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off stage sound problem

Discussion in 'Sound, Music, and Intercom' started by cisgrig, Dec 17, 2008.

  1. cisgrig

    cisgrig Member

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    Location:
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    Our theater has a sound system for the booths, and off stage for hallways, dressing rooms, etc. There have been on going issues with it working intermittently over the years. We've had it worked on but the problem keeps popping up. Now the 'leg' that serves the off stage areas just stopped working. The back hall system has 6 speakers with volume controls. In a previous life I installed telecom systems so I have an understanding of what is suppose to be going on. Here is what I’ve done so far: physical check for damaged cable – ok. Music on the circuit, works fine in the booths, nothing in the halls. then when I used my tone unit to trace the circuit, the inductive amp picked up the music that was playing. But no sound through those speakers. It then quit working too. I’ve isolated this leg from the other two (all three were coming from one com port at the amp) and tried both output ports, nothing. I’m getting 45 ohms on that circuit. Any ideas, suggestions and comments will be appreciated.[media]http://www.controlbooth.com/forums/images/smilies/icon_redface.gif[/media]
     
  2. museav

    museav CBMod CB Mods Departed Member

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    This probably should be in the Sound forum.

    Is this an intercom system or a distributed background music type audio system? I got confused when you mentioned an amp with a com port and multiple output ports as that seems to be referencing both an intercom system and a speaker system, which are two different things.

    Can you tell us the actual equipment and how the system is wired? For example, what is the "amp" noted and what feeds it? Is there any labeling on the output connections of the amp that are used? Are all the output devices speakers or are any intercom stations?
     
  3. cisgrig

    cisgrig Member

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    Thanks, here is some more info:
    equipment: QSC model 1200 stero amp
    2 inputs (com 1 & com 2)
    2 outputs (come 1 & com 2)
    input feed is from our mixer board.
    At the outputs: " 4 ohms min per channel. 8 ohms min in mono bridge mode. Power per channel: 100w 8 ohms / 150w 4 ohms"
    The output is wired with one cable that is then spliced into 3 others that feed different areas of the building. (control booth (1), spot booth (1), off-stage (7)).
    The devices are all speakers with volume control at each one.
    One of the 7 in the back area is a JBL #8130 w a JBl HX95 transformer. All the units I checked earlier had the transformer. Speakers in the booths are JWDavis model #D5805 (no transformers).
    If this is something I can get done over the next month, my boss will think kindly of me.
     
  4. museav

    museav CBMod CB Mods Departed Member

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    That really helps! It sounds like you have a mixed distributed speaker system with (7) 8" ceiling speakers each with a maximum 5 Watt, 25/70V transformer and (2) J.W. Davis speakers which apparently do not use transformers but about which I can find nothing online (I know of J.W. Davis but can't find anything on that particular model).

    The amp sounds like a QSC Model 1200 and there are a few questions that might help understand better how the amplifier is actually being used. To which input connection(s) on the rear of the amplifier is the feed from the audio mixer physically wired? Next to the inputs and above the barrier strip on the rear of the amplifier is an eight position input programming dip switch, how is that set? How are the gain controls above the input connections on the rear panel set? How is the output wiring connected, is it wired to one set of red and black binding posts or is it connected to both red binding posts and not to either black post (the latter reflects bridge mono operation)? Also related to bridge mono operation is the mono bridge switch located next to the inputs and below the barrier strip, is it in the "Bridge" or stereo position?

    You mentioned that the three lines (the two booth speakers and the speakers back stage) were all tied together, are they literally all connected directly to the back of the amp with all three red wires on one binding post and all three black wires on the other?

    This may be difficult, but if you can easily access the JBL ceiling speakers, the transformer attached to the speaker should have several different color wires coming out and a little chart on the top of the transformer showing what those represent. The wiring coming in should attach to two of those internal wires, one going to the common wire and the other to one of the other wires connected to the transformer. Knowing what the transformer says the connection used is (e.g. 5W/70V or something like that) could help. This is not as critical as some of the other information so don't spend a lot of effort if it is difficult to do.

    When you say you measured 45 Ohms, was that with a voltmeter or an impedance bridge? Audio is an AC signal and speakers are a complex load, while a simple DC resistance reading may help in identifying an open or short it does not really reflect the actual AC impedance. However, if the 45 Ohms was measured with an impedance bridge then it could be very useful information.

    Sorry for all the questions but it sounds like you have a system that was either very carefully designed to support a specific load and thus sensitive to any changes or very poorly designed, which that is depends greatly upon the answers to some of these questions.
     
  5. cisgrig

    cisgrig Member

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    "on which input connection(s) on the rear of the amplifier is the feed from the audio mixer physically wired?" #2
    dip switch panel, ON: 1,2,7,8
    Gain controls: both on +3
    out put connection: #2 black and red posts, about 36" it splices into the ones going to CB, SB & off stage. They are all black to black, red to red, no ground.
    mono Bridge switch is in STEREO position

    I'll have the JBL speaker info tomorrow, there is a meeting going on back there.
    the 45 ohms is on a multimeter, ohms setting.

    Keep asking, I'll get answers, can use all the help I can get. I suspect that orginially (15 years ago) this may have been a well designed system, but its been 'worked on', beat on and generally misused since then. I know that most of the volume control switches don't work well. In the long run, can the individual volume switches in the dressing rooms, etc. be taken out? We can control the volume from the mixing board.

    This came to a head when I took out the monitor speaker in the sound booth, it and the light booth were on one of the 3 'legs'. Or at least that's when it wasn't working and I decided to get this fixed.
    Thanks..
     
    Last edited: Dec 18, 2008
  6. Anonymous067

    Anonymous067 BANNED USER

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    Is there a common "building intercom" system that overrides or joins onto your booth feed to the hallway speakers?

    We have a set up like this at our hs, and its a pain in the &^%.
    I've never had the time to look at our individual system, but we have this great lady (sarcasm) who works in the media center who thinks she controls our speakers in the booth/theater system, and she likes to "play" with what she thinks is the "main patch" (which is nothing more than a few plugs in a 2RU panel for the intercom/bell system. Problem is, theres actually locked control boxes that feed our hallway speakers too.
    Like I said, idk how this runs into our amps etc...another story.

    So my question is...is there something else that runs into the amplifier or joins it pre-amp connection (ie...volume controls in the hallway that are before the amp, a building electrical room, etc etc), which might be screwing up your sends?

    Great...now I'm curious how OUR system is set up....thanks. :]. lol.
     
  7. museav

    museav CBMod CB Mods Departed Member

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    Okay, that seems pretty clear that the amp is being run in normal two channel mode.

    It would be unusual but the intent may have originally been to present a load to the amp that equated to a 25V output and then run the corridor speakers as a 25V system. However, that would place a slightly lower impedance load on the amp than recommended. It is also odd that they used Channel 2 and set both gain controls at the same level if it was intended to run the system on a single channel. Nothing really wrong with that but most people would probably just instinctively use Channel 1 and this makes me wonder if there was a problem at some point and someone simply moved everything to Channel 2.

    Because the systems seems to be mixing constant voltage (25/70V with transformers) and low impedance (4/8Ohm) speakers, my first thought is to separate the backstage speakers from the booth speakers across the two amp channels and put a transformer on the amp output feeding the backstage speakers. The dip switches on the back of the amp let you parallel the inputs so what is coming in on either input appears on both outputs, just engage dip switches 5 and 6 for this, so that would get the signal to both channels without any rewiring of the inputs. However, this approach depends on two things. First, it assumes that both channels of the amp are working properly, which would have to be confirmed. Second, what transformer to use for the amplifier output depends on whether the speakers are run as 25V or 70V, although in many cases they might be able to run as either just with a different 'tap' or power rating. This is what I hoped to confirm by seeing how the existing speakers are wired. But something like a Rane TF170 or QSC OT-300A transformer would probably work with the Model 1200 and are about $60 to $90.

    An impedance bridge is a measurement device that applies an audio signal to an internal reference circuit that when connected to an external device or circuit can then be used to measure the impedance of that external device. Here is one of the more common models, Impedance Meter.
     
    cisgrig and (deleted member) like this.
  8. cisgrig

    cisgrig Member

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    Location:
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    for Blah067 - it is a stand alone system. good luck with yours!

    Museav:
    here is what is on the speaker transformer
    C25v 70.7V
    JBL HX95
    5w 2w 1w 0.5w

    the leads into transformer:
    red to red black to black green/brown/brown (from other side of transformer) to blue

    output side:
    gray to volume control blk, comes back on grnd to speaker. red/Red to speaker.

    my first thought is to separate the backstage speakers from the booth speakers across the two amp channels and put a transformer on the amp output feeding the backstage speakers.
    Seemed like a logical thing to do. Can I just put a speaker on out 1 and see what I get? And can I leave out the volume controls at the speakers?

    Thanks
     

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