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Will USB provide Phantom Power? Condenser Microphones
Old 24th March 2016
  #1
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Will USB provide Phantom Power?

I want to buy my first audio interface. I was thinking cheap but reliable and good for beginners. I heard about the Focusrite Scarlett 2i2, which is a great product by the looks of things.

All I want to know is, will the interface (which is powered by a USB cable from the computer, handle two condenser microphones well? Will there be enough power through the USB to the interface? Or will I notice a weak signal or anything like that? Because then I might consider buying one with a full separate power supply...
Old 24th March 2016
  #2
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jp22's Avatar
 

Quote:
Originally Posted by Sethroph View Post
I want to buy my first audio interface. I was thinking cheap but reliable and good for beginners. I heard about the Focusrite Scarlett 2i2, which is a great product by the looks of things.

All I want to know is, will the interface (which is powered by a USB cable from the computer, handle two condenser microphones well? Will there be enough power through the USB to the interface? Or will I notice a weak signal or anything like that? Because then I might consider buying one with a full separate power supply...
Although devices such as the one in question claim to provide 48v phantom power, a USB port only provides 5v and a standard XLR condenser microphone is 48v, so I have no idea how they actually pull that one off..

To me, something isn't quite right about it all.. so personally, I would never use one to record with.
Old 24th March 2016
  #3
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Quote:
Originally Posted by jp22 View Post
Although devices such as the one in question claim to provide 48v phantom power, a USB port only provides 5v and a standard XLR condenser microphone is 48v, so I have no idea how they actually pull that one off..

To me, something isn't quite right about it all.. so personally, I would never use one to record with.
Magic...



Or simple voltage conversion. Most condensers require between 12 and 48V and draw about 3mA of current. USB2 provides 5V and 500mA, and USB3 900mA. The rest is simple math.

https://en.wikipedia.org/wiki/Ohm's_law

In short, it has more than enough power to charge a condenser mic.
Old 24th March 2016
  #4
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Quote:
Originally Posted by DPower View Post
Magic...



Or simple voltage conversion. Most condensers require between 12 and 48V and draw about 3mA of current. USB2 provides 5V and 500mA, and USB3 900mA. The rest is simple math.

https://en.wikipedia.org/wiki/Ohm's_law

In short, it has more than enough power to charge a condenser mic.
Well, according to my "simple math"..

At 5 watts per amp and since 500mA (USB2) would be 0.5A, that would be 2.5 watts.

So, 2.5 watts x 500mA (0.5A) = 5V / 10 Ohms

Have we learned anything yet ?
Old 24th March 2016
  #5
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Quote:
Originally Posted by jp22 View Post
...At 5 watts per amp and since 500mA (USB2) would be 0.5A, that would be 2.5 watts. So, 2.5 watts x 500mA (0.5A) = 5V / 10 Ohms...
You were doing well until you multiplied watts-x-amps. Phantom power at +48v (created via a voltage multiplier circuit inside the interface) is by standard sent through 6.8k-ohm resistors through the XLR connector to the mic. The equality of the resistors' values is more important than their absolute values, but the phantom standard is intended to power microphones up to 10mA of load (just under 1/2 watt).

Most microphones don't use more than 4mA, but some famously (like the CAD M179) use 8mA. I found that factoid in the literature on the M179 when I bought a batch of them, and looked into the mysteries of phantom power as a result.

So, in round numbers, a pair of high-draw microphones could use up to 1-watt of phantom power, leaving a mere 1.5 watts from the USB port for all the other stuff the interface has to do.

I don't know if Focusrite has designed for this worst-case possibility, but at least those other threads that worry about the power supply load of a mixer running 32 concurrent phantom-powered miss should rest easy about the rather small probable total of under 16 watts.

HTH
Old 24th March 2016
  #6
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Quote:
Originally Posted by MediaGary View Post
You were doing well until you multiplied watts-x-amps. Phantom power at +48v (created via a voltage multiplier circuit inside the interface) is by standard sent through 6.8k-ohm resistors through the XLR connector to the mic. The equality of the resistors' values is more important than their absolute values, but the phantom standard is intended to power microphones up to 10mA of load (just under 1/2 watt).
I just wrote that last line of figuring backwards, since it doesn't really matter either way.

Basically what you're saying though is - you could run two condenser microphones on just a few AA batteries (if they were wired for it).

Quote:
Originally Posted by MediaGary View Post
So, in round numbers, a pair of high-draw microphones could use up to 1-watt of phantom power, leaving a mere 1.5 watts from the USB port for all the other stuff the interface has to do.
Regardless, something about it all still doesn't add up with these boxes. Like I previously said, these type of USB devices are doing things in a very strange and artificial way, especially when it comes to providing more volume and gain.


.
Old 24th March 2016
  #7
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The Zoom H4n Handy recorder runs off only 2 AA 1.5vdc batteries in series (3.0vdc) and can provide 48v phantom power to two condenser mics
https://www.zoom-na.com/products/fie...order#overview
Old 24th March 2016
  #8
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Quote:
Originally Posted by MarkF48 View Post
The Zoom H4n Handy recorder runs off only 2 AA 1.5vdc batteries in series (3.0vdc) and can provide 48v phantom power to two condenser mics
https://www.zoom-na.com/products/fie...order#overview
Point made. They may as well just make a deal with Focusrite and throw a "Handy recorder" in the box for free too.
Old 24th March 2016
  #9
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Quote:
Originally Posted by jp22 View Post
Like I previously said, these type of USB devices are doing things in a very strange and artificial way, especially when it comes to providing more volume and gain.
.
If you think about it, transducing air pressure into electrical current, and then sampling that current into digital 1s and 0s is very strange and artificial.

Yet, it's all music in the end.
Old 25th March 2016
  #10
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Quote:
Originally Posted by DPower View Post
If you think about it, transducing air pressure into electrical current, and then sampling that current into digital 1s and 0s is very strange and artificial.

Yet, it's all music in the end.
So magic then. Let's go with that

:D
Old 25th March 2016
  #11
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antichef's Avatar
In my experience, a bus powered interface can provide enough juice for two pencil mics (which usually state that they can be happy within a wide range of voltage topping out at 48), but not enough for two beefy mics like C414s

It has been a long time but I remember measuring the voltage at close to 48 but seeing a sag when I plugged a C414 into another channel.
Old 25th March 2016
  #12
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Quote:
Originally Posted by jp22 View Post
Basically what you're saying though is - you could run two condenser microphones on just a few AA batteries (if they were wired for it).
I do that exact thing all the time. Sound Devices Mix PreD powered by two AA's driving 2 U87's, 414's or C451's. And... the mixer too. Of course, eats batteries like a butcher's dog eats hamburger.

Last edited by kennybro; 25th March 2016 at 04:38 AM..
Old 25th March 2016
  #13
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kafka's Avatar
Quote:
Originally Posted by MarkF48 View Post
The Zoom H4n Handy recorder runs off only 2 AA 1.5vdc batteries in series (3.0vdc) and can provide 48v phantom power to two condenser mics
https://www.zoom-na.com/products/fie...order#overview
Yup. Mine does. Works great.
Old 25th March 2016
  #14
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duetpc's Avatar
 

I have one the these for a laptop rig. Gets power from the USB bus and powers condenser mics @ 48v without a problem.

IK Multimedia | iRig Pro DUO

-- Guy.
Old 26th March 2016
  #15
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zvukofor's Avatar
Babyface, Duet, USBPre - works perfect, modern DC-DC converters doing their magic!
Old 2nd April 2016
  #16
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Thanks for all your answers, I think I might save up for one that is plug powered, just to be on the safe side. But it all depends on budget!
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