I'm not suggesting a do-it-yourself solution would be appropriate
, but ....
a) this is the correct way to resolve the problem.
Not quite. An LPAD is commonly used to attenuate amplifier outputs. Here is a tutorial:
L-pad Attenuator Tutorial for Passive Attenuators
b) an appropriate value for that resistor.
Referencing the above web page, here are some values for R1 and R2 (ohms) for a few attenuation levels:
3db ... R1 = 75, R2 = 150
6db ... R1 = 110, R2 = 50
10db .. R1 = 130, R2 = 16
Use 5% carbon resistors (avoid wirewound), rated at 1/2 watt.
You will need one LPAD for each headphone channel (left, right). I would suggest building this in its own box with appropriate plugs and jacks so it can be removed if necessary. Don't hack a perfectly good headset.
Better yet, check around for a TSO approved in-line attenuator. Better to CYA if found plugged into an aircraft circuit.
