A transformer specified with this ratio is optimum for the detector and headphone impedances being used. A transformer that has factory specified impedance levels as much as four times lower than desired, but with the correct transformation ratio, and a frequency response range much wider than 0.3-3.3 kHz will probably work very well. Basically, it should have a JFET input and a gain-bandwidth product of about 4 MHz. The 22 uF electrolytic caps should have a voltage rating between 10 to 25 Volts. The 10 uF caps can have a voltage rating of 5-10 volts 5.
Calibration is simple.
Set SW2 to the 0 dB position, SW3 and SW4 to their 10k Ohm settings and potentiometer P for zero gain. To do this, load J2 with a 10k Ohm resistor and feed a 1 kHz signal from an audio generator into J1. Adjust P so that the output voltage at J2 equals the voltage at J1. Tune in a station and adjust potentiometer P so that the volume is the same as when the detector output feeds the headphones directly. This setting does not have to be changed in the future.