On Mon, 18 Feb 2013 12:49:51 +1100, "Phil Allison"
wrote:


"Jeff Liebermann"
klem kedidelhopper

The music is distributed via 70V lines at very low level
throughout the building.

Methinks not. The voltage on a 70v constant voltage speaker system is
(insert drum roll...) 70.7 volts rms.

** Absolute nonsense.

70V is the maximum voltage output from the amplifier and there is nothing
constant about it.

Please tell that to the Wikipedia authors and system vendors that
insist on calling it a "Constant voltage speaker system".
http://en.wikipedia.org/wiki/Constant_voltage_speaker_system
However, you're correct, the voltage is not constant, but that's what
it's called, and what I've been calling it for many years (without
really thinking much about the implications).

No way is it "very low" unless
something is broken or you're running at drastically reduced volume.

** Volume pots allow the level to be set at the operators convenience.

It is very common to have a low, background music level and a much higher
speech level for announcements.

From the original question:
"Each speaker is connected to the line with a small 1 watt 70V to 45
ohm transformer"
So, that means that at maximum volume setting, and at maximum
amplifier power output, this speaker will be delivering a fabulous 1
watt rms. That's about right for background music, or in-room
intercom, but not enough to shake the hallways for an announcement.

The big advantage to 70v and 100v systems is that they can use cheap,
junk, small gauge wiring, such a CAT3 telephone wire for relatively
high power speakers.

** More nonsense.

The real advantages include the use of long cable runs with low losses and
having many distributed speakers - which do not need to be all of the same
rating type or level.

Agreed. I didn't want to itemize the benefits and limitations in
detail. That's why I included the links with the details.

From my experience, the principal advantage to the service company was
the ability to sneak the wiring past the inspectors disguised as telco
wire, alarm wire, doorbell wire, "thermocouple" wire, barbed wire, or
whatever else could be found or scrounged for the occasion. It kinda
balances the added cost of the xformers.





--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558