[sdiy] 24V PSU Question

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In a message dated 11/12/03 4:43:26 PM, prototek at optonline.net writes:

<<     Hey guys and girls. I have a quick question about power supplies. The
supply is meant for a mic preamp project I'm doing so I hope everyone will
forgive the "un-synthiness" of this post.
    The preamp functions off of 24V but also needs 48V for phantom power.
Since I have no "hands on" experience with psu's this puzzles me. Is there a
way to tap both voltages off of one supply, or do I need two seperate
supplies (24V and 48V respectively)? Any methods for doing this would be
greatly appreciated.
 >>

Hello John,

Just to be clear, I think you mean that "the preamp functions off of 24V, but 
also needs a 48V supply to provide power for microphones", correct?  If this 
is the case, you might consider simply using the 24 volt supply to provide 
phantom power for your mics.  Many of today's condenser mics will happily run 
just fine on anything from 18 volts to 48 volts (sometimes even less than 18 
volts!) - and it usually doesn't even seem to effect the mic's output level or 
headroom, either (I don't know why...).  Believe it or not, there are a lot of 
"professional" commercial mixers and mic pres that have low voltage (less than 
48 volts) phantom power. 

If you know what mics you will be using, try them out with the 24 volt power 
source.  It certainly can't hurt the mic.  The 6.8K current-limit resistors 
that most phantom circuits use will probably also be OK, too.  If it doesn't 
work, or the mic signal is weak, try dropping the resistors down to 4.7k or 3.3k 
before you give up on the idea and build a separate supply or a (potentially 
noisy) voltage doubler.

BTW, what mic preamp circuit are you building?

Michael Bacich