I had the same problem with my Saffire: the phantom power voltage quickly drops when switched on, causing the mic to cut out. This problem is common in older Saffires, and is well known to Focusrite support (a modification in newer-production units solves the problem).
The problem is that a tiny 100 ohm surface-mount "chip" resistor in the phantom power circuit fails due to insufficient power-handling capability. (This is "R196" in the phantom circuit.)
The fix is to replace the "chip" resistor with a "normal" 1/2 watt metal-film resistor
(with wire leads), mounted upright on the circuit board.
I replaced the resistor myself, following directions (and photos) sent to me by Focusrite Tech Support.
The procedure is somewhat fiddly (a Swiss watchmaker could probably do OK). If you've never soldered before, DON'T EVEN TRY THIS, and if you've never worked with surface-mount components, I would ask your electronics-hobbyist buddy to do it for you.Here is the procedure sent to me by Focusrite:
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SAFFIRE PHANTOM POWER MODIFICATIONS
Remove the 7 screws on the rear panel.
Remove the 4 screws on the underside of the
Lift away the metal cover.
Remove the 4 small black screws securing
the MIDI In/Out sockets.
Remove the single screw securing the S/PDIF
Remove the two screws securing this PCB to
Gently pull the PCB away from the rear
panel and move it through 90 degrees towards the front panel.
Place the unit on its front and support the
PCB with some suitable non-conductive item.
All the components mentioned are located in
the same area on the same PCB (see photo).
It is best to remove the coil and capacitor
(as shown in the second photo), as this gives easier access to the components.
Remove R196 and measure its resistance. If
it does not measure 100 ohms, replace it.
Use a conventional type (100 ohms metal
film, .5 or .6 watt) mounted upright as
shown in the photo. Before fitting this resistor, check that the three zener
diodes DZ11, DZ12 (22v) and DZ9 (3.3v) are OK and have not shorted out. (Diodes
are 500mW, 5% tolerance)
units for Phantom Power problems
Connect a condenser Mic or suitable dummy
load* to Channel 1.
Engage +48 switch.
Measure the DC voltage between XLR pins 2
& 1 on Channel 2 Mic Input XLR.
Faulty units usually read around +28Vdc and
the voltage gradually drops.
Good unts will measure +48Vdc +/- 0.7Vdc.
* Dummy Load for testing: Solder a 20,000
ohm resistor between pin 2 and pin 1 of a male XLR plug and another 20,000 ohm
resistor between pin 3 and pin 1 of the same XLR plug.
Here are the photos:
My Saffire has worked perfectly since I replaced the bad resistor.