Is there a Phantom Power "canceller"?

Posted: Fri Apr 04, 2008 1:00 pm

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... Wink

Posted: Fri Apr 04, 2008 2:34 pm

Wow this thing really took off! I wasn't expecting that Smile

Hey Cucco, are you referring to the rode D-Power Plug? http://www.rodemicrophones.com/accessory.php?product=DPower_Plug

Posted: Sat Apr 05, 2008 5:44 am

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.

Posted: Mon Apr 07, 2008 6:35 am

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.

Posted: Mon Apr 07, 2008 6:40 am

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.

Posted: Mon Apr 07, 2008 7:06 am

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?

Posted: Mon Apr 07, 2008 7:19 am

Sure. But I don't think there was 25 mA across the TRS. I'm guessing that I got a good .5mA if that.

Posted: Mon Apr 07, 2008 8:07 am

You won't know for sure though, what each person feels is different. And anyway, who is going to volunteer for some electrical contact testing?

Posted: Tue Apr 08, 2008 4:16 am

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.

Posted: Tue Apr 08, 2008 3:09 pm

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

Posted: Thu Apr 10, 2008 1:26 pm

I never worked on the ring stuff but I believe its 85vac. but, I have been wrong in the past.

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