R7, R8, and R9 determines the gain of the preamplifier circuit. The gain of the circuit is shown in Equation 6:

Equation 6.

Resistors R7 and R9 are selected with a value of 2 kΩ to avoid loading the output of the OPA1678 and producing distortion. The value of R8 is then calculated in Equation 7:

Equation 7.

Capacitors C3 and C4 limit the bandwidth of the circuit so that signals outside the audio bandwidth are not amplified. The corner frequency produced by capacitors C3 and C4 is shown in Equation 8. This corner frequency must be above the desired –3-dB bandwidth point to avoid attenuating high-frequency audio signals.

Equation 8.

C3 and C4 are 390-pF capacitors, which places the corner frequency approximately 1 decade above the desired –3-dB bandwidth point. Capacitors C3 and C4 must be NP0 or C0G type ceramic capacitors or film capacitors. Other ceramic dielectrics, such as X7R, are not suitable for these capacitors and produce distortion.