paging system problem

captainvideo462002 · 2007-07-28T03:09:07+00:00

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 thoughtthat given the large amount of speakers perhaps the 35 watt amplifierwas a bit light so I replaced it with a Bogen 70 watt unit.Bogen gives resistance notations on their output taps. The 70 volt tapis printed 10.3 ohms. I'm assuming that regardless of how you tap thespeakers, the load should not be greater than that.The newly installed 70 watt unit does not trip out on page now but ifyou try to get any kind of decent level out of it it becomes verydistorted.There are three speaker trunk lines coming down to this amplifier. Idisconnected them and they read 20, 16, and 5.5 ohms respectively. IfI run the amplifier with just the 16 ohm trunk the portion of thebuilding 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 Idon't push it too hard. However if I add the 5.5 ohm line, (most ofthe remainder of the building) to the mix the amplifier can't handleit. It becomes very distorted. In fact it was little surprise to notethat the amplifier will not drive just this 5.5 ohm line by itselfeither.We looked at some of the speakers on this 5.5 ohm line and most aretapped a 2 watts while some are tapped a 4 watts.I took a typical 70 volt line to voice transformer and looked at theprimary. The specs are as follows:Tap (W) Resistance (ohms)---- ------------4.0 1152.0 1701.0 250.50 450.25 700To employ multiple speakers in the following scenario I wouldcalculate RT. From the above transformer readings using ohms law itseems that I can employ a maximum of 16 speakers tapped at 2 watts fora total resistance of 10.625 ohms. Thats only asking a 70 wattamplifier to produce a maximum of 32 watts.Now I would have thought that I could use a 70 watt amplifier, tap myspeakers any way I would like to, not to...

Jul 28, 2007 21 Replies

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TMI 18 years ago

Hi all,

Years ago TOA had the sidearm of distributed PA impedence meters. I also have an Sennheiser and they are the right way to do things. If you look in Don Davis' Sound System Engineering (given to me by BD at Sound Equipment Corporation 31 years ago) there is a plan for a simple circuit a filiment transformer with a light bulb in series that causes speakers without the correct transformers to "climb out of the ceiling" an make themselves known. It is more durable and less expensive than using amplifiers.

Get some flavor of audio impedence meter and build one of these if you make your living on 70v and 25v systems.

Tom Maguire

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captainvideo462002 18 years ago

Well I've learned to never again take a used "working" paging amplifier out to a job again with out first bench testing it for RMS power output. Or worse yet as I guess I did, assume that one already in service was ok.. Lenny.

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