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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Installed Sound/Contracting => Topic started by: Scott Hofmann on August 29, 2019, 02:48:33 pm

Title: DC Relay in a 70v System?
Post by: Scott Hofmann on August 29, 2019, 02:48:33 pm
I want to incorporate a TOA  S-04S(T) chime module in an existing TOA 900 series mixer/amp in an auditorium’s 70v show relay/paging system. The chime will obviously go to all the areas of the theatre that have show relay speakers already. The catch is that the lobby does not have any existing 70v speakers and that’s what really needs the chime signal for start of show and intermission.
There is a show relay speaker in the lobby box office though, so the wiring is accessible and it would be relatively easy to add a couple of speakers in the lobby.

But here’s the issue…. I don’t want the show relay and paging constantly going thru the “new” lobby speakers, just the chime. So my idea would be to send a low-voltage DC signal on the speaker wires (system-wide) which would operate a relay to close the speaker connection to the new speakers only when someone pushed a DPST momentary chime button. The 70v transformers would block the DC from getting to all the speakers.

Is there a reason this wouldn’t work and would I need to use capacitors to keep from feeding the DC back into the mixer/amplifier’s output terminals?
Title: Re: DC Relay in a 70v System?
Post by: Scott Holtzman on August 29, 2019, 02:55:10 pm
I want to incorporate a TOA  S-04S(T) chime module in an existing TOA 900 series mixer/amp in an auditorium’s 70v show relay/paging system. The chime will obviously go to all the areas of the theatre that have show relay speakers already. The catch is that the lobby does not have any existing 70v speakers and that’s what really needs the chime signal for start of show and intermission.
There is a show relay speaker in the lobby box office though, so the wiring is accessible and it would be relatively easy to add a couple of speakers in the lobby.

But here’s the issue…. I don’t want the show relay and paging constantly going thru the “new” lobby speakers, just the chime. So my idea would be to send a low-voltage DC signal on the speaker wires (system-wide) which would operate a relay to close the speaker connection to the new speakers only when someone pushed a DPST momentary chime button. The 70v transformers would block the DC from getting to all the speakers.

Is there a reason this wouldn’t work and would I need to use capacitors to keep from feeding the DC back into the mixer/amplifier’s output terminals?

I would put the DC blocking cap on the amp for safety.  Other than that you have described how a phone works, DC signalling AC audio....Should work fine
Title: Re: DC Relay in a 70v System?
Post by: Kurt J Nemer on August 29, 2019, 03:10:58 pm
Probably will get an audible pop when you apply the DC, might not be problem though.
 I"d be concern about the DCR of all those 70V transformers in parallel, might look a lot like a short circuit to DC.

If both speaker leads are ungrounded maybe set it up like phantom power. Apply the DC from a transformer "center tap" to ground.
Title: Re: DC Relay in a 70v System?
Post by: Marc Sibilia on August 29, 2019, 03:12:53 pm
Is there a reason this wouldn’t work and would I need to use capacitors to keep from feeding the DC back into the mixer/amplifier’s output terminals?

You can't apply DC directly to the transformer.  While it is true that the DC won't pass, it can cause saturation in the primary of the transformer, which will at a minimum cause distortion, and if the saturation of the iron is complete, will cause the amplifier will see a near dead short (the very low DC resistance of the primary instead of the transformed by the turns ratio squared impedance of the load connected to the secondary).  You will need a large enough capacitor to pass the audio into the transformer and block the DC.
Title: Re: DC Relay in a 70v System?
Post by: David Allred on August 29, 2019, 03:32:56 pm
My solution 1) Play a chime track through the system.
My solution 2) Buy a wireless doorbell (chime).
Title: Re: DC Relay in a 70v System?
Post by: Hal Bissinger/COMSYSTEC on August 30, 2019, 02:26:48 am
How hard would it be to run the wiring for the new speakers back to the amp location and provide a relay there?
-Hal
Title: Re: DC Relay in a 70v System?
Post by: Scott Hofmann on August 30, 2019, 10:17:42 am
How hard would it be to run the wiring for the new speakers back to the amp location and provide a relay there?
-Hal
After reviewing all the great posts, I think the best solution would be to add a small amp with the chime module at the amp rack location and run a speaker line from it to two new lobby speakers, which will not put show relay/paging into the new speakers. Also take the line out of this amp and feed into an input on the existing mixer/amp to get the chime signal to all the existing 70v show relay/paging speakers.
Thanks for the great discussion!
Title: Re: DC Relay in a 70v System?
Post by: Scott Hofmann on August 30, 2019, 10:22:31 am
My solution 1) Play a chime track through the system.
My solution 2) Buy a wireless doorbell (chime).
I appreciate your suggestions but....
Solution 1: show relay/paging will still go to the lobby which is not desired.
Solution 2:they have just put about a dozen of these all over the place (without my input) and you can barely hear them, especially in the lobby! Hence the need for a new plan.
Title: Re: DC Relay in a 70v System?
Post by: David Allred on August 30, 2019, 01:05:08 pm
I appreciate your suggestions but....
Solution 1: show relay/paging will still go to the lobby which is not desired.
Solution 2:they have just put about a dozen of these all over the place (without my input) and you can barely hear them, especially in the lobby! Hence the need for a new plan.

Maybe you could put a relay to turn on and off those speakers.   ;D

The doorbell would work in my local community theatre lobby.  You must be on a larger scale. 
Title: Re: DC Relay in a 70v System?
Post by: Scott Hofmann on August 30, 2019, 03:26:24 pm
Maybe you could put a relay to turn on and off those speakers.   ;D

The doorbell would work in my local community theatre lobby.  You must be on a larger scale.
1500 seat auditorium.
Title: Re: DC Relay in a 70v System?
Post by: Hal Bissinger/COMSYSTEC on August 30, 2019, 10:22:52 pm
After reviewing all the great posts, I think the best solution would be to add a small amp with the chime module at the amp rack location and run a speaker line from it to two new lobby speakers, which will not put show relay/paging into the new speakers. Also take the line out of this amp and feed into an input on the existing mixer/amp to get the chime signal to all the existing 70v show relay/paging speakers.
Thanks for the great discussion!
I was going to suggest that but I was trying to save you a few bucks!  :) No problem with doing it that way.
-Hal
Title: Re: DC Relay in a 70v System?
Post by: Scott Hofmann on August 30, 2019, 11:46:51 pm
I was going to suggest that but I was trying to save you a few bucks!  :) No problem with doing it that way.
-Hal
I was initially trying not to run new speaker cable and get another amplifier, but most of the time the best solution is not the cheapest, as you know!
Title: Re: DC Relay in a 70v System?
Post by: brian maddox on August 31, 2019, 02:55:15 pm
I was initially trying not to run new speaker cable and get another amplifier, but most of the time the best solution is not the cheapest, as you know!

I feel like a lot of my conversations here can be summed up...

Me:  I want to do something clever, but kinda lazy/cheap.

PSW:  Don't

Me:  ok