Phantom Power

capacitor mics on USB soundcards

if you are thinking about shelling out $$$$ on a new Capacitor microphone to go with a USB soundcard (with phantom power and a XLR connector) dont be suprised if it does not work -

Most capaciror microphones require the phantom power supply to be at +48 volts - this is not a problem when using dedicated OUTBOARD preamps) but can cause major problems when using a USB soundcard, the problem is that a PC does not have a +48 volt supply in it, so depending on the manufacturer and how much you pay for the unit depends how (IF AT ALL) the 48 volts is generated -

the usual way to get a +48 volts supply is to have an invertor on the soundcard that takes its input from the PC psu and increases it to 48 volts, problem with this is that invertors are expensive, VERY NOISY and require a lot of shielding, so to overcome this a manufacturer may tap onto the outside voltage rails of the PCs 12volt +/- supply as this gives a 24 volt supply, this is then used as phantom power, but not all capacitor mics will run that low, -

so before you buy a capacitor mic to use with a USB soundcard check with the manufacturers specs to see what is the minimum phantom power it will run at -

Dr J

Interesting. Is this based on experience or did you read this somewhere?

Obviously USB only provides a single low current 5v rail so the problem is even more interesting with a USB soundcard. Saying that, most modern electronic devices actually need very low voltage internally so as long as there is enough potential difference across the device’s internal voltage regulator it should probably work. I’ve seen marginal USB ports/device combinations where the port can’t supply enough current for the device but whacking a powered hub in-line usually solves that problem

I don’t know off hand what the voltage specs are for Phantom Power - I’ll go and look it up… it may be the mic’s fault rather than the soundcard if the phantom power is less than 48v but still in spec????

Many mics can run off batteries (eg my AT mics - ATM33a) again suggesting that the full 48V may not be a requirement sometimes.

Also, there’s a bunch of pretty respectable entry level preamps (eg the revered Studio Projects VTB-1) that run from wall-warts (9-12v), and I’m pretty sure that my Soundcraft desk runs from only 18V, so the problem of stepping up the voltage to 48V is one that has been solved in many places before.

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Hi, Dr J is quite right, standard phantom power is 48V and a lot of mics need something close to this to work. There was discussion on the net about the M-audio audio buddy giving out a bit less than 40v and the problems it caused some people. It is difficult to make 48v from say 5v or 12v DC from USB or a computer PSU. Its much easier to make it from an AC PSU such as the one used with the VTB-1 as you can use a simple “tripler” type circuit of diodes and Capacitors as the current demand is low.
Some Mics specify a range of Phantom power the Rode NT1A specifys P48 and P24 so with this knowledge I’ve used mine on a home made preamp with 24V phantom power.

Nick

Quote (nick @ Jan. 07 2007,11:53)
Hi, Dr J is quite right, standard phantom power is 48V and a lot of mics need something close to this to work. There was discussion on the net about the M-audio audio buddy giving out a bit less than 40v and the problems it caused some people. It is difficult to make 48v from say 5v or 12v DC from USB or a computer PSU. Its much easier to make it from an AC PSU such as the one used with the VTB-1 as you can use a simple "tripler" type circuit of diodes and Capacitors as the current demand is low.
Some Mics specify a range of Phantom power the Rode NT1A specifys P48 and P24 so with this knowledge I've used mine on a home made preamp with 24V phantom power.

Nick

Yeah, I'd forgotten that the VTB-1 wall wart outputs AC rather than DC. That does make things somewhat easier.

Interesting subject.

I still haven't got around to looking up the specs for Phantom power. It would be interesting to know what the tolerences are on the voltage.

You may search in vain for a “standard” for phantom power. What is standard about phantom power is that it is a DC voltage supplied over the top of the AC audio signal in a mic cable, for the purpose of polarizing the capacitor and powering the head amplifier that make a condensor mic go.

Older tube condensors use an outboard power supply to feed the capsule and head amp. Once the FET became popular as an amplifier, they were used in mics. Again, the early ones have an outboard power supply, and some took batteries as well. I have some Sony C37FET mics like that. I think they run quite a bit less than 48VDC. Some of the early Neumann SDC mics run at some crazy big voltage, like 54VDC. Lotsa small, cheaper condensor designs will work with lower voltages, in fact are designed to work in the range of 12VDC.

I think you’ll find a common range for phantom power. I guess in thinking about it, the IEEE may have codified the term “phantom power”, but I’m not sure about setting any absolute voltage limits. You can make a product called a “condensor microphone” that operates on 12VDC, and can be called a “condensor microphone”. I think that much is clear. “Phantom power” is a DC voltage delivery system using the microphone wires as carrier. Is there a requirement that such a power system be operated at 48VDC +/- some range in order to be called “phantom power”? I don’t honestly know.

I know there’s a defacto standard. It’s my belief that it is based on what it takes to run the most common microphone head amplifier designs. I know when I rack an old mic amp and add phantom power, I alway set up my power system to deliver 50VDC, just to account for losses. Generally, unless the mic came with it’s own dedicated power supply, more phantom voltge will not hurt your mic. If the mic has a power supply, DON’T use another phantom source too.

<!–QuoteBegin>

Quote
In 1966, Neumann company of Berlin, Germany, presented a new series of transistorized microphones to Norwegian Radio. For compatibility reasons, Norwegian Radio requested phantom power operation.

Due to the limited daylight hours during winter months, the studios used an auxiliary lighting system fed by a central 48 volts power supply. This was therefore the voltage used for powering those mics and is the origin of 48V Phantom Power.

This was later standardized in DIN 45596. The prevailing international standard, IEC 61938 - clause 7.4, defines 48 volt and 12 volt phantom powering; a 24 volt version of phantom powering was included in the DIN standard for several years, but was never widely adopted by equipment manufacturers.


See here for more info:

http://www.microphone-data.com/pdfs/Powering%20mics.pdf

So, blame it all on those vikings. :)

WALL WARTS and inline PSUs only (as far as i know) have a single output, to operate a mic at +48 volts then the wall wart/psu output would have to be at either 48vdc or the USB card would have to have a switch mode PSU built in -

the problem with the frst is that OK you would have the correct mic voltage present but the card would have to dissipate +43 volts to obtain the +5v that most of it runs on and dissipating that much voltage would generate a LOT of heat -
the problem with the second is that a switchmode supply could cost more to add to the card than the rest of the components put together -

using a capaciror mic designed to run on a +48v supply at a lower voltage can nullify the quality of the mic -

Capacitor mics are still a rareity in most domestic (home studios) where dynamics still rule the roost but as more people will/are using USB units so more people will be tempted to buy a capacitor mic, so i posted this topic with the aim of making those about to undertake using a capacitor mic aware of the pitfalls that may occur - to me, (and having my own recording studio helps) i only use dedicated preamps which supply their own phantom power for the capacitor mics, and the standard mixer 1/4inch jacks for dynamic mics (not using the phantom power on the mixer at all) as the mixer has more than enough gain for the dynamics a seperate pre is not needed -

problems i have had with phantom power is that on some units the phantom power is supplied to both the XLR connectors and to the mic 1/4inch jack inputs - it should only go to the XLR connectors - as i said in another post, theory says that if you apply phantom power to a DYNAMIC mic the phantom power should have no effect on it - BUT if you plug tham in with phantom power switched ON and they go PING then thats them ruined - there are some units out there that when the phantom power is switched on it goes to both sets of mic inputs, if you only use capacitor mics then thats no problem, if you only use dynamic mics that dont require phantom power then thats no problem - but what happens when you want to record using dynamic and capacitor mics at the same time -

well i suspose reading the specs on the unit helps, but if you dont know about phantom power, how it works and what are its complications then all the specs in the world wont make any sense to you till your dynamic mics go ping. or pop, or your new $500 capacitor mic when plugged into your new $1000 PRO USB unit sounds like a tin can on a piece of string - then its to late -

Dr J

thanks for the link, Gizmo…

The phantom spec I have seen most often is to provide +48V dc through a 6.8k resistor, to each of pins 2 & 3 of the XLR connector.

Inverter circuits aren’t that difficult to design, and don’t really pose more of a noise source than the rest of the digital parts of the typical audio interface card.

I’m playing with a friend’s USB interface (Edirol UA-25). It’s powered just by the USB connection which carries just +5V dc. Inverter circuits onboard create a -5v dc to provide +/-5V for opamps, and +48v for phantom. I just measured the phantom voltage - it was +45.6, which to me is bloody close to +48, so I wouldn’t hesitate to use any phantom-powered mic with this interface.

EDIROL - a Roland by any other name plays as sweet -

Dr J