ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: [1]   Go Down

Author Topic: 70v question  (Read 3373 times)

RogerOwens

  • Newbie
  • *
  • Offline Offline
  • Posts: 18
70v question
« on: July 17, 2005, 03:19:13 pm »

I've been getting some hum for our 70v speakers in the lobby and cry room lately. It has happened when the amp was on and off. Both times nothing was being sent to the amp and turning the console off had no effect. Usually I can run some music through the speakers and it stops but inexplicably comes back usually about an hour later. I don't know if this makes a difference but the speakers in the cry room have an inline volume control. The amp is a crown reference and doesnt show any faults.  
Logged
Roger Owens
FOH Engineer/Tech Director
New World United Methodist Church
Arlington, TX

Ivan Beaver

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 963
Re: 70v question
« Reply #1 on: July 17, 2005, 04:25:09 pm »

I would look for any connections between the 70V wires and ground-such as conduit runs.  Disconnect the 70V wires and measure with an impedance meter (use an ohmmeter if you do not have an impedance meter) and measure resistance/impedance to electrical ground.  I have seen systems hum even when off when there is a ground loop between them and the building AC.  I actually had this happen to me (it took awhile to figure it out) in which the monitors were humming when the system was off.  Turned out one of the floorpockets had a non-insultated 1/4" jack for the monitors and the pocket was screwed into the wooden stage deck.  1 screw went all the way through to the steel studs below and there was my loop!  Removed it and the system was fine.
Logged
Can I have some more talent in the monitors--PLEASE?

Ivan Beaver
dB Audio & Video Inc.

Jimbob

  • Newbie
  • *
  • Offline Offline
  • Posts: 2
Re: 70v question
« Reply #2 on: August 19, 2005, 06:44:29 pm »

 The source of your hum is from an input device or a bad/unshielded or defective input cable.  Amplifier outputs to speakers (8 Ohm or 70V) are impervious to almost all interference.  70V outputs are subect to ground loops. Isolate your 70V speakers from electrical ground.
Logged

John Roberts {JR}

  • Full Member
  • ***
  • Offline Offline
  • Posts: 106
Re: 70v question
« Reply #3 on: August 19, 2005, 08:04:07 pm »

Jimbob wrote on Fri, 19 August 2005 17:44

 The source of your hum is from an input device or a bad/unshielded or defective input cable.  Amplifier outputs to speakers (8 Ohm or 70V) are impervious to almost all interference.  70V outputs are subect to ground loops. Isolate your 70V speakers from electrical ground.


Dear Jimbob: Please read Ivan's response just before yours. He has given the original poster good advice. Hum and even interference can find it's way into long speaker runs. Hum is most likely from ground contamination. For example if the building's ground currents comingle with the speaker ground run a noise voltage proportional to that current will be added effectively in series with the amplifier output.

Stranger things than speakers that hum even when turned off can and do occur.

JR
Logged

ProSoundWeb Community

Re: 70v question
« Reply #3 on: August 19, 2005, 08:04:07 pm »


Pages: [1]   Go Up
 



Page created in 0.083 seconds with 24 queries.