Strange Level / Phantom Power problems

moink123 · 21st November 2009, 04:27 PM

I have a strange problem while using unbalanced split stereo signals into remote boards. I've experienced this with the Cooper 208D, Sound Devices 302, and the Audio Developments AD114.

When adapting a stereo 1/8" input (from an iPod, laptop, etc...) to L & R unbalanced XLR, the signal often comes through extremely thin and tinny. For some reason, this is remedied by turning on the phantom power on both channels.

Can anybody tell me why phantom power fixes my very strange problem?

Jim vanBergen · 22nd November 2009, 04:53 AM

The voltage being applied actually provides a differentially balanced circuit. It buffers the signal, and provides common mode rejection.

Here's a decent link that is accurate:

Phantom Power Explained

Hope this helps!

ghellquist · 22nd November 2009, 12:43 PM

Want to understand this as well.

Could you elaborate on how your cables are connected? I would probably make cables as this:

First channel
tip of 1/8 goes to pin 2 of XLR
ground of 1/8 goes to pin1 and 3 of XLR

Second channel
middle ring of 1/8 goes to pin 2 of XLR
ground of 1/8 goes to pin 1 and 3 of XLR.

Sometimes it may be better to connect to pin 1 of only one of the XLR-s as it does create a (very small but still) loop. Very rarely it is better to not connect pin 1 at all, generally when both units share a ground connection some other place. This would also be the option to use if you cannot control phantom power separately.

Note that cables connected like this should not have phantom power on -- they shortcircuit pin 1 and 3 (nothing will be destroyed, but the power supply might sag in voltage, not that it should but things are not always up to spec). There is no free lunches really, the only way to avoid the problem is to have transformers in the cable.

// Gunnar

21st November 2009, 04:27 PM	#1
moink123 Gear Head Join Date: Mar 2008 Posts: 35	Strange Level / Phantom Power problems I have a strange problem while using unbalanced split stereo signals into remote boards. I've experienced this with the Cooper 208D, Sound Devices 302, and the Audio Developments AD114. When adapting a stereo 1/8" input (from an iPod, laptop, etc...) to L & R unbalanced XLR, the signal often comes through extremely thin and tinny. For some reason, this is remedied by turning on the phantom power on both channels. Can anybody tell me why phantom power fixes my very strange problem?

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22nd November 2009, 04:53 AM	#2
Jim vanBergen Lives for gear Join Date: Apr 2005 Location: New York Friggin' City Posts: 2,525	The voltage being applied actually provides a differentially balanced circuit. It buffers the signal, and provides common mode rejection. Here's a decent link that is accurate: Phantom Power Explained Hope this helps!

22nd November 2009, 12:43 PM	#3
ghellquist Gear addict Join Date: Jan 2006 Location: Stockholm Sweden Posts: 335	Want to understand this as well. Could you elaborate on how your cables are connected? I would probably make cables as this: First channel tip of 1/8 goes to pin 2 of XLR ground of 1/8 goes to pin1 and 3 of XLR Second channel middle ring of 1/8 goes to pin 2 of XLR ground of 1/8 goes to pin 1 and 3 of XLR. Sometimes it may be better to connect to pin 1 of only one of the XLR-s as it does create a (very small but still) loop. Very rarely it is better to not connect pin 1 at all, generally when both units share a ground connection some other place. This would also be the option to use if you cannot control phantom power separately. Note that cables connected like this should not have phantom power on -- they shortcircuit pin 1 and 3 (nothing will be destroyed, but the power supply might sag in voltage, not that it should but things are not always up to spec). There is no free lunches really, the only way to avoid the problem is to have transformers in the cable. // Gunnar