Unbalanced XLRs and phantom power

[nargalzius]

Hi guys, I was wondering if it's safe to connect an unbalanced XLR to the mixer when it's got Phantom power turned on.

I ask because I have a regular mono intrument cable (1/4" jack) which I intend to connect to the XLR input of the mixer via a Proel DHMA320 (female mono to male XLR) adapter. No particular reason as to WHY I want or need it this way. I guess I just want to know once and for all hehehehe)

The thing is, phantom power will be ON for my condenser mic as well, I was wondering what are the consequences of plugging in the guitar via the adpator (which probably means pins 2 and 3 are bridged) on that XLR input with phantom power on. Will it fry the guitar, fry the mixer, or will nothing happen? Can I simply ignore that nuance and go about interconnecting and recording like normal? Or if you're familiar with the adaptor, is it indeed bridged, or is it constructed in a way that converts the unbalanced 1/4" to a balanced XLR (if ever possible) - is there any way to find that out beforehand?

And while we're at it, is there any difference in input levels when I use that said fmxlr adaptor on an XLR input compared to just connecting it directly to the 1/4" bal/unbalanced secondary inputs?

[sonicdeviant]

   For safety sake, I would avoid plugging in anything that doesn't need phantom power when you have phantom power activated.  It may or may not fry something.    

Paul and Derek may know more about the technical specifics of this, but I've just been taught not to do it.

[nargalzius]

but I've just been taught not to do it.

Though I wasn't taught anything, I'm pretty much thinking the same thing with regards to the issue.

However, I did try plugging it in with phantom power off, and can say that the input level on the XML is higher than when using the 1/4" jack. So I guess this is the closest to a "legitimate" reason I can get to use the XLR instead of the 1/4" jack.

Hope either of the two can answer this soon, they don't seem to be as active as they were in the forms as they were in the beginning. Hopefully it's just because of projects they have to finish

[sonicdeviant]

Yep, I think they're both busy folks, and I believe Derek just went on vacation.  He mentioned something about being sparse for a bit.   

[sldghamr]

So your mixer doesn't have a 1/4" jack for the channel you want to go into? The best way is to use a direct box, which typically uses a transformer that provides a path for the phantom power on the XLR side that's isolated from the 1/4" side. I wouldn't go straight in with that adapter. If the adapter plumbs pin 2 straight through to the tip on the 1/4", that could be trouble. I'm not exactly sure though.

[miazmaticdotcom]

Phantom power was designed with the idea that it would only be there for devices that needed it, but that it would NOT affect those that didn't.

However in order to do that, it relies on the three-pin XLR cable. If you short two of the pins by converting to mono 1/4", with phantom power on, you might toast something.

[penmachine]

The reason I wasn't so active is that I was on vacation, with little Internet access. I'm surprised I posted to the blog as much as I did.

Anyway, while I personally shut off phantom power when unplugging or plugging in cables, I don't do it for any well-researched reason. I have done a lot of plugging and unplugging with phantom power on, usually at live gigs, with no nasty results -- but then again, that's usually with bulletproof dynamic mics, and a single tough AKG C1000s condenser, too.

In general, it's probably wise to cut phantom power when connecting or disconnecting, but not necessary, especially since XLR connectors separate the pins so widely that they're difficult to short by accident. The previous comment about possible danger from bridged-to-mono 1/4" connectors may be an issue, but also keep in mind that, while we're talking about 48 V, the actual current draw is quite small. The only devices I've heard that risk damage from phantom power directly are some very old ribbon microphones, but if you have any of those vintage things, you probably know to be careful with them, and likely have special connectors and such to work with them anyway.

Wikipedia has a good, if a bit messy, article on the topic:

http://en.wikipedia.org/wiki/Phantom_power

And here's a good piece from Mackie (PDF) that covers phantom power, unbalanced-vs.-balanced, and other stuff:

http://www.mackie.com/pdf/arcane_mysteries.pdf

[sonicdeviant]

Wow.  Cool.  Thanks Derek.  So it's okay to plug my SM57 into the pre with phantom on???!!!  Where the heck did I ever learn 'don't do it?'

I think it was Mackie!  I used to own a CR1604...and it had switchable phantom power.

[nargalzius]

The previous comment about possible danger from bridged-to-mono 1/4" connectors may be an issue, but also keep in mind that, while we're talking about 48 V, the actual current draw is quite small. The only devices I've heard that risk damage from phantom power directly are some very old ribbon microphones

Thanks for the link and this certainly clarifies stuff regarding microphones. But just to make everything clear: the reason I asked is because I wan't to know if it's safe to plug in my Taylor T5 in such a way. As you know such guitars:

1. Have their own preamps
2. Are expensive.

So given, number 1, I'm just concerned wether phantom power would be a problem say in case I go to a venue where I have no choice but to hook directly to the input without an intermediate direct box. Is that power, while small, enough to actually go through the cable, back to the guitar and do something bad to the preamp (hence making #2 a very important factor in my asking)

I understand tha microphones are built in a way that if they don't need phantom power, they simply ignore it. I'm wondering if this holds true as well for instruments with 1/4" jacks that are simply adapated to XLR (should the need arise)

Furthermore, isn't it reasonable to assume that such an adaptor (1/4" to XLR) should already terminate whatever current phantom power brings? I mean I don't think I know of phantom powered devices that use 1/4" jacks... so doesn't the mere fact that they made an adaptor for it mean that they have already included the whole phantom power issue in the equation?

Here's the adaptor BTW:

[penmachine]

I'm neither an electrician or the representative of any manufacturer of this stuff, but my guess is that if it were a serious risk, there'd be all sorts of warnings on the packaging for these things, and I've never seen that.

[sldghamr]

Just curious why you wouldn't always keep a direct box with you? A direct box contains transformers (typically) which also ensures proper impedence matching which can/does effect the tone of the instrument (it's not just a gender bender.) Get a small one and store it in your guitar case, never leave home without it.

[Thusfrommylips:]

I've never seen a mixer without a 1/4 in. input. haha. wow.
I know that I have used phantom power on my mixer with guitars pluged in and nothing has hapened. Likewise with phantom power turned on on my interface.

[nargalzius]

Just curious why you wouldn't always keep a direct box with you? A direct box contains transformers (typically) which also ensures proper impedence matching which can/does effect the tone of the instrument (it's not just a gender bender.) Get a small one and store it in your guitar case, never leave home without it.

You know what? You're right! So the first thing I did this morning was to get me one I got a Behringer GI100 Ultra-G. At least now I can certainly connect it to the balanced XLR input now without having to worry about anything

I've never seen a mixer without a 1/4 in. input. haha. wow.

It's not really an issue of the mixer not having 1/4", take this scenario for instance: You happen to be in a crappy venue, like a really old church in a 3rd world country (where I am) and the choir loft only has XLRs snaked to the mixer at the opposite end of the church because we're pretty big on vocal choirs here instead of full band church music.

[Thusfrommylips:]

ahhh i see, but hahahaha odd chances. but i totally get you

[DaveDudek]

Wow.  Cool.  Thanks Derek.  So it's okay to plug my SM57 into the pre with phantom on???!!!  Where the heck did I ever learn 'don't do it?'

I think it was Mackie!  I used to own a CR1604...and it had switchable phantom power.

Funny, in another thread about mixers, I said exactly the same thing about my SM-57 and my Mackie mixer.  Mackie's documentation warns agains using phantom power with mics that don't require it, or at least that's how I interpreted it.  I'll have to look up the exact wording they used.

[sonicdeviant]

Yes, it may be completely safe to do it, but my workflow style isn't hindered one way or the other; therefore, I switch on phantom power only when my MXL is plugged in.  Better safe than sorry, so I go with the Mackie approach. 

[penmachine]

There may be a slight (very slight) danger when PLUGGING or UNPLUGGING mics with phantom power live, just because of the off chance of shorting a connection, but I know of no modern mic that can't handle phantom power once the connectors are seated. And as I said on the other forum, it's pretty damn hard to break an SM57 under any circumstances, sometimes even if you try.