25 Volt lines are more common, in my experience, than 100 volt distributed systems, but everything said so far still holds true. The amp output would be configured for the appropriate voltage and each speaker would have a transformer. As long as you do the same, with the additional speakers, it should be fine.

    If you were to hang 8 or 16 ohm speakers on the distributed line, it would essentially short it out and get very quiet.
  4. fx120

    fx120 Member

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    100V is pretty much standard outside of the US.

    70V is the most prevalent system inside the US from my experience. I've been doing service work for about 5 years now and have run into a grand total of 3 25V systems.

    Anyway, just make sure that the system is in fact a constant voltage system. Look at the amplifier and see where the output leads are connected, or verify what output mode the amplifier is set to. If it is a constant voltage system you can probably safely just tack on another ceiling speaker in parallel with the others. Just make sure that the speaker you purchase is designed for operation as part of a constant voltage system, and you choose the correct transformer tap. For a typical 8" ceiling speaker I start at 5W and see if that is enough poop compared to the rest of the system, but it will vary depending on the tap positions of your other speakers and the amplifiers gain setting. It may be worth trying to find out the tap positions are on the existing ceiling speakers.
  5. NHStech

    NHStech Member

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    I found the manual to the amp. It is a University Sound MA605B. It says it has a 600 ohm transformer-isolated line input for paging, with automatic muting for all other inputs. So, it is an amp that can be used for regular purposes (mics, etc), but does have paging as well. The speakers are 8 ohms, and are your usual white ceiling speakers that have a volume control.
    Attached is a pdf file from the manual. The first page are the specs, the second is what the back of the amp looks like, the third is the front. I don't really know enough about sound to the point where I can answer the questions you folks have posed, as well as even how to wire "in parallel." Thanks for any help you can give. Hopefully the pdf will help answer some of your questions about the system.

    Attached Files:

  6. museav

    museav CBMod CB Mods Departed Member

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    The MA605B is typical of a number of 60W 'utility' mixer amps that can be used to drive 4 Ohm, 8 Ohm, 16 Ohm, 25V or 70V lines. So without knowing how the speakers were wired it is impossible to tell which way it was being used.

    When you say the ceiling speakers have a volume control are you referring to a separate wall mount volume control or a knob in the center of the speaker grille? The latter are typically 25V or 70V speakers with a transformer. It would help if you could provide a model for the speaker, it could be from any of a number of manufacturers (Atlas Sound, University, Lowell, Quam, Dukane, Bogen and so on) all of whom make, or made, a variety of ceiling speakers.
  7. NHStech

    NHStech Member

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    I looked at them yesterday and couldn't find anything as far as brand. I will check again in the next few days. There is a knob in the center. I'll check the wiring and get back to you.
  8. cutlunch

    cutlunch Member

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    Hi

    So you actually had the speaker out of the roof and were looking at the back of it?
    If it is on the 70V system there are probably two wires that go from the speaker itself to something that looks like a solid block with wires from that going to the volume control.

    This link shows a speaker with the transformer on the side.
    OSD-C1090VK 8" Ceiling 70V Speaker w/Volume Control

    Hope this helps.

    Brent
  9. Chris15

    Chris15 CBMod CB Mods Premium Member

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    Can you get to the back of the amp?
    If you look at the connector labelled as 9 on the manual you posted and can tell us which terminals the wires are attached to, we can probably answer the questions relatively easily... Expect one to be connected to "COM".
  10. NHStech

    NHStech Member

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    chris15 and cutlunch,
    first off, thanks for your patience with me. Yes, I did go up there and take the speaker down. It looks very similar to the picture you gave me, albeit the volume control knob is a little larger on mine. But, yes, the block was on the side as shown.
    I'll get to the amp tomorrow and look at where the wires are on the back. Thanks again.
  11. cutlunch

    cutlunch Member

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    That sounds like a 70V system.

    There is another thing you could check but I wouldn't take the speaker down for it.
    That is the volume control. How it turns ie smoothly around like a volume control on a stero or does it click around from position to position like a selection switch.

    On a 70V system it is more likely to be a switch with a limited number of positions. Because to change the volume on a 70V system they have to select different Taps on the transformer.

    Brent
  12. Chris15

    Chris15 CBMod CB Mods Premium Member

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    Agreed it sounds like a constant voltage system. Not agreed on need to change taps to volume control on a CV system. You can do it with a stock pot easily enough, but depends on the power tap of the speaker, something 2.5W or less is easy. This covers most office installs from experience...
  13. NHStech

    NHStech Member

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    Chris 15, I checked the back of the amp. The wires are coming from the "com" and 70v. There are no wires hooked up to the paging input.
    Cutlunch, The volume control is smooth. It doesn't click.

    What are the taps and a stock pot?
  14. cutlunch

    cutlunch Member

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    Chris15 is right about the volume control.

    Taps just are where you can connect into the transformer coil in different places to get a different wattage loading. They could be wires coming out of the transformers or maybe metal tags that you can solder wires to.

    Here is an example showing both the physical transformer and the schematic representation.
    Pure Resonance Audio SD64T Transformer & more 70v-100v Transformers within Commercial Audio Accessories at www.ProAcousticsUsa.com

    On the schematic the taps are those wires that lets us set different wattage levels to use on the primary side of the transformer e.g. Orange (wire colour) 4W(atts) is one tap, Brown 32W(atts) is another tap. (these are the wires at the back of the transformer in the picture.)

    So to set it up you decide what wattage you want,then on the primary side wire that tap ( lead ) to the Signal (70V on back of amp) side of the 70V line and wire the black lead to the ground side ( common on back of amp).

    On the secondary side the two wires go the loudspeaker. (these are the wires at the front of the transformer in the picture.)
    But since you have a volume control that has to be wired in somewhere. Chris will tell us if it is more likely in the primary side or secondary side.

    But now you have seen what a 70V transformer looks like you should be able to work out how your volume control is wired. Remember primary side has the more wires and they should be labeled with the different wattages. The primary side will just have the two wires coming out.

    So you need to identify the taps on your speakers. So find the line coming from the amp then see which wire goes to the Com side and the other lead should go to one of the leads marked with a wattage rating. Then add up the wattage used on both speakers and take away from 60 Watts to work at how many more watts of speakers you can add. Don't push it all the way to 60 Watts as there are losses so maybe use 55 Watts as totaled allowed. Chris will tell us the typical figure used to allow for these losses.

    Since you already have both speakers working you know you could move one without overloading the amplifier.

    Stock pot just means a standard potienometer (also known as a variable resistor) used for a volume control. The stock part just means it has a resistance value that is commonly used for this job.

    Hope that helps.
    Brent
  15. Chris15

    Chris15 CBMod CB Mods Premium Member

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    The various transformer taps are there to allow you to set the volume effectively for a given area. Individual volume controls in a normal office paging system aren't always included, so you tap different speakers at different power levels to try and get the coverage (volume etc.) patterns you are after. You don't need as much power in a small room, say a storeroom or bathroom as you do in say a school classroom. So you might tap the storeroom at say a watt and the classroom. This is partly because you can't change the master volume to fix issues in one place without it changing everything else. Please let us know if the whole concept of tapping still doesn't make sense.

    By a stock pot, I mean something you can get at Radio Shack or whatever other retail electronics outlet you choose as an item off the shelf.

    I don't know how speakers with internal controls are wired, but I would be putting the volume control on the line side. There are two ways to do it, depending on whether you want to have off as a setting. Before you go and say why wouldn't you, there are some applications where you don't want the user to be able to completely turn the speaker off, rather just subject it to background noise.

    If people want me to talk more on volume controls (and ask) I will, but I think that's enough before I start boring you.

    As to spare capacity. On a certain Australian brand, the generally accepted loading figure is 120W constant on a 100W amp. Most things are overengineered. Older gear you can be less sure of. You also balance the factors depending on the likelihood of expansion and need to increase taps later. It would be unwise in a greenfield install to leave less than say 25% spare capacity, a number in line with things like distribution boards.

    Remember that at 100V, a 500W line has an impedance of 20 ohms. So an ohm or two of resistance is not dire. Especially because it's not appropriate at that point to treat the line as a lumped impedance, you will have a greater power loss on the sections of cable closer to the amp and as you pass through each (ceiling) speaker you drop part of the total load and thus you have less current continuing along the main line and thus less cable loss.

    As to the original question, with a 60w amp, it is unlikely (but possible) that your current speakers are tapped to 30w each. I would expect 5-10w is more likely. That being the case you can easily add another speaker or speakers. You might be better off adding 2 new ones to serve the extra rooms rather than needing to patch a hole in the ceiling from whence you remove one of the existing speakers... If you've got a volume control on it, you can turn it off if you don't want it on...
  16. museav

    museav CBMod CB Mods Departed Member

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    70V systems do not typically use common pots for volume controls, they use autoformers with ratings equal to or greater than the attached speakers. A simple pot will present a varying load to the amp, the opposite of what is desired in a 'constant voltage' system.

    Common practice here is to load the amp at less than rated capacity in order to accommodate line and other losses, obviously the run lengths, power being transfered and cable gage affect what those losses are. It also depends on the transformers, some "1W'" transformers represent a 1W 'load' to the amp while others actually deliver 1W to the speaker and thus represent a greater than 1W 'load' to the amp. Finally, one must consider future changes, such as are being discussed here. If you had twenty speakers on an amp then adding another one is a small percentage change, but with just two speakers as here then adding a third is a 50% change, so this is also application dependent.

    Perhaps a better way to look at this is from the speaker end. A typical direct driven speaker is a nominal 8 Ohm impedance. A 1W, 70V (which is actually 70.7V) speaker is a 4,998 Ohm load. A 10W, 70V speaker a 500 Ohm load and a 30W, 70V speaker a 167 Ohm load. You can see why you can thus parallel multiple speakers with this approach without presenting too small of a load to the amplifier. And while the line losses may drop after each speaker, the load seen by the amplifier is the lumped impedance of the transformers and cabling.
  17. NHStech

    NHStech Member

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    An end note to this thread - we went ahead and ordered two new speakers. Our school district's electrician has had experience installing 70v systems, so this was nothing new to him (unlike this poster). The speakers (four of them) are where they should be and functioned well in our testing. Thank you to all of you who helped in this endeavor. We now have a paging system that is actually useful.

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