Public address system

Sorry, but your question does not compute. What voltage? What current? What source are you using? If you are an engineer, you must be either in software or digital design... ;-)

Charlie - who has designed many PA systems and never used power speakers!

Maybe the OP means powered speakers, as in locally powered, signal passed at line levels? ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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On a sunny day (Wed, 07 Oct 2009 13:16:16 -0700) it happened Charlie E. wrote in :

Agreed, I designed one for in office building, and the speakers were 5 W at the most,. and the amps were LM380 IIR. You need good acoustics, The way I did that, is mount the speakers like this: Supply was simple linear with a small transformer, did supply a remote driven LCD clock too.

double pane wooden wall | | | | | | /| ||

I haven't used power speakers either. However, from experience I'd say I'd try it. The main reason are the power volume controls. Even expensive ones don't last. At our church they begin to crackle after 1-2 years, later the audio cuts out across large chunks of potmeter range. Powered speakers fit the same 8" ceiling tile cut-out:

Then you don't have to string fat cable around, CAT5 suffices. Some have IR controls, others can be controlled by regular low-power potmeters and not those failure-prone wirewound rheostats. Another nice feature on some active PA speakers is that you can individually adjust the frequency response so it acoustically matches the furnishings of each room area. Most are now class D so heat isn't an issue.

Of course, the fire marshall will have a word as well here and that could make commercial installations of powered speakers quite cumbersome, because of the power cable.

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Hi Jim, I assumed that. But, it is so much easier in a PA situation to have a single power amp, and have multiple un-powered speakers, that only a newbie or amateur would even consider it. If he has many stations, he just uses 100V or 70V transformers for the line. To have a powered speaker at each location means you have to have separate power supplies at each location, or run a power feed with your audio feed, unless you get really crazy and decide to run power over the audio line! 8-)

Charlie

Yeah, power in a plenum needs to be in conduit, or at least armored cable.

Charlie

I have actually considered rolling my own powered speakers, simply because it offends me to have amplifier damping on the end of 120' of zip cord... rear speakers are that far from my present amplifier ;-)

For me, local power would be trivial.

Signal via a current loop to avoid ground issues. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
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Some are LV-powered, for example at 24V. But yeah, it's a hassle to do proper power runs up there. The reward comes later, by not having to replace rheostats all the time. According to Murphy's law this stuff dies at odd times like when pastor just starts his sermon. BT :-(

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120ft? That must be one heck of an entertainment room.

It becomes non-trivial in public buildings because the fire marshall and his opinion is law. If he thinks something looks odd it gets written up no matter how safe it is :-(

Luckily he wasn't there when I hauled in my electric saw and connected it via a cable that had a NEMA plug on one end and a Euro outlet at the other.

Just do diff pairs with true line level (or more) and you'll be fine.

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24' x 24' room x 16' ceiling. Ceiling/roof structure forces going around the south side of that raised area using the attic over the entry court then back over the master suite :-(

I tend toward the Greek approach ;-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
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I love to cook with wine.      Sometimes I even put it in the food.

Euww ...

Might want to invest in shielded CAT5 then, depending on whether you must cross paths with some huge mains trunk. And keep the code distance from that ;-)

But this is America ...

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Many inspectors are just confused on the issue of low voltage power wiring. They know the AC stuff just fine but never can make up their mind about the low voltage power.

If there was a way to do it with CAT5 and POE (Power Over Ethernet) that'd be cool, because most inspectors now understand CAT5 network cabling if done right.

Wait, they already have speakers with VoIP and POE:

Probably does not work well for sound reinforcement due to codec delays, I see the suggested application is paging where the timing is much less important.

Tim.

Yeah, the speakers I mentioned can also take power over CAT5 but it's about an amp max per speaker. That gets old when you must install a dozen.

We also use the speakers for overflow area sound at our church. It's best to have the least amount of digital stuff in the way, too much delay, we do it all analog.

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On a sunny day (Wed, 07 Oct 2009 15:31:00 -0700) it happened Charlie E. wrote in :

Bull. All depends on the situation. In my case I also had a box full of electronics, huge LCD (clock and call number) with CFL backlight, per unit. The few watts for the speaker amp was nothing, and not even continuous. And that allowed me to use multicore twisted unsrceened differential feeds for audio and data. All serially connected for about 50 boxes. Redundancy, if one fails the others keep working, if *your* main amp fails then all is silent. Good speaker damping, excellent sound, many times better then a 100V system can do. Remote digitally controlled muting of each audio amp, so you can address a single location / room.

Audio tube amp freaks have limited technological imagination.

That is ancient technology from when all there was, was one big 100W tube amp.

Q

number)

audio and data.

all is silent.

do.

single location / room.

Ok, so you aren't talking about a PA system, but a custom paging and other use system. If you have a lot of other equipment at each location, and cabling that involves a lot more than just the PA signal, then locally power amplification is just another feature. If you are talking PA, then having to install and power multiple amplifiers is usually a lot more trouble than necessary. Using a single, inexpensive amplifier connected by 16 guage speaker cable is a lot easier than even CAT5.

Charlie

On a sunny day (Thu, 08 Oct 2009 08:16:09 -0700) it happened Charlie E. wrote in :

number)

audio and data.

then all is silent.

can do.

single location / room.

Oh, I agree with that. OTOH when you are in a building, and have to lay cables, open up walls to lay them, assign some people to do it, the man-hours cost may be amazing, putting the cost a little extra hardware in the shadow. I am taking real buildings here, maybe needs concrete drilling too.

Hi thanks for the replies. The loudspeakers have an amplifier incorporated in them as I said in my earlier post. Its not a HiFi system but a Public Address system or as some of you prefer calling it a paging system to call operators to an Intercom line for urgent communications or warn them of dangers to the environment like poisonous gases , fire and the like.

The loudspeakers each have an amplifier incorporated I repeat ... and each one of these amplifiers absorbs 25W, though the data sheet says the maximum value is 40W. The input voltage to each loudspeaker is 24 to 48Vdc but looking at the loudspeakers specs I prefer using 48V as distortion at 24V(2% versus1.5%) is high as is the quiescent current through the amplifiers (45mA versus 25mA). Input impedance of the loudspeakers = 10kOhms, S/N is about 90dB. The amplifiers are Class D type. I prefer a power supply with a 230V to a 115V input option.

I can't use a single amplifier because with the number of speakers we're dealing with it would need more then a 1000W speaker with huge distances involved its complicated. The source is an intercom system so at a first estimate 0.6mm telephone wire should suffice.

The source amplifies the input voice to 7 dB but I reverse engineered that with an inductor coupling to 0 to make it OK for the loudspeakers amplifiers.

Till tomorrow.

Martin

And hey I forgot to include inj my last post the question is about powering these things :

what type of Power supply? SMPS , linear or mixed? What S/N , distortion figures should I be asking from the supplier? What other specs are important. Thanks again to all in advance.

Somebody was asking about the reliability of powered speakers. These are robust things ... cost a small fortune a bit but they blare out louder then the work environments background noise and are in conformity with the norms for Industrial environments where they will be fitted.

Martin

I would certainly go for SMPS (primary switchers). Linears become toasty. Try some out, to see if they cause any serious noise. Modern ones should not be much of a problem in that respect as they operate well above the audio range. Best to find an "international power" type that takes anything between 90VAC and 260VAC without a need to re-configure. Most contemporary SMPS will be like that. That way you can sell to just about anywhere, from Japan with its 100VAC grid to the UK with their 240V grid.

It's a pity that the speakers don't work with a voltage around 18VDC because then you could possibly use cheap laptop brick supplies if code and safety regulations allow that. 24VDC or 48VDC is industrial, meaning expensive.

It will be important to distribute the audio feed differentially.

[...]

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