I read in sci.electronics.design that Jaye Gallagher
2park-reader-02.sydney.pipenetworks.com.au>) about 'Generating
[email protected]
from 5V for microphone phantom power...', on Tue, 28 Dec 2004:
Gee, that's lots of lines of attribution. Keep reading, my original
content is further down...
That may refer to powering simple electret microphones that draw less
than 1 mA.
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.
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.
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.
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.
Some mics are designed to work over the 12V to 48V range, but there
may be lower performance at 12V.
