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Could phantom power damage a PC's speaker output?

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Could phantom power damage a PC's speaker output?

Postby aekoi » Sat Mar 21, 2020 11:54 am

Hi there.

I am using a small mixer for online tuition. I have a condenser mic which requires the desk's universal phantom power to be turned on. I am also feeding another mic input from the speaker output of my PC, using 'one side' of a minijack to twin XLR male cable. (PC has bog standard motherboard outputs, I have no soundcard).

Could this method allow phantom power to damage the speaker output on my PC? I have plenty connectors to make alternative cables if required.

Thanks for any help you can give.
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Re: Could phantom power damage a PC's speaker output?

Postby ef37a » Sat Mar 21, 2020 12:48 pm

Yes, I think you probably could damage the headphone/line drive chip in the laptop.

This is about the only area where phantom power can cause a problem. Mostly people worry about a non-problem but feeding ~48 volts into things that were never intended to get it is asking for trouble.

There will be an AC coupling capacitor (though maybe not as you might think be in the 'hot' pin) but it will be the 'wrong way around' i.e. phantom power will put +V on the -ve end and the voltage rating will be far too low. I doubt any laptops fit 63V non-polarized caps?
Best solution is a transformer. Is the quality critical? If not a wee 1200 from OEP would do. Or, Amazon do a twin ch earth isolator for about a tenner and you could mod that.

You could of course put a capacitor in the line. 2.2mfd electrolytic at 63V should do assuming a mic input impedance of 2k and you don't want a lot of bass? The cap of course should have the +V end connected to phantom power and a 'drain' resistor of about 22k connected to the -ve end and ground. But! I still think there is the possibility of a voltage surge and I doubt laptop jacks are protected very well, if at all?

Dave.
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Re: Could phantom power damage a PC's speaker output?

Postby aekoi » Sat Mar 21, 2020 12:50 pm

Many thanks Dave. Putting any DI box between the mic pre and the PC speaker out would isolate it from phantom also wouldnt it?
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Re: Could phantom power damage a PC's speaker output?

Postby aekoi » Sat Mar 21, 2020 1:01 pm

I've just raked out my old Joe Meek MQ3 box which can deliver phantom, so I can leave my desk phantom off.
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Re: Could phantom power damage a PC's speaker output?

Postby ef37a » Sat Mar 21, 2020 1:06 pm

aekoi wrote:Many thanks Dave. Putting any DI box between the mic pre and the PC speaker out would isolate it from phantom also wouldnt it?

Yes it would but you would still have to be careful how you 'started things up' IMHO as a pulse could get to stuff designed to live with just five volts!

NO spook juice at all as you are now going to do is best of all.

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Re: Could phantom power damage a PC's speaker output?

Postby Hugh Robjohns » Sat Mar 21, 2020 1:15 pm

Yes, damage is a real possibility. A di box is the best solution, or use a separate preamp as you now are.
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Re: Could phantom power damage a PC's speaker output?

Postby aekoi » Sat Mar 21, 2020 1:39 pm

Thank you both for the quick and helpful replies.
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Re: Could phantom power damage a PC's speaker output?

Postby James Perrett » Sat Mar 21, 2020 3:11 pm

I don't know which mixer you are using but does it not have a line input on the channel you are using? On most mixers phantom power isn't fed to the line input - only the mic input (but it is always best to check in case yours is one of the odd designs where phantom power is fed to both).
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Re: Could phantom power damage a PC's speaker output?

Postby MOF » Sat Mar 21, 2020 3:25 pm

On most mixers phantom power isn't fed to the line input - only the mic input (but it is always best to check in case yours is one of the odd designs where phantom power is fed to both).

I’ve never come across a mixer where phantom power is applied at the line input stage.

The title of this post is confusing, my initial thought was ‘No’ because where would the phantom power come from since mixer outputs don’t have phantom power. Closer reading showed that it was the PC headphone output going into the mixer’s mic’ input that was the issue.
Is it a separate PC being fed for the online function, if not then won’t there be some feedback issues here?
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Re: Could phantom power damage a PC's speaker output?

Postby Hugh Robjohns » Sat Mar 21, 2020 3:37 pm

MOF wrote:I’ve never come across a mixer where phantom power is applied at the line input stage.

I can't instantly recall a mixer like that, although I have a nagging thought that I have reviewed at least one that did... but I've definitely come across that problem on more than one preamp.

...And I've edited the thread title to avoid further confusion.
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Re: Could phantom power damage a PC's speaker output?

Postby Kwackman » Sat Mar 21, 2020 4:03 pm

Hugh Robjohns wrote:I can't instantly recall a mixer like that, although I have a nagging thought that I have reviewed at least one that did

This one?
https://www.soundonsound.com/reviews/sc ... ular-mixer
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Re: Could phantom power damage a PC's speaker output?

Postby Hugh Robjohns » Sat Mar 21, 2020 4:53 pm

:clap: Well remembered! :D

This was one of the preamps that had phantom on line inputs

https://www.soundonsound.com/reviews/trident-audio-s20
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Re: Could phantom power damage a PC's speaker output?

Postby MOF » Sat Mar 21, 2020 6:42 pm

Hugh Robjohns wrote:
I can't instantly recall a mixer like that, although I have a nagging thought that I have reviewed at least one that did

This one?
https://www.soundonsound.com/reviews/sc ... ular-mixer

Well I never!!! Having just bought a ribbon mic’ I’m going to have to be careful. The instructions do say not to plug it in until at least a minute after phantom power has been switched off.
In theory they should be fine with power on but I do remember working on AW Pet series two when Jimmy Nail was singing into an old ribbon mic’ we had and we’d get loud banging noises as the ribbon arced across to the magnet, once power was switched off it was fine.
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Re: Could phantom power damage a PC's speaker output?

Postby Hugh Robjohns » Sat Mar 21, 2020 7:02 pm

MOF wrote:I do remember working on AW Pet series two when Jimmy Nail was singing into an old ribbon mic’ we had and we’d get loud banging noises as the ribbon arced across to the magnet

As you say, the vast majority of ribbon mics are absolutely fine with phantom power on the line. I must have hot-plugged hundreds of Coles 4038s over the years and they all worked! ;-)

Most ribbon mics employ a step-up transformer to boost the output level, which would keep the phantom power away from the ribbon itself. In the case you describe the two ends of the ribbon must have been connected directly to the output XLR, with the magnet assembly connected to the cable screen. Consequently, if the ribbon got too close to (or more likely, actually touched) the magnet assembly, the phantom supply would be shorted out with arcing and audible thumps, splats, and bangs.

With an old and abused ribbon mic it's possible the ribbon itself could have sagged or stretched and so would be at greater risk of hitting the metalwork around it!
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Re: Could phantom power damage a PC's speaker output?

Postby MOF » Mon Mar 23, 2020 7:43 pm

the ribbon got too close to (or more likely, actually touched) the magnet assembly, the phantom supply would be shorted out with arcing and audible thumps, splats, and bangs.

With an old and abused ribbon mic it's possible the ribbon itself could have sagged or stretched and so would be at greater risk of hitting the metalwork around it!

I’ve just done a quick image search Hugh https://www.proaudioeurope.com/info/cla ... s/stc-4033 and I think it was a STC 4033, I think they were used in booms at ATV before it became Central TV, we used the AKG 451 by the time I joined.
It was an old mic then, and maybe the ribbon had sagged, I don’t ever remember being told to keep it upright which is what I was instructed to do with my recent ribbon mic’ purchase.
That was the only time I remember it ever being used.
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