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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Audio Measurement and Testing => Topic started by: Mark Wilkinson on November 18, 2015, 02:16:54 pm

Title: Phantom power ok feeding back into mixer outputs?
Post by: Mark Wilkinson on November 18, 2015, 02:16:54 pm

I'm using a UMC 404 for Smaart I-O.   Phantom power is either on or off for all 4 of the unit's inputs.

So if I use Input 1 as the reference channel with a line level signal coming from a mixer or a processor,
and use the remaining channels for measurement mics with +48v engaged,
aren't I also sending +48 v back into the mixer's or processor's output?


It looks like it must not hurt the mixer or processor based on the Smaart I-O devices I see people are using....
....but it it completely harmless, and if so, how so?   

Many thx.......continue to realize how little I know.......!
Title: Re: Phantom power ok feeding back into mixer outputs?
Post by: Steve M Smith on November 18, 2015, 03:22:34 pm
Yes, but there should be capacitors in line with the outputs which will not pass the dc.


Steve.
Title: Re: Phantom power ok feeding back into mixer outputs?
Post by: John Roberts {JR} on November 18, 2015, 04:00:03 pm
Most well engineered professional products will tolerate several kinds of abuse, in general it is not a good practice.

JR
Title: Re: Phantom power ok feeding back into mixer outputs?
Post by: Keith Broughton on November 18, 2015, 04:05:49 pm
I'm using a UMC 404 for Smaart I-O.   Phantom power is either on or off for all 4 of the unit's inputs.

So if I use Input 1 as the reference channel with a line level signal coming from a mixer or a processor,
and use the remaining channels for measurement mics with +48v engaged,
aren't I also sending +48 v back into the mixer's or processor's output?


It looks like it must not hurt the mixer or processor based on the Smaart I-O devices I see people are using....
....but it it completely harmless, and if so, how so?   

Many thx.......continue to realize how little I know.......!
I am using the same device and I send the signal in on a TRS 1/4 connector instead of the XLR.
The TRS is a line/instrument level input.
While the phanton may not be a problem, in this case, I have had problems with other devices getting phantom on an output.
Title: Re: Phantom power ok feeding back into mixer outputs?
Post by: Rob Spence on November 18, 2015, 07:16:52 pm
My LS9 goes a bit crazy with awful noises if I put phantom into an omni out. I don't do that anymore and, at least for me, nothing got damaged.


Sent from my iPad using Tapatalk HD
Title: Re: Phantom power ok feeding back into mixer outputs?
Post by: Mark Wilkinson on November 19, 2015, 09:39:08 am
I am using the same device and I send the signal in on a TRS 1/4 connector instead of the XLR.
The TRS is a line/instrument level input.
While the phanton may not be a problem, in this case, I have had problems with other devices getting phantom on an output.

Thanks all,

Keith, great!
 
I always pictured  XLR/1/4 combination connectors  as simply having the TRS's tip-ring-sleeve  in parallel with XLR pins 2-3-1.
It's nice to finally realize that the TRS is just for line level and doesn't connect. phantom.

Is this generally true for all XLR/1/4 combo connectors, other manufacturers?
Title: Re: Phantom power ok feeding back into mixer outputs?
Post by: Arthur Skudra on November 19, 2015, 01:41:27 pm
Likewise, going the other way I discovered that the line output of my Roland Octacapture is *not* happy when phantom power is present on the connection, it goes into a really weird oscillation.  So if you are plugging the output of some measurement interfaces either to a mic input on a mixing console, or a DSP input, make sure the phantom power is turned off for that input.  YMMV.
Title: Re: Phantom power ok feeding back into mixer outputs?
Post by: Steve M Smith on November 19, 2015, 03:59:40 pm
A properly designed piece of equipment with both XLR and TRS inputs on the same channel should not have the phantom power appear on the TRS input.  It's quite trivial to use the internal switching contacts within a jack socket to accomplish this.

However, that's not to say that it never happens.  There is most likely equipment around with TRS just parallel connected to the XLR.


Steve.
Title: Re: Phantom power ok feeding back into mixer outputs?
Post by: Primoz Vozelj on November 19, 2015, 07:00:06 pm
Mixed a show with X32 rack on stage sending LR to XLR inputs of analogue house console with global phantom power switch turned on. No problems.
Title: Re: Phantom power ok feeding back into mixer outputs?
Post by: Lyle Williams on November 20, 2015, 04:43:14 am
In one application where I don't want 48v, I have a bit of plastic jambed into the circle of the XLR combo jack to prevent accidental insertion of a connector that would carry 48v.
Title: Re: Phantom power ok feeding back into mixer outputs?
Post by: TJ (Tom) Cornish on November 26, 2015, 10:37:20 am
I'm using a UMC 404 for Smaart I-O.   Phantom power is either on or off for all 4 of the unit's inputs.

So if I use Input 1 as the reference channel with a line level signal coming from a mixer or a processor,
and use the remaining channels for measurement mics with +48v engaged,
aren't I also sending +48 v back into the mixer's or processor's output?


It looks like it must not hurt the mixer or processor based on the Smaart I-O devices I see people are using....
....but it it completely harmless, and if so, how so?   

Many thx.......continue to realize how little I know.......!
Depends on the mixer. My personal experience - A&H GLD is OK, Yamaha 01v96 OK but sometimes makes pops, Soundcraft Spirit 8 - very not OK - blown output.
Title: Re: Phantom power ok feeding back into mixer outputs?
Post by: Jerome Malsack on December 02, 2015, 01:40:09 pm
When in doubt use the hum blockers. 

Ebtech, Behringer HB100,  transformer 1:1 line.