How to REMOVE phantom from XLR cable

[3rd world order]

I'm very interested in this passive summing idea... but I'm trying to avoid having some idiot pull a "hey what's this +48v button do?" and blow something up down the line...

is there any kind of connector (something like a Shure A15AS.. yes i know it's a pad.. i mean the form factor) i can plug in-line to prevent this from happening?

[7 Hz]

The only thing that stops 48v on the line is capacitors. Better get some high quality ones!

[paully]

Originally Posted by 7 Hz

The only thing that stops 48v on the line is capacitors. Better get some high quality ones!

>>>>>>>>

Don't forget a quality transformer. Paul

[JonCraig]

maybe i'm missing something... but if you're mixing the outputs of your daw, why would phantom be an issue? +48 will only appear on the mic input side... on its way out to the mic.

high quality polypropelene (spell) caps supposedly sound best. you'll have to do a bit of research, as the value of the cap & the other circuit impedences (output from DAW and input into summing box) all combine to create a hi-pass filter (i think... maybe lo-pass...) so you have to choose a value of cap that puts that freq drop somewhere where you won't hear it. (like 20Hz or something.)

--jon

[Telecastr]

Originally Posted by JonCraig

maybe i'm missing something... but if you're mixing the outputs of your daw, why would phantom be an issue? +48 will only appear on the mic input side... on its way out to the mic.

My thoughts exactly.

[3rd world order]

it's specific to passive summing with the folcrum... you use mic preamps for makeup gain after the folcrom summing box.

[JonCraig]

hmmm... interesting. have you double-checked with fulcrom to make sure they haven't already put a phantom-defeating circuit (i.e. caps or a transformer) on the outputs of the box? seems like the sort of thing they might think of and build in for ya.

--jon

[3rd world order]

Originally Posted by JonCraig

hmmm... interesting. have you double-checked with fulcrom to make sure they haven't already put a phantom-defeating circuit (i.e. caps or a transformer) on the outputs of the box? seems like the sort of thing they might think of and build in for ya.

yep... it's a straight passive summing box... very minimalist. the phantom thing is one of the "issues".. i'm not exactly certain that engaging the phantom power would blow anything up.. i just don't want to find out the hard way. yaknowimean?

I guess i could use my neve 3114s as makeup gain safely... they don't have phantom... but i was kinda hoping to find an easy solution.

[Third Eye]

Couldn't you just use a ground lift on the XLR (pin 1). Phantom can't work without a ground right??

[keepsound]

Originally Posted by Third Eye

Couldn't you just use a ground lift on the XLR (pin 1). Phantom can't work without a ground right??

You have 3 solutions:
1. Ground Lift on the pin 1 of the XLR (only good if you have to ground lift the signal for hums too)
2. use a 1:1 transformer
3. use two ELCO 100uF 63Volts capacitors on pin 2 and 3 and after them two 4K7 resistors connected to ground. To be shure you can add other two ELCOS wit inverted polarity to the output (first two ELCOS with + to input, then the two resistors to ground, last two ELCOS with + to output)
It's a simplified circuit of this one:
http://www.uneeda-audio.com/abswitch.jpg
Other sources:
http://www.rane.com/note110.html
http://www.aes-section.nl/project%20...Connection.pdf
http://www.mackie.com/pdf/arcane_mysteries.pdf
Ciao

[ulysses]

Of course the 4th good solution would be to just not worry about it. Yes, we built the Folcrom without any blocking caps because it is an intentionally full-tilt minimalist design and we didn't want any filters of any kind in the box. We just don't see phantom power as a significant risk. We put a note about it in the manual so people would know that phantom will pass back through to the Folcrom inputs as kind of a "cover our ass" statement in case anybody ever had a problem with it. So far nobody has had a problem, and I don't expect anybody to have a problem. If you're worried about it, then just don't turn on phantom power on a preamp that's fed by a Folcrom that's fed by gear you're worried about. If it accidentally gets turned on, then just turn it off. Not a big deal. I'd be happy to go into excruciating detail about the technical details of this question, but I'm afraid it will filter through the grapevine and come out the other end as a Grave Danger, just like the overreaction to the ribbon microphones/phantom power question. Remember, it's called phantom power because it goes unnoticed. It works surprisingly well. If you need more information, feel free to give me a call or an e-mail. Thanks.

[3rd world order]

Originally Posted by ulysses

Remember, it's called phantom power because it goes unnoticed. It works surprisingly well.

OK... i guess i was just being cautious. I've got a new baby daughter who LOVES to push buttons... you know how that goes.

[keepsound]

Hi,
phantom power does nothing to dynamic microfones, but can be dangerous to electronic devices like Dats (Tascam at first place) and audio interface cards (Like the PCMCIA VxPocket2 from Digigram).
I've just got underdimensioned condensator explosions (with a little nice white smog inside the device) after a 48 volt powered connection.
So a "ghost" phantom box could be a good idea to prevent damages on devices from where you're not shure if they can survive.
Regards
P.S. I'm sorry form my bad english (I'm italian...)
Italo

[paully]

Originally Posted by ulysses

...Of course the 4th good solution would be to just not worry about it. Yes, we built the Folcrom without any blocking caps because it is an intentionally full-tilt minimalist design and we didn't want any filters of any kind in the box. We just don't see phantom power as a significant risk...

>>>>>>>>>

The risk is not to the external hardware, but to the power supply itself when patching to other than a mic. As the +48 rides on the tip(and ring), the first thing it sees when patching into, say a commercial patch bay, is the ground. Bye bye power supply. As far as I know, it's unavoidable unless you remember to shut the +48 off prior to patching. I've discussed this with reps from Switchcraft and Neutrik, suggesting that they design a ground-isolated 1/4" / TTL TRS jack that would keep ground broken until full insertion of the patch jack. What most of us do is put a big red sign up that says "SHUT IT OFF, STUPID"!!

Paul

[JonCraig]

this is the case when patching anything w/ p48 on a patchbay. i've even ****ed up before and done it. you hear some sparks, if the chan is up in the control room, you get a pop. people give you shit, you get on with life. i've never had (or heard of) a power supply getting cooked because of this. besides, with a short on the patchbay, you'd burn up the 6.81k resistors long before you cooked a power supply. that's part of what they're there for... to act as little fuses.

--jon

[paully]

Originally Posted by JonCraig

this is the case when patching anything w/ p48 on a patchbay. i've even ****ed up before and done it. you hear some sparks, if the chan is up in the control room, you get a pop. people give you shit, you get on with life. i've never had (or heard of) a power supply getting cooked because of this. besides, with a short on the patchbay, you'd burn up the 6.81k resistors long before you cooked a power supply. that's part of what they're there for... to act as little fuses.

--jon

>>>>>>>>>>

Well, Jon, march to your own drummer. Don't say you can't cause damage.. talk to virtually any studio operator. I'm not sure what limiting resistors you're talking about, unless they're germaine to the design mentioned earlier. Even blowing something like that is a time-consuming affair to fix; IOW, the supply is 'cooked'. That was the point of my post.. mistakes happen, but our big red sign is there to help prevent it. Ground-lift switches on bay or module channel sends and returns is a good idea(barring hum), but name me a patchbay or module maker that offers them. That's the reason that I talked(in particular) to Switchcraft about full-insert jacks. That would solve the problem on TRS 1/4" / TTL bays.

Paul

[JonCraig]

Originally Posted by paully

I'm not sure what limiting resistors you're talking about,

they're part of the phantom power standard. 6.81k resistors, 1% or better tolerance. they act as current limiters for the phantom current going to the mic. (if you do the math, they're the reason that mic manufacturers have to work to make their mic run on ~14 mA). behold the attachment.

Originally Posted by paully

Even blowing something like that is a time-consuming affair to fix;

very true. but i've never seen or heard of it happening. but, yes... in theory, if it did happen (and it probably has happened to some dumbass) it would be a pain.

Originally Posted by paully

IOW, the supply is 'cooked'.

no, rather, the resistors would be cooked, and the supply would be fine. that's what the resistors are there for!

--jon

[brianroth]

As JonCraig has pointed out, the 48V rail is isolated from the mic line via a pair of 6.81K resistors. My friend Rick Chinn has written some papers on the subject:

http://www.uneeda-audio.com/#phantom

The WORST CASE scenario is tying tip to ring to sleeve. Then, the two 6.81K resistors are effectively in parallel, for a net of approx. 3.4K. Ohms Law tells us that 48V across 3.4K is approx 14.1 mA. It would have to be a VERY lame power supply that blows up under a 14 mA load!

As for the resistors smoking, each 6.8K R would be dissipating approx. 0.34 Watts with a dead short as described...a potential argument for using 1/2 Watt series resistors.

Bri

[ulysses]

First of all, phantom power has no business being carried on TRS connectors. It's for microphones, which belong on XLR connectors. What, are you running your mike lines through your patchbay? I hope not. Those popping noises are one of several good reasons why not.

Secondly, as others have pointed out, the phantom power build-out resistors limit the current to significantly less than any legitimate 48V supply is able to deliver, so you're not going to hurt any phantom supply unless it was already defective. If you ground one leg of a phantom-powered mike input, you'll pull about 7mA and you'll dissipate 338mW in the resistor. That's not a lot of power - about what some condenser mikes consume. Good designers will use a 1/2W build-out resistor, and/or spread the resistance across more than one resistor (typically there are three build-out resistors: a low-value feed resistor followed by a shunt capacitor, then a resistor feeding each leg). So long as there's a total of 3400 ohms from P48 to ground through the matched pair of signal legs, you can only get a total of 14mA out of any microphone input channel. I know that the preamps I've designed are able to withstand a shorted input indefinitely. In fact, you should be able to short every channel except one, and still measure 48V on that last channel (or power a microphone with it). But even a poorly designed preamp can handle shorting an input for a few seconds. If it can't, it's not just poorly designed. It's defective.

Getting back on topic - there is absolutely no way for the Folcrom in particular to damage or even strain a phantom supply. Not an issue at all in any way. There is no connection between signal legs and ground legs inside the Folcrom under any normal conditions. The signal path is completely floating, so it wouldn't pull any phantom current at all. The only real risk is reverse-biasing the electrolytic blocking caps on the outputs of the gear feeding the Folcrom.

[paully]

Originally Posted by ulysses

First of all, phantom power has no business being carried on TRS connectors. It's for microphones, which belong on XLR connectors. What, are you running your mike lines through your patchbay? I hope not. Those popping noises are one of several good reasons why not.

>>>>>>>>

I'm not going to hi-jack this thread by arguing past this reply but yes, that's a common way to break out the mike lines from an iso booth or the live room to a specific set of outboard preamps, compressors, etc. ; NOT the main patchbay, but a seperate, half-normalled bay designed for just that purpose. Sorry, but there's nothing wrong with using 1/4" or TTL PBs that carry low power. For a lot of people, it's the only economical and readily available choice. That's why the inadvertant shorting bothers engineers using 1/4" or TTLs. Use full XLR(or nothing) if that's your choice. Apparenty others on this thread have been bothered by the shorting problem, which is why I addressed it in the first place. One point that should be made. Not all phantom power supplies are made equal. If you do go the extra mile in your design, good. Some don't, and those units will not withstand a continuous or even a momentary short. Now, let's let everyone get back to the main topic, and in that spirit, I'd like to change all previous references from 'cooked' to 'damaged'.

Paul

[JonCraig]

Originally Posted by ulysses

First of all, phantom power has no business being carried on TRS connectors. It's for microphones, which belong on XLR connectors. What, are you running your mike lines through your patchbay? I hope not. Those popping noises are one of several good reasons why not.

i know of plenty of places that run mic lines through a TT patchbay. no problem. i have heard that sonically the mic signal suffers from this due to lack of connection area on the connector. is that what you're referring to?

--jon

[JonCraig]

Originally Posted by paully

I'm not going to hi-jack this thread by arguing past this reply.

well i am.

Originally Posted by paully

Not all phantom power supplies are made equal. If you do go the extra mile in your design, good. Some don't, and those units will not withstand a continuous or even a momentary short.

i just shorted the inputs on an m-audio buddy for five minutes. still works fine (ie still sounds like shit). sorry, but you're wrong here. don't know who told you that a power supply couldn't hold 14 or 112mA of current, but i've never heard of one that couldn't. 14mA can be supplied by a 9V battery. even a switching power supply (which you'd never find in a piece of audio gear, for obvious reasons) could hold its own.

--jon

[ulysses]

Originally Posted by JonCraig

Even a switching power supply (which you'd never find in a piece of audio gear, for obvious reasons) could hold its own.

What are these obvious reasons? There is some very high-end audio gear out there with very high-performance switching power supplies in them. Now that switching supplies can be made to run at very high frequencies, they can be small, efficient, inexpensive, and noiseless. It's a whole lot easier to shield 1MHz EMI than 60Hz. They are a bit more work to design.

[JonCraig]

Originally Posted by ulysses

There is some very high-end audio gear out there with very high-performance switching power supplies in them. Now that switching supplies can be made to run at very high frequencies, they can be small, efficient, inexpensive, and noiseless.

hmmm.. didn't realize that. if that's the case, then i'll stand corrected. what kind of current output are they capable of?

--jon

[ulysses]

Historically, you're right, switching supplies have been uncommon in high quality audio gear primarily because of problems with switching noise leaking into the audio. If you have a switcher operating at 10kHz, for example, this will be difficult to deal with. It's only in the last few years that the technology has really been developed. The main reason it has been developed is first of all high-frequency circuitry has improved, driven by the computer industry; and secondly the portable electronics market (cell phones) has driven development of compact, highly efficient, and inexpensive power management. Switching supplies can be more than 90% efficient, whereas linear supplies are typically more like 70% efficient. Since a lot of audio gear is being built by computer companies now, you're seeing switching supplies in a lot of interfaces and that kind of gear. Not necessarily super high-end stuff. But then there's folks like Great River who are willing to put in the effort to design a supply that performs to their standards.
I don't know that current capacity is a meaningful limitation in choosing a switching supply over a linear supply. In fact, a high current requirement would make a switching supply an even better choice. A large, high-current 60Hz transformer will be heavy and expensive, and will emit 60Hz EMI that requires distance and ferrous mass to shield against. A switching supply operating at 1MHz uses a much smaller transformer, and its leakage can be shielded with a thin piece of copper or aluminum. At the moment, linear supplies are still the norm here in the audio world but you can expect that to change more and more in the next several years. I think the only real obstacle right now is the additional complexity in terms of design work and parts count. Even that is rapidly changing, as semiconductor manufacturers are integrating more and more of the components into a single chip.

[3rd world order]

Originally Posted by ulysses

The only real risk is reverse-biasing the electrolytic blocking caps on the outputs of the gear feeding the Folcrom.

this is the only thing i'm worried about...

[Ziggy!!]

Im sure your converters won't like it... I know a few people that have popped some capacitors this way. It often troubles them at first.


It kind of sounds like an accident waiting to happen...