Public address system

[Andrew Edge]

Hi to everybody
I'm designing a public address system for a work place .The
loudspeakers I'm using have an internal amplifier and use up to 25W
at maximum power. I'm wondering what specs to look for the power
supply which will power the speakers. Apart from the total power
which should be about 100W for each position as they will be about 2-4
speakers ... 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 to all in advance.

Martin.

 

[Charlie E.]

Hi to everybody
I'm designing a public address system for a work place .The
loudspeakers I'm using have an internal amplifier and use up to 25W
at maximum power. I'm wondering what specs to look for the power
supply which will power the speakers. Apart from the total power
which should be about 100W for each position as they will be about 2-4
speakers ... 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 to all in advance.

Martin.

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!

 

[Joerg]

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!

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:

http://www.cambridgeaudio.com/summary.php?PID=79&Title=Summary

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.

 

[Charlie E.]

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

...Jim Thompson

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!

Charlie

 

[Charlie E.]

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:

http://www.cambridgeaudio.com/summary.php?PID=79&Title=Summary

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.

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

Charlie

 

[Joerg]

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

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 :-(

 

[Joerg]

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 ;-)

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

For me, local power would be trivial.

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.

Signal via a current loop to avoid ground issues.

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

 

[Joerg]

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 :-(

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 ;-)

I tend toward the Greek approach ;-)

But this is America ...

 

[Joerg]

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:
http://www.ctdistributing.com/images/VoIP_Speaker.pdf

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.

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.

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.

 

[Charlie E.]

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

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

 

[Andrew Edge]

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

 

[Andrew Edge]

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

 

[Joerg]

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.

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.

[...]

 

[Joerg]

One should be careful about powering Class D amp from a switcher. Unless
the Class D and SMPS are synchronized, the intermodulation tones will be
all over the place, and I can guarantee that some of the tones will be
clearly audible.

That's why I suggested to try it out first, before issueing a large
purchase order. I found it can be done. You might need to use a couple
wee ferrites and a capacitor but Martin could cross that bridge when he
gets there, and if he can't get it quiet ask here again.

 

[Nico Coesel]

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

You've got to be joking...

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.

Sorry, but the only way to do this right is using 100V speakers which
are fed by several amplifiers at strategic places. There is no sense
in designing such a system. It is already available as an off-the
shelf product. Go ask at the local football stadium or an amusement
park. They deal with long distances as well. Besides, I don't know if
you want to design a safety critical system. I'd see if I can buy such
a system so I can blame someone else if things go wrong.

 

[Adrian Tuddenham]

[...]

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.

Is there any requirement for the system to continue to work if the mains
fails? If you need emergency battery backup, a centralised system or
one with a few large amplifiers will be much easier to install and to
maintain over the years.

 

[Ross Herbert]

:
: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
:
:
:
:On Wed, 07 Oct 2009 21:55:57 +0200, Andrew Edge
:
:>
:>Hi to everybody
:>I'm designing a public address system for a work place .The
:>loudspeakers I'm using have an internal amplifier and use up to 25W
:>at maximum power. I'm wondering what specs to look for the power
:>supply which will power the speakers. Apart from the total power
:>which should be about 100W for each position as they will be about 2-4
:>speakers ... 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 to all in advance.
:>
:>Martin.


Andrew, I don't wish to be overly critical, but I fear that you are embarking
upon a project about which you have little real-world knowledge.

From your description you are dealing with a large number of speakers over a
huge distances. My first recommendation would be to ditch the idea of powered
speakers. Each powered speaker is a fault liability in its own right due to the
individual electronics in both PSU and amplifier, and fault isolation may not be
as easy as simply saying "oh, speaker number 123 is faulty". Your statement that
each speaker "absorbs 25W" tells me you are new to this subject and I really
think you need to talk to a PA specialist who is conversant with covering large
areas such as sports stadiums or college campuses.

 

[Andrew Edge]

You've got to be joking...


Sorry, but the only way to do this right is using 100V speakers which
are fed by several amplifiers at strategic places. There is no sense
in designing such a system. It is already available as an off-the
shelf product. Go ask at the local football stadium or an amusement
park. They deal with long distances as well. Besides, I don't know if
you want to design a safety critical system. I'd see if I can buy such
a system so I can blame someone else if things go wrong.

I make a living designing these things ... so just answer my question
if you can ... same goes to Tuddenham and Herbert.

Thanks to Joerg and Vladimir for their constructive comments.

Martin

 

[Charlie E.]

I make a living designing these things ... so just answer my question
if you can ... same goes to Tuddenham and Herbert.

Thanks to Joerg and Vladimir for their constructive comments.

Martin

Martin,
Are you saying that you design PA systems for a living? Or industrial
systems for a living? And, are you an electronic designer? Or
mechanical? I have to ask because your question is so basic, and
wrong, that I suspect that you have little to no electronic or
electrical experience, unless it is in a field such as digital design
or software where practical problems are far from reality.

As to the answer to your question, it is quite simple. Where are
these units located. What is the spacing? Is power available at each
location? If they are relatively close, you get one really beefy
power supply, and run a cable from there to the units in parallel. If
they are far apart, then you buy smaller power supplies, and put one
at each unit. If some are far, and some are near to each other, you
use a separate power supply for each area. You also get to find out
what a ground loop eliminator is... ;-)

In other words, your question is meaningless. There is not enough
information to give a reasonable answer, and it seems that most of the
assumptions you have are wrong.

Charlie

 

[Nico Coesel]

I make a living designing these things ... so just answer my question

FYI: We are in the same business. I'm not saying the old way is the
good way and I endorse to think about new ways of doing things instead
of following old habbits. But in this case the 100V systems make a lot
of sense. I'm quite sure if you do the math on a 100V system and your
system, the 100V system will be 4 times cheaper and much more
reliable.