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Brian Eno Third Speaker

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Brian Eno Third Speaker

Postby forumuser931182 » Wed Nov 04, 2020 11:43 am

Just listening to a bit of ambient music to calm the nerves and remembered a diagram on the back of a Brian Eno record that showed how to connect a third "ambient" speaker. I did this back in the day - feeding the positive output of both channels of a power amp to third speaker. Very cool.
Now I only have active speakers so not so easy I guess. Would I be able to replicate this by sending line level to an xlr input on a third speaker - left channel to pin2 (+) and the right channel to pin3 (-) ? Would I perhaps need to invert one of the channels first ?
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Re: Brian Eno Third Speaker

Postby Hugh Robjohns » Wed Nov 04, 2020 12:13 pm

The 'ambient speaker' thing worked by sending the stereo difference signal to the third speaker. As it was wired across the two positive amp terminals, current only flowed through the third speaker when there was a difference in voltage at those two terminals.

Central sounds (vocals, kick drums, bass, etc) produce the same voltage at both terminals, so they don't appear in the third speaker, but anything panned hard to one side, or anything with a large out-of-phase element (like reverbs and some chorus effects) would appear in it, and thus add to the 'ambient' or 'immersive' effect.

With active speakers you just need to generate the stereo difference signal, and feed that to the third speaker.

If you're using a DAW the easiest way to do that is with an MS plugin. Duplicate the stereo source track in some way and insert an MS plugin across the duplicated tracks. The original stereo track goes to the main speakers as usual. The output from the MS plugin will be Mid and Sides signals -- and the Sides signal is the 'stereo difference' feed needed for the third speaker. So ignore the Mid and route the Sides signal to your third speaker.

Obviously, you'll need to arrange for the LR and S signals to track level together somehow if you want to adjust replay volume.

You can do the same thing in the analogue world with a small stereo mixer. Input splits of the left and right channels, flip the polarity of the right channel (you may need to use an adaptor cable with the wiring to pins 2/3 or tip/ring swapped over for that), pan both channels central and route that stereo difference signal to the third speaker.

If you're happy with DIY cable soldering, it is also possible to achieve the same thing in the way you describe, by routing the +ve outputs from the left and right channels to the +/- inputs of the XLR/TRS connector on the speaker (left to + and right to -). The fact that the speaker has a balanced/differential input means it will extract the stereo difference signal in exactly the same way as the old amp-speaker setup.
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Re: Brian Eno Third Speaker

Postby The Elf » Wed Nov 04, 2020 1:53 pm

Some AV surround amps have options for 'four speaker stereo' (at least the Yamaha ones do that I've had over the years), or similar. I have no idea what they are doing, but maybe they are working on similar principles?
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Re: Brian Eno Third Speaker

Postby Hugh Robjohns » Wed Nov 04, 2020 1:56 pm

Yes... and the old Dobly ProLogic (and the DTS equivalent) do much the same too with a stereo difference signal feeding the rear channel(s)...

For four speakers the usual idea is to send L-R to the left back corner and R-L -- or the inverted polarity (L-R) -- to the right back corner...
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Re: Brian Eno Third Speaker

Postby forumuser931182 » Wed Nov 04, 2020 10:12 pm

Thanks for the explanation Hugh. I seem to remember that Brian Eno’s sleeve notes said he didn’t know why it worked but it just sounded good.
I will try the MS trick and then make a permanent xlr lead to make it all easier to function - a special “ambient” lead. I think I found those subsequent fake stereo/quad effects a bit gimmicky, best to stick to the Eno albums probably rather than the entire record collection.
You didn’t mention the extra requirements of candles and incense.
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