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Old 18th March 2004, 07:53 PM   #1
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Default Phantom Menace

If you know of a better forum for this with enough technical knowledge please tell me

I am putting together a small reinforcement gig for my band. I just bought a small Yamaha 16/6FX mixer,and wanted to use my old 30meter stage cable with it. On test, it works just fine -passively, but if I try +48V, noise and mains ripple hum ( no mics connected).
Q1 - does anyone actually USE phantom power live?
Q2 - why is it happening?

Notes output - Yamaha 46.2V (compared with, Folio Notpad 47, JoeMeek 42V). Tried the Yam on an osciloscope - no ripple at all. Just some white noise. Cable is a 30m 25way cable with a single screen from a metal stage box at one end. It breaks out to 12 XLRs at the other. Screens are joined to a central point. pin1- earth, pin2 hot, pin3 cold. Phantom +ve on 2 & 3, -ve on 1. No shorts between wires. Resistance end to end =1.2ohms.

I don't get a hum with 5m microphone cables. The hum reduces slightly if I connect a condencer mic. This cable does not hum if I use a different desk.
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Old 18th March 2004, 09:10 PM   #2
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You bought a sick mixer.

Does this happen with only one channel or do all channels act the same?

Might try recapping the power supply. Old electrolytic caps dry out and stop filtering hum.

Phil
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Old 18th March 2004, 09:45 PM   #3
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Well that is a possibility that occured to me. But In what regard "sick"? The scope showed me No AC ripple on the DC supply. Its not under volts conpared to other sources, and It doesn't do it on other cables.
The mixer is under guarantee so it can go back but, before we jump to conclusions, can you figure out what is actually happening?
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Old 19th March 2004, 11:19 AM   #4
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I suspect that you old snakes earthing scheme is incompatable with the desk. You might want to take your desk back to the shop and try it with a proper individually screened multicore, and see if this makes a difference.
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