• Registered: 2022-06-16
  • Posts: 2

Topic: FF800 line inputs and phantom power.

I want to add phantom power to a pair of line inputs on my Fireface 800 , and potentially also on my FF400

This is to accomodate drum mics with very high output that my mic preamps can't handle.

Normally to do this one would use a pair of 6.8k resistors and a pair of capacitors to block the DC from the preamp side of the circuit.

However the FF800 line inputs are already AC coupled - ie they have a capacitor to block DC.

So my question is, what size and voltage rating is the input capacitor on a FF800 line input and will it be OK with 48v being applied, if I don't add additional blocking capacitors?

2 Reply by parenthetical (edited by parenthetical 2022-06-16 13:18:43)

  • Registered: 2022-06-16
  • Posts: 2

Re: FF800 line inputs and phantom power.

To answer my own question. I found some photos online that seem to show 22uF 25v capacitors at the line inputs.

I will confirm this by opening up my own unit when I get time.

However it looks like applying 48v phantom power to these inputs without additional blocking caps would be a Very Bad Idea.

3 Reply by cyrano (edited by cyrano 2022-06-16 20:45:28)

  • cyrano
  • DAW Mastermind
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  • From: Belgium
  • Registered: 2015-04-28
  • Posts: 701

Re: FF800 line inputs and phantom power.

Here's the schematic for a typical phantom power feed:

https://sound-au.com/p96-f2.gif

The 6,81 kOhm resistors are the most important. Usually, they are 6,8 kOhm, as that is available as a standard value. The 22 uF caps aren't as critical, but don't overdo the value, as these are the ones that discharge into the input when hot plugging.

I'd also avoid hot plugging, as TRS jacks will short-circuit for a brief moment when plugging in or out.

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