Current for phantom power?

[substitute]

Hey folks,
I'm looking at some inexpensive 48vdc power supplies to add phantom to my drip 47 and wondering how much current is needed to power a mic.

THANKS

 

[Gus]

look at microphone specs

 

[substitute]

sometimes the obvious is sort of illusive...

Plugging 48v and 250 ohms into Ohm's law I came up with a draw of about 0.2 amps, seems like something in the neighborhood of 10 watts should do the trick.

 

[jdbakker]

250 ohms

Where does that number come from? 10W into a mic body would make it nice and toasty, no?

Have a look at the Mic Pre META, phantom current requirements have been discussed here a few dozen times.

JDB.
[a Search gives quite a few useful hits, too]

 

[substitute]

Thanks for steering me in the right direction, and sorry for slouching on the searching.

 

[JohnRoberts]

Phantom build out resistors are 6.8k, so worst case is both mic inputs shorted to ground, or 2x 48V/6.8k.

JR

 

[pucho812]

isn't the standard 10 miliamps per channel?

 

[PRR]

> isn't the standard 10 miliamps per channel?

Originally 2mA, then 10mA.

As JR says, worst-case is 48V/3.4K or 14mA. You should not smoke if a shorted mike or cable sucks 14mA.

Of course a shorted Phantom load is probably stupid. We need to get some power to the load. The nominal 10mA spec will drop 34V in the 3.4K, leaving 14V for the mike.

At least one version of AKG414 is a 2mA current limiter to a 9V regulator. It will never pull more than 2mA, and does not care a bit if the raw "48V" supply is as low as 16V.

Another class of mike uses the 48V as capsule bias. To keep a decent voltage at the capsule despite the 3.4K, current draw for the amplifier must be quite low, like under 2mA (which is why the original spec assumed 2mA).

Another low-price class of mike is liable to do ANYthing that "works" on most inputs found in Banjo World. Your protection here is the 3.4K resistance in your board, and the concept that pulling much more than 10mA is self-defeating for the mike designer.

> 48v and 250 ohms... 0.2 amps

I had a self-designed system like that. 36V, 470 ohms 2 Watt load. As Phantom became more common I had to be careful not to let wires get mixed up. Perhaps fortunately, I often ran it in stuff that didn't look at all like mike-wires: CAT5 cable, speaker cable, intercom wire, magnet wire.

 

[substitute]

Another low-price class of mike is liable to do ANYthing that "works" on most inputs found in Banjo World

This part was hard for me to follow, the other stuff was kind of mind blowing.

 

[drpat]

...

 

[raysolinski]

To cut to the chase the simple answer is...not much...48 volts (regulated), 14ma per channel will be more than enough for any mic....

Cheers,
Ray

 

[substitute]

Thanks again Ray.

I think he was referring to most of the crap that Guitar Center tries to cram down our ears.

I kind of knew this too, as far as why we knew that Banjo World meant T.G.I.Guitars, who knows, but from now on I will grumble about why I need to drive all the way to Banjo World in new jersey to get a mic cable.

 

[87Gerhard87]

Hi guys,

I'm wondering about the DC to DC converter. Will this be able to supply 48V Phantom power for 10 channels?

https://www.mantech.co.za/ProductInfo.aspx?Item=15M1645
Thanks!

 

[ccaudle]

Hi guys,

I'm wondering about the DC to DC converter. Will this be able to supply 48V Phantom power for 10 channels?

https://www.mantech.co.za/ProductInfo.aspx?Item=15M1645
Thanks!

Not really the appropriate class of device. For 10 channels you need 5W max, that device claims to be a 250W power supply.

 

[87Gerhard87]

@ccaudle Okay thank you! Anything that you might recommend?

 

[JohnRoberts]

10 channels of 14mA worst case (all mics shorted) is only 150 mA@ say 50V or < 10W. I haven't tried this asan add on supply but have heard issues with DC switchers behaving poorly when lightly loaded. You can throw a LED load on the output to insure some minimum current draw.

Search harder, I have seen discussions about using digital supplies for phantom around the internets.

JR

 

[Bo Deadly]

SMPS will modulate output if they are not under some load. This presents a particular challenge for phantom because the load might be 10 x 14mA or 0 depending on how many mics are loading. The proper solution would be to use the smallest 48V SMPS you can find (APC-16-350 was at one point and I know that one can be super quiet) and use TL431 to make a shunt regulator so that it's always loaded with at least 50mA or so. Just find the threshold at which it stops modulating the output and then adjust for a little more than that. Not a project for a beginner though.

 

[ccaudle]

APC-16-350

That is a very weird supply to use for phantom power. It is constant current; the output voltage is not regulated and can be anywhere from 12V to 48V depending on the load impedance.

 

[Bo Deadly]

That is a very weird supply to use for phantom power. It is constant current; the output voltage is not regulated and can be anywhere from 12V to 48V depending on the load impedance.

I know it's confusing but that's actually not how these supplies work. At least none of the MeanWell LED constant current ones that I've tried.

Notice how 48V * 0.35A is the full 16W but 12V * 0.35A is only 4.2W.

So in practice, they DO put out a constant voltage up to the rated current. I'm not sure what the exact difference in behavior between CC and CV is. My guess is that it has to do with the specifics of the feedback mechanism and the sharpness of the knee. Maybe CC is designed to operate below the rated voltage so that LEDs have a constant brightness.

When I originally started fiddling with these, I knew very little about SMPS and selected CC ones only because they were used in a SoundSkulptor PS and I reasoned that they probably used the CC ones for a reason. As in maybe they studied the various options and found these specific supplies to yield the best noise performance.

CC does seem to be the more natural feedback mechanism. Maybe with CV, the output just quickly shuts off and maybe gets really noisy just prior (this is pure speculation on my part however).

 

[moamps]

I know it's confusing but that's actually not how these supplies work. At least none of the MeanWell LED constant current ones that I've tried.

The constant current source has a defined current while the voltage across it is not defined and depends on the load. A real current source has a limit of that "floating" voltage and in this case it is 48V. This also means that the output voltage at the CC source will be 48V for all loads less than 350mA in this case. This is not a normal work area for CC source but an end point, for which this SMPS is not actually designed. And very often that voltage is not accurate and can fluctuate.

CC does seem to be the more natural feedback mechanism...

It sounds like a new audiophile concept has been born.