>48v on passive Fet preamp

[Dilettante]

Say there is a device that supplies ~54v of phantom power instead of the conventional 48v.
If one plugs Cloudlifter, FetHead, etc. into the chain, can these be damaged by the excess voltage?

 

[sergeauckland]

Was the 54V is measured with a phantom device plugged in? Phantom power is supplied through two resistors, so the voltage you measure at the XLR input depends on the current drawn. If you measured the 54v on the XLR input with nothing plugged in, 54V seems reasonable and depends how well stabilised the nominally 48V supply is.

S.

 

[Dilettante]

Was the 54V is measured with a phantom device plugged in?

Live and learn! Voltage was measured with a multimeter set to DC and plugged into XLR inputs 2 and 3.

depends how well stabilised the nominally 48V supply is.

Could you elaborate here?

 

[sergeauckland]

Live and learn! Voltage was measured with a multimeter set to DC and plugged into XLR inputs 2 and 3.



Could you elaborate here?

A 48v phantom supply is not very critical of actual voltage, as most phantom-powered devices will work perfectly well from, perhaps, 24v upwards. The main difference is that with a low supply, the maximum headroom on microphones will be limited. This matters if the microphones are on a drum-kit, it may also matter if they're vocal mics with somebody with a very powerful voice. Tom Jones was claimed to get normal line-levels out of a Neumann U87 mic...

For a purist recording of a string quartet, or normal speech, having a low phantom supply may not matter at all.

Consequently, phantom supplies are only nominally 48V, and whether they start off at 54v as your seems to do, which will drop to 48v or less on load, or whether they start at 48v and drop lower depends on the manufacturer. Phantom supplies usually feed through a pair of 6.8k resistors, so effectively 3.4k in series with the supply. Consequently, the voltage will drop by 3.4v for every 1mAmp of current drawn. As a microphone can draw typically 3-5mA, that means that the voltage will drop by anything from 10.2v to 17v from whatever voltage it starts at. This means that your starting (off-load) voltage of 54v could drop to 37v if you're using a microphone that draws 5mA. That's still perfectly OK and normal.

Hope this makes sense.

S.