On 6/20/2012 12:14 AM, josephkk wrote:
On Sun, 17 Jun 2012 05:32:47 -0400, "Michael A. Terrell"
wrote:
On 6/16/2012 10:22 PM, Charles wrote:
http://www.rane.com/note136.html
What a long way to get to a place, when you can simply do the math
in your head. The third schematic won't work, because the drawing shows
both amps in phase. There will be 0 volts at the outputs of the amps,
as drawn. I've also seen a system with severe distortion and overheating
amps that were wired in parallel, but out of phase. I was doing this in
the early '60s, and owned a commercial sound business for a decade after
that.
Web pages like that are pure marketing, nothing more. 350 watts
loss in a 1 KW system is **** poor design. It was written by a
marketing dweeb, who thinks that they are a design engineer.
The amps or transformers are simple, and three formulas cover 25,
70.7& 100 volt systems:
Oooh, i have a units issue he
V=IZ
P=VI
Full signal voltage system relates as: V^2/P = IZIZ/IZI = Z
The amplifiers are rated at full power, continuous service, with a
sine wave that the Constant voltage out.
25*25 = 625 ohms per watt. A 100 watt amp would be 6.25 ohms.
Make that watt*ohms
70.7*70.7 = 5000 ohms per watt. A 100 watt amp would be 50.0 ohms.
100*100 = 10,000 ohms per watt. A 100 watt amp would be 100 ohms.
You use the ohms/watt over the desired watts and it gives you the
load impedance. Or just total all the loads and be within +/- 20% of
the amplifier's rated output. This has been done for almost 100 years
and it's still a valid method. I always preferred the error to be on
the +20% side so a few additional speakers could be added at a later date.
This is no more rocket science than wiring in any power distribution
system is.
The only issue is just how high power, at 10 kW not bad, at 1 MW power
distribution becomes incorporated. At 1 MW the power distribution system
for the PA system (many amplifiers) itself is in arc flash hazard
territory, very not funny.
A properly designed high power sound system uses multiple
amplifiers, and usually distributed to minimize power loss by placing
each amp near a cluster of speakers. Then one amp feeds all the remote
amps at the same distribution voltage. It is then matched to the line in
of the power amps. Transformer isolation reduces chances of lightning
damage.
The worst I ever saw was at an amusement park where some clown put
all the amps at on end of the park, and ran overhead and buried 70.7
volt lines all over the park. Some runs were over two miles. On top of
that, all of the amplifier inputs and outputs were paralleled, and they
used those old cheap 300 ohm TV antenna knife switches to connect or
disconnect each amplifier for troubleshooting. It was no wonder they
lost amplifiers on a regular basis. The system would get very
distorted, so they would start flipping switches till it cleared up,
then add the standby amp, if it wasn't away being repaired.
This is the type switch I'm talking about.
http://www.mcmelectronics.com/conten.../s4/26-790.jpg
The local Lockheed-Martin facility has over 10 KW distributed audio
that I understand is a complete hairball. One amp feeding the next in a
daisy chain, and from building to building. Speaker wires run all over
the place, and not always to the closest amp. Then the work space has
been modified repeatedly for decades. I ran into the head of
maintenance one day and overheard him talking with one of his techs
about the mess and tried to explain how it work, then realized I was
talking to a couple electricians.
KISS, so people don't clutter their minds with useless classroom
type crap. You could waste days doing that way, instead of minutes, and
that's a great way to price yourself out of business. Instead, know
what you have to work with, and carry everything you need to the job in
as few trips as possible. Have extra parts and hardware on hand.
Stopping work to go buy a couple screws or electrical box can cost you
several hours of lost productivity. Very few customers are foolish
enough to sign a T&M contract.