Switching jacks on PA head

Discussion in 'Bad Dog Cafe' started by trxx, May 28, 2020.

  1. trxx

    trxx Tele-Afflicted

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    I'm looking at a block diagram of an old PA head trying to figure out if plugging into the power amp inputs (marked in red) breaks the connections from the mixer outputs to the internal amps' inputs. It looks to me that the outputs from the mixer are at the same nodes as the left and right line outputs, so I don't see what is actually being switched. What am I missing?

    I'm trying to figure out if I can send the mixer line outputs to a crossover and back in to the internal amps without having the unprocessed signals (no crossover) mixed with the processed signals (crossover signals). It seems that those switching jacks should work that way, but when looking at the diagram I don't see how that would happen.

    switching_jacks.png
     
  2. Rich_S

    Rich_S Friend of Leo's Silver Supporter

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    It’s kinda fuzzy, but yes... plugging into the power amp inputs will disconnect them from their assigned mixer busses.

    The diagram is made more confusing by the extra “tip” connections shown. I’m not sure why they put those on there. They almost look like TRS connectors, but if you try to figure out the circuit that way? It makes no sense.
     
  3. trxx

    trxx Tele-Afflicted

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    I think I see, and those jacks aren't going to do what I want. They seem to just allow for injecting external signals to be mixed with the internals signals from the mixer section. To do what I want, I would have to somehow break the traces between the mode select switch and the power amp inputs when plugging into the main line outputs. Like this.

    switching_jacks_mod.png
     
  4. trxx

    trxx Tele-Afflicted

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    I'm not so sure. I would think it should work that way, but according to the diagram, it doesn't appear so to me. But then, what would be the point of having switching jacks at all if they don't break the signals from the mixer? I'm waiting on the thing to arrive, so I'll see when it gets here.
     
    Last edited: May 28, 2020
  5. trxx

    trxx Tele-Afflicted

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    I guess I could go ahead and get a crossover. Worst case, I have to resell the thing.
     
  6. trxx

    trxx Tele-Afflicted

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    Looking at the manuals for similar powered mixers of the same brand (older and newer) I see the same switching jack symbols being used, and some of those manuals mention that the power amp in switching jacks do actually break the signal from the mixer to the amps. So I think it will be good to go. I guess it's just an odd choice of schematic symbol for those jacks that doesn't really show how those jacks work.
     
  7. Peegoo

    Peegoo Friend of Leo's

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    The 'extra tip connection' in the diagram is there to show actuation of the switch contact that disengages the mixer bus when there's a plug in the jack. It's not how a two-conductor switching jack appears in most schematics.

    I agree--it is clumsy artwork that creates confusion.
     
  8. trxx

    trxx Tele-Afflicted

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    Well, damnit. Now I went and bought a real mixer too. The deal was too good. I could always sell the PA head and buy another power amp, or just keep it around as a grab and go mixer/amp, use it as an amp, and a backup.
     
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