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I had a gig last week and played (guitar) over a shared 2x12" speaker. After plugging in my stereo amp the sound was "strange" and delayed for about half a second. The sound guy blamed my effect board, but after some investigation I noticed that the speaker had three inputs labeled input, parallel and serial. I accidentally plugged in my amp in the serial and parallel sockets. Taking no chances I switched to a mono setup and plugged into the input input. Sound was back to normal.

Now I am wondering: how is such a three input speaker wired? And how can the strange delay effect be explained?

  • How were you plugged in before you made the change? I'm pretty sure the "parallel" and "serial" jacks were actually outputs, not inputs. – Todd Wilcox Mar 1 '16 at 15:15
  • @ToddWilcox Into the parallel and serial sockets. From my point of view they were the only ones visible and I assumed them to be the "normal" connections. I actually have not considered them to be outputs ... that's probably correct. – fho Mar 1 '16 at 15:18
  • Shared as in one band provided the speaker, all bands used it. – fho Mar 2 '16 at 13:30
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Ok, I think here's what happened:

  • I'm 99% sure the "Parallel" and "Serial" jacks are actually outputs, and they are mutually exclusive options one would use to chain an additional speaker cabinet. Chaining a second cabinet in parallel would lower the total impedance of the system, and chaining one using the serial output would raise the impedance. As impedance matching is important, both options were available.
  • Even though they are outputs, they are connected to the input in a certain way, so you could accidentally use them as inputs and get some sound from the speaker.
  • Because of how you originally plugged in and how those jacks are probably wired, I suspect any audio that was the same in both the left and right channels (i.e., panned center) would be cancelled out, and audio that was different between the two channels (i.e., panned left or right) would still be heard.
  • If you had a delay effect on your rig with the dry sound panned center and delay taps to the sides, then you might have cancelled out the dry sound and only heard the delayed sound, hence the delay in hearing your sound.

Here's how a serial output jack is wired:

enter image description here

Here's how you can have both jacks in one cabinet (imagine the parallel in not being there and the serial out being a switched jack as above):

enter image description here

Images Source

Note that if the input is wired a certain way, the outputs can be used as inputs instead, depending on how you plug in. Even with that in mind, it's better to consider the "Parallel" and "Serial" jacks to be outputs, in general.

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    I would guess the parallel and serial jacks are neither inputs or outputs but alternate ways to connect speakers in series. As such they could function as inputs or outputs depending on which of two connected speakers is receiving the input from the source. They simply allow for chaining two speakers together either in parallel or serial mode. They are probably not designed to be used at the same time and are certainly not for left and right stereo input. – Rockin Cowboy Mar 2 '16 at 1:24

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