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Is there a Phantom Power "canceller"?

Member for

14 years 8 months
So I'm just a little curious if anyone knows of a device, like a DI or something that acts as a safe guard against Phantom Power? For use with Ribbons basically. I ask because its like a recurring nightmare of mine to just slip on the phantom power button, and then "boom".

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Member for

21 years

Member Tue, 04/08/2008 - 03:16
Your salt content has alot to do with your resistance to DC. Also how sweaty you hands might be (moisture). I worked in Phone CO's for a while and mostly in the DC power plant (48v) and I have been shocked many times. It is unpleasant but has not had any negitive side effects. I have heard that if you have a heart condition you may want to avoid contact with low voltage DC or any voltage for that matter. I would imagine that any inadvertant contact with phantom power would have no ill effects (other then a surprising shock) with 99.999% of people unless you were doing some underwater recording or recording someone singing in the shower.

Member for

15 years 10 months

Kapt.Krunch Fri, 04/04/2008 - 08:57
pmolsonmus wrote: But I have them all marked with tape that says "PHANTOM ON??????" and before I plug that mic in I check and then double check.Phil
Well...hmmm...as long as YOU know, and somebody else doesn't grab them.

Ned Newby.. "I guess he wants to make sure phantom power is on? :? "

I'm guessing you make sure it's impossible for that to happen? :wink:

Kapt.Krunch

Member for

15 years 11 months

RemyRAD Fri, 04/04/2008 - 10:50
I just knew that Boswell was some kind of crazy genius. I just didn't realize he was more sane than the rest of us? Fabulous Boswell! Please keep us posted on your wonderful new circuit. TAA! You British guys are just incredible! When I grow up... I want to be British!

Learning to fake my British accent. Actually I can do Canadian better eh? Nothing you can do a boat that eh.
Ms. Remy Ann David

Member for

17 years 6 months

Cucco Fri, 04/04/2008 - 12:00
I'm a HUGE sucker for brunettes with British accents. Cockney and Irish accents too...Mmmm.

I'm sitting here watching the first 30 seconds of the video at work and realize..."uhhh...perhaps I shouldn't watch this at work."

Despite the fact that we're a highly evolved species, parts of me are still a dumb animal... ;-)

Member for

15 years 11 months

RemyRAD Tue, 04/08/2008 - 14:10
Back in the day when installing your own telephone was not encouraged by the telephone company, I was installing one for a friend. Of course there is a small quiescent DC voltage always on phone lines. I was twisting up some connections when I realized her phone was ringing. Mind you, the phone was not yet connected. But that 90 volts of pulsed DC must have made my eyes look like the ringer of the Bell! Good thing I didn't have a pacemaker installed. But then it was still close enough to your heart beat to keep you going. I finished connecting the phone on the fifth ring and handed it to her.

I still have that ringing in my fingers. HEY GET THE PHONE!
Ms. Remy Ann David

Member for

20 years 6 months

MadMax Sat, 04/05/2008 - 04:44
Boswell (et al),

I had my SF12 die awhile back. Blown ribbons.... bonus.

When I talked to the good folks at Royer, they said it looked like a power spike got the ribbons, and probably from phantom power being on at connect/disconnect.

I asked them about any protection devices they could recommend, if they didn't make anything.

The comment was a simple set of series non polarized blocking caps and at least a 1M loading resistor.

I've got a prototype I'm working on, with enough supplies to build about 12 or so. If anyone is interested in either building one on their own, or if you would be interested in trying one out, let me know.

The prototype is a two channel job for the SF12, since it's a stereo mic and fits in a box the size of a DI.

It's been on the back burner for a couple of months since the studio build has taken off, but if there's any interest I'll get back on it.

The components are reasonably cheap... like a coupla' bucks. The expensive parts are the XLR's and the enclosures, which are a few bucks each. My estimate is somewhere in the $25-$30 range complete including labor. Parts alone, somewhere in the $15-$20 range... less if you go with plastic enclosures.

Not hawking a product here... I had a minimum order to get the parts, so I figured what the heck, make a dozen.

If your interested, gimme a shout.

Member for

15 years 5 months

Boswell Mon, 04/07/2008 - 05:35
You have to be a little bit careful about coupling capacitors. They can be both your friend and the enemy at the same time.

A few sums:

(1) to get a -3dB point at 10Hz working into the 6K8 input resistance of a PP circuit, the capacitance value needs to be 2.4uF.

(2) a 2.4uF capacitor charged by the PP circuit to 48V holds 112 uC (microCoulombs) of charge.

(3) Under fault conditions, such as someone puncturing the cable with a chair leg and causing one of the signal conductors to short to the screen ground (this has happened to me!), the coupling capacitor can discharge through the microphone. The initial current surge value will be the full 48V through the (say) 200 Ohms of the microphone, giving 240mA. The energy of discharge is approximately 2.65mJ.

(4) If the discharge is through a 25mm ribbon microphone having a 1T magnet, the impulsive force at the ribbon is about 6mN (milliNewtons). Most ribbon assemblies would not survive this.

A simpler comparison can be made by comparing the generating and motoring emfs on the ribbon. A typical ribbon microphone can generate around 25mV r.m.s. at the preamp input when placed in the highest continuous sound field that the ribbon assembly can tolerate before damage. This is about 2000 times less than the 48V peak of the discharging capacitor when motoring.

This all sounds a bit scary, but, as I mentioned, capacitors can be our friends too, as long as we respect their properties and take into account their capacity (sic) for doing damage. So, if we can spot the fault conditions and take action in time, we can use capacitors to do the job of isolating us from phantom power. That was what the contract design work was all about.

I'm doing some direct-to-stereo recordings this week where I will have the opportunity of running a pair of condenser microphones and a stereo ribbon microphone side-by-side, making two separate stereo mixes. I'll be trying out my protection circuit/preamp on the ribbon, the idea being to see if it survives critical sonic examination, although I don't intend to rupture microphone cables just to test its protection properties.

Member for

17 years 6 months

Cucco Mon, 04/07/2008 - 05:40
Just curious -
Obviously that amount of current at the microphone would be lethal to the mic, but any of it seeking a ground through, say, someone holding it would also be (VERY) lethal.

What do you foresee as the possible conditions for this? Say the intern is placing the mic as someone ruptures the cable inadvertently??

Pardon my ignorance about this - especially if it's a dumb question.

Member for

15 years

taxman Thu, 04/03/2008 - 13:50
Check out the Royer web site. The do's and don'ts section says don't plug in or out when phantom power is on. Other than that, is seems phantom power not an issue. Some of their mics require phantom power, so those can't be damaged by pluggin in with the power on.

Don't know about other brands.

Member for

15 years 5 months

Boswell Mon, 04/07/2008 - 06:07
Cucco wrote: Just curious -
Obviously that amount of current at the microphone would be lethal to the mic, but any of it seeking a ground through, say, someone holding it would also be (VERY) lethal.

What do you foresee as the possible conditions for this? Say the intern is placing the mic as someone ruptures the cable inadvertently??

Pardon my ignorance about this - especially if it's a dumb question.
I would judge that there would be no human danger. You might just feel it, but in human terms the total stored energy is relatively small and skin to metal contact resistance is usually not less than about 1K Ohm.

Edit: J - just seen your post about grabbing hold of a TRS cable with 48V on it. Maybe I should revise my remark about just being able to feel it, but dangerous? You survived, didn't you?

Member for

18 years 3 months

pmolsonmus Thu, 04/03/2008 - 20:44
I'm going to go out on a limb and suggest that he already knows the ins and outs.

What he's rightfully fearful of doing is plugging in a ribbon mic into a phantom powered line.

I've got a much cheaper solution -

Masking tape - I only have a few old ribbon mics around and don't use them that often because they're noisy. But I have them all marked with tape that says "PHANTOM ON??????" and before I plug that mic in I check and then double check.
It's worked for me so far, but I'm don't use these out in a classical concert venue.
My .02

Phil
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