atcomsystems.ca/forum Forums Wiring - Telecom Carriers - Trunks & Services Security, Alarms and Paging 70v BGM system

I got called in to a job site to resolve a problem with a back ground music system.

Of course this problem started with the electrician. They used standard 8 ohm speakers and volume controls, amplifier with no impedance matching control. When this didn’t work they brought in a “home audio expert”. He quickly realized this is a commercial application so he changed the volume controls to 70v attenuators and the amplifier to 70v line. This appeared to fix the problem except occasionally there is a distortion.

Okay, so this is where I come in. I listen to the music and realize the distortion is on the peak of the bass. So I go and look at the amplifier, see that it is 70v with a bunch of home runs wire nutted together. So I figure I’ll check the taps on the speakers add them up to see if they are exceeding the 30 watt rating of the amp. Well to my surprise there are no taps on the speakers, they are plain old 8ohm speakers. I was really surprised that this amp. is still running at all with 18 speaker.

So now my plan is to install taps on all of the speakers and set them for 1 watt. This should put an 18 watt load on the 30 watt amplifier. This should be good.

My only questionable thought is: Are the 70v volume controls providing the proper load to the amplifier by themselves? Will installing the taps on the speakers cure the trouble?

I’ve always used and understood that you use 70v amp, 70v volume controls (when needed) and 70v taps set to work with the amplifiers rating. I've never had a problem using this foumula but I've never tried to use only 70v volume controls with no taps.

I think the taps will cure it.
I am too.

I do not think that having 70V volume controls without the appropriate speakers would do anything. I had an 8 Ohm amp hooked up to a regular speaker and a 35 watt volume control and even at the highest setting I did not get a whole lot of volume.

I don't beieve it's possible for 8ohm speakers to work on a 70volt line. It's possible that there's an 8 ohm-70v transformer (or more than one) someplace in the cable run. Transformers should be mounted on the speaker, but I've seen crazier things before.

Sam

Electrically, the 8 ohm speakers could work on a 70 volt line without 70V to 8 ohm transformer, but the distortion should have been worse. Only thing I can think is those attenuators are protecting everything. If you put in the transformers, and tap down to 1-2 watts, you can, and probably will have to, pull the attenuators. Do 1 drop at a time, you'll probably find other oddities. Like several 8 ohm speakers in series, or parallel, etc. I'm glad it's you and not me! John C. (Not Garand)

I am just wondering, would 70 volts AC not damage a straight 8 ohm speaker?

I don't beieve it's possible for 8ohm speakers to work on a 70volt line.

I am just wondering, would 70 volts AC not damage a straight 8 ohm speaker?

Well do the math and tell me haw many watts an 8 ohm load is going to try to pull with 70 volts across it. Hint- power is cheap these days and amplifiers in excess of 612 watts are a dime a dozen.

I remember hearing about an old trick to find those "rogue" speakers- those that somebody added to a system by connecting them directly to the line without a line transformer. Simply disconnect the feed from the amplifier output and put 120 volts on it for a few seconds. The speakers with transformers will be unharmed but those 8 ohm voice coils will never bother you again.

SST, I would check those attenuators that the idiot supplied. Atls AT series (or equal)autoformer type attenuators are what you should be using. Since this is a music system I would remember that crap speakers and crap transformers equal crap music. You want big iron in the transformers and quality speakers. Don't expect to do your shopping at Radio Shack, MG Electronics and others.

Distributed systems always get a bad rap for anything other than paging. I'm here to tell you that they can sound just as good as the stuff sold by "home audio experts" for many times the cost.

-Hal

Hal, I am not sure what you mean/how you got 612 watts?
What I meant was something like your taking 120 volts and going right into a speaker, it would kill it...

Put the transformers on all the speakers.
You may get slightly better quality by stepping the Amp output down to 25 volt which it may or may not have.
As for 25 or 70 volt working on 8 ohm speakers,
I see this all the time from DIYs. :rofl:
MrG

You may get slightly better quality by stepping the Amp output down to 25 volt which it may or may not have.

Not true. It all depends on the quality of the transformers then the speakers. The more iron in the core of the transformer the lower the frequency at which it will start to saturate. So heavy transformers will have a better low frequency response. Quality manufacturers like Quam and Atlas will spec the frequency response of their transformers.

Hal, I am not sure what you mean/how you got 612 watts?

Jeff, Jeff. Have you been sleeping in math class again? No more Tech Talk on school nights!

P= E squared over R

P= power in watts
E= voltage
R= resistance in ohms

-Hal