Generating 48V@50mA from 5V for microphone phantom power...

I read in sci.electronics.design that Jaye Gallagher wrote (in ) about 'Generating 48V@50mA from 5V for microphone phantom power...', on Tue, 28 Dec 2004:

If your 5 V-48 V converter were 100% efficient, you would draw 480 mA from your battery. Can you supply a current significantly larger than that?

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Why pay the efficiency hit twice, one on the 5V switcher and one on the phantom power supply? Why not just power the phantom supply straight from the batteries?

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

You can certainly do this, but you'll pay a cost in efficiency, component cost and size -- a switching power supply takes a certain standing power whether it's delivering power or not, and that standing power generally increases as the design power output of the switcher goes up. In addition you'll need larger, more expensive magnetics and semiconductors to implement the thing for 2.5W instead of 0.5W -- but requirements is requirements.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

That may refer to powering simple electret microphones that draw less than 1 mA.

The famous IEC/EN 61938, Table 11. Specification 48 V +/- 4V, maximum current 10 mA (per microphone). Typically, the 48 V supply is fed to the microphone via two 6.8 kohm resistors that are effectively in parallel. Many microphones do not draw as much as 10 mA. but some do.

There is a significant problem with phantom supplies, even in some mixing desks, that have inadequate current capacity. Also, there are two lower voltage supplies standardized in IEC 61938 Table 11:

12 V +/- 1 V, maximum current 15 mA; 24 V +/- 4 V, maximum current 10 mA.

The 24 V standard is no longer used, it appears, by any microphone manufacturer.

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

the feeble phantom, bottom of the page

gives a lot of P48 info

martin

Serious error. All shortcuts have disappeared. Screen. Mind. Both are blank.

Hi there,

I need to generate a fairly clean 48V at around 50mA to be used for phantom power for up to four microphones. The source voltage I need to step up somehow is the 5V output of a battery driven DC-DC step up. Whilst I've looked around for parts and schematics for this, any help or comments would be much appreciated.

Thanks, Jaye.

Gee, that's lots of lines of attribution. Keep reading, my original content is further down...

And that's different from the phantom power standard, it's usually called "bias power" and is used for electret condenser mics plugged into soundcards and minidisc recorders with their 1/8" miniphone connectors, as well as in built-in mics for telephone answering machines.

A few microphones may have a center-tapped transformer output, which would effectively ground both resistors (these of course should not have phantom power applied, but that's another topic). This would give 14mA per microphone, or 42mA total if three such microphones are plugged in (thus the OP's 50mA spec isn't too far off for absolute worst-case conditions). Also, each of these 6.8k's will be dissipating

0.34 watts with the full 48V across them, so it's a good idea to use 1/2W resistors.

Some mics are designed to work over the 12V to 48V range, but there may be lower performance at 12V.

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No-one claimed that it was the same as *standardized* 48 V phantom power. Some such mics are powered on the unbalanced signal conductor, some have 'A-B' phantom power.

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Hmm. You're right, that's a lot. Perhaps I'm confused about the amount of power required for a single phantom power supply? I've seen phantom power supplies that claim to power a mic. for 10-30 hours from 2 AA batteries. (Or thought I had in my net trawling.)

I haven't been able to find any hard specs on the current that should be available via phantom power. Anyone have any idea?

Thanks, Jaye.

Have just trawled around a bit more. Seems that 2mA per mic. is a more reasonable estimate of the average current drawn by professional condenser microphones. Some draw more, some less (see

So, let's say that I want a supply circuit that can set up +5V to +48V and provide probably around 10mA in total, but have capacity to provide up to

50mA (if the end-user plugs in power hungry microphones, that's their business).

Thanks, Jaye.

also available in multi-resistor SIP with these power ratings. There is ceramic line too.

It's possibly a bad idea to use 1/2W resistors, as these aren't usually available in the close tolerances required. The match (but not the actual value) of the two resistances needs to be very accurate.

It is possible to make up the desired power rating by using a number of lower powered precision resistors in parallel or series.

Regards, Allan

.... if anyone knows of a source of matched 6.8kohm 1/2W resistors, please let us know.