The pre's on a 424 can run a dynamic microphones, and not a
condenser because it does not provide phantom power.
The only exception would be a condenser "electret" microphone
that can take a battery to provide its own phantom power.
Another good mic pre, (that costs less than the RNP) is the
Studio Projects VTB-1 at around $180. It has phantom power.
www.bpmmusic.com has has been offering free 2 day shipping on it. Be aware that not all of the outboard pre's offer full 48 volt
phantom power, and it can cause some condenser microphones
to not operate to their proper spec's.
Pre's can run into the thousands of dollars BTW.
How much were you planning to spend?
Chris