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I drawn the iRig circuitThank you for your very sharp eyes.
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WTF are the components marked as FB? Circuit breakers?Ferrite Beads to reduce radio frequency signals (an iPhone is full of digital and radio signals).
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Can someone read the code for Q1? Is it a JFET, MOSFET or what?If R5 is missing, then it has to be a JFET.
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The capacitors C1 and C2 form a band pass filter. Can someone specify those using filters theory?Not without values.
And I bet it turns out that C1 cuts subsonics, C2 cuts supersonics. Technically it may be a 50Hz-15KHz bandpass, but that is such a wide band that we can usually consider it as a hi-pass and a low-pass which hardly affect each other.
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C3 is also meant to filter high freq signal.Yes.
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For its acurate calculation we need to know what semiconductor Q1 is.Maybe not. At higher voltages all the semiconductor devices are "high" impedance, circuit impedance is dominated by resistors. This FET is working at quite low voltages, so may not be very-high impedance, but not much less than the R4 VR1 iPhone resistor network.

R2 can't be 39 ohms. Could it be 39K?
R1 should be a resistor and probably larger than R1 because guitar signals are bigger than the working voltages we can get from an iPhone jack. The maximum clean JFET gate signal is probably 0.1V-0.3V, guitars often exceed 0.1V and can go as high as 0.5V.