ProSoundWeb Community

Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => SR Forum Archives => Installed Sound/Contracting FUD Forum Archive => Topic started by: Bradford "BJ" James on November 06, 2008, 12:14:25 pm

Title: 70 Volt system com vs 70v
Post by: Bradford "BJ" James on November 06, 2008, 12:14:25 pm
Hi,
I'm going over to a local service club to replace their fried 70v mixer/amp. Just curious: since I don't actually have access to the speakers(in the suspended ceiling)is there a way to check to make sure what wire is the common and which is the 70v? All I have access to is the 2 wires coming out of the wall, and they're not labelled. Can I f*** anything up having the wires crossed?
Title: Re: 70 Volt system com vs 70v
Post by: Joseph Macry on November 06, 2008, 04:28:04 pm
Shouldn't matter.  If you get them backwards, you've simply reversed polarity and no one will notice.  As long as everything is equally out of polarity....
Title: Re: 70 Volt system com vs 70v
Post by: Phil LaDue on November 06, 2008, 04:35:45 pm
By suspended ceiling do you mean grid and tile?
Title: Re: 70 Volt system com vs 70v
Post by: Bradford "BJ" James on November 06, 2008, 05:35:42 pm
re ceiling:Yes.
And ya once I got thinking about it, it would only reverse polarity....no biggie.
Thanks,
BJ
Title: Re: 70 Volt system com vs 70v
Post by: Brad Weber on November 06, 2008, 06:06:10 pm
Perhaps a little obvious, but couldn't you simply look at what is connected to what on the existing amp or look at the system documentation?
Title: Re: 70 Volt system com vs 70v
Post by: Hal Bissinger/COMSYSTEC on November 06, 2008, 07:08:36 pm
When I install a system I normally prefer to use wire that has black and red conductors, however I have used black and white if necessary. Either case black is always the common, (-) or ground. This allows you to get the speakers wired the same and in phase. It doesn't matter which way you connect it to the amp  output (assuming only one amp) but good practice is to follow the same convention.

Of course the typical hack job uses lamp cord or something just as bad so if that's the case all bets are off.

-Hal
Title: Re: 70 Volt system com vs 70v
Post by: Bradford "BJ" James on November 06, 2008, 07:10:32 pm
Amp is fried, I'm not sure it was connected properly to start with. It's just a crappy little radio shack amp though- failure was imminent.
Title: Re: 70 Volt system com vs 70v
Post by: Hal Bissinger/COMSYSTEC on November 07, 2008, 11:08:06 am
You just might want to check to see that there are line transformers on each speaker and they are connected properly before you replace that amp. But then again that's just me...

-Hal
Title: Re: 70 Volt system com vs 70v
Post by: Don Boone on November 07, 2008, 03:00:32 pm
It's TOA Z-104 time.

Don
Title: Re: 70 Volt system com vs 70v
Post by: Rain Jaudon on November 07, 2008, 03:20:42 pm
http://www.markertek.com/Product.asp?cat=TESTTOOLS&subca t=&prodclass=AUDIOTEST&baseitem=ZM-104&search=0& amp;off=0&showreturn=0&utm_source=gbase&utm_medi um=shoppingengine&utm_term=&utm_content=&utm_cam paign=sf

TOA ZM-104
Title: Re: 70 Volt system com vs 70v
Post by: Craig Hauber on November 08, 2008, 04:01:49 pm
I've been having good results with this product

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=390- 804

It's intended for loudspeaker box builders, but it will produce a complete frequency versus impedance chart for the entire audio range.
You have to do the math to figure out how many "watts" the line equals but it also shows your exact results including line-loss and attenuators as well as showing the best spot to put your HPF if you want to maximize efficiency.

Another place the device excels is checking resultant impedances on those nasty 8-ohm house installs with all the speakers running off of a conventional home-stereo receiver.



Title: Re: 70 Volt system com vs 70v
Post by: Al Clayton on November 13, 2008, 09:52:27 pm
Don't forget the Goldline ZM1. Actually reads in watts...
My techs prefer it. I prefer the ZM-104 myself. both will do the job!