Re: paging system problem
- From: Ken Weitzel <kweitzel@xxxxxxx>
- Date: Sat, 28 Jul 2007 03:54:55 GMT
captainvideo462002@xxxxxxxxx wrote:
I'm working on a 70 volt system that has a great deal of speakers.
There was a 35 watt amplifier that was tripping out on page. I thought
that given the large amount of speakers perhaps the 35 watt amplifier
was a bit light so I replaced it with a Bogen 70 watt unit.
Bogen gives resistance notations on their output taps. The 70 volt tap
is printed 10.3 ohms. I'm assuming that regardless of how you tap the
speakers, the load should not be greater than that.
The newly installed 70 watt unit does not trip out on page now but if
you try to get any kind of decent level out of it it becomes very
distorted.
There are three speaker trunk lines coming down to this amplifier. I
disconnected them and they read 20, 16, and 5.5 ohms respectively. If
I run the amplifier with just the 16 ohm trunk the portion of the
building that is on this line, (about 10 or 12 speakers) sounds good.
If I parallel the 16 and 20 ohm lines I can still make it work if I
don't push it too hard. However if I add the 5.5 ohm line, (most of
the remainder of the building) to the mix the amplifier can't handle
it. It becomes very distorted. In fact it was little surprise to note
that the amplifier will not drive just this 5.5 ohm line by itself
either.
We looked at some of the speakers on this 5.5 ohm line and most are
tapped a 2 watts while some are tapped a 4 watts.
I took a typical 70 volt line to voice transformer and looked at the
primary. The specs are as follows:
Tap (W) Resistance (ohms)
---- ------------
4.0 115
2.0 170
1.0 250
.50 450
.25 700
To employ multiple speakers in the following scenario I would
calculate RT. From the above transformer readings using ohms law it
seems that I can employ a maximum of 16 speakers tapped at 2 watts for
a total resistance of 10.625 ohms. Thats only asking a 70 watt
amplifier to produce a maximum of 32 watts.
Now I would have thought that I could use a 70 watt amplifier, tap my
speakers any way I would like to, not to exceed 56 watts and have a
system with a 20 percent safety margin regardless of what the total
parallel resistance works out to be. Is this correct or am I missing
something?
With old wiring, lines teed in everywhere, and improperly insulated
splices through out the building this place is a nightmare. However it
is a nursing home and I'm due to leave on vacation 10 days, so I've
got to get this fixed this upcoming week. So any suggestions or advice
would be most sincerely appreciated. Lenny Stein, Barlen Electronics.
Hi Lenny...
I suspect that you're confusing resistance with impedance... :)
The dc resistance that you're reading on your speaker lines is just
the resistance of the wiring to those speakers (transformers),
because after all it is a transformer, so the dc resistance should
be virtually zero ohms, right?
The math is indeed simple, divide your 70 watts by the total of all
of the tap wattages - add a little margin - and if it fits you should
be OK.
If that works out, then afraid you're gonna have to check each and
every transformer primary individually - perhaps by sub'ing - or maybe just disconnecting the primary looking for a large change in
level - until you find one that's gone shorted, or until you find shoddy workmanship at a transformer or a splice that's a dead short.
Wear your oldest clothes, cause you're gonna be crawling around in
an awful lot of really dirty dusty crawl spaces. Did it myself 20
years ago in a hospital, and wouldn't like to do it again.
Take care.
Ken
.
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