The amplifier's gain is nominally 20 dB. Its frequency response is determined primarily by the value of just a few components-primarily C1 and R1. The values in the schematic diagram provide a response of 3.0 dB from about 120 to over 20,000 Hz. Actually, the frequency response is flat from about 170 to well over 20,000 Hz; it's the low end that deviates from a flat frequency response. The low end's rolloff is primarily a function of capacitor C1, since R1's resistive value is fixed. If C1's value is changed to 0.1 mF, the low end's corner frequency-the frequency at which the low end rolloff starts--is reduced to about 70 Hz.
If you need an even deeper low end rolloff, change C1 to a 1.0-mF capacitor. If it's an electrolytic type, make certain that it's installed into the circuit with the correct polarity--with the positive terminal connected to Q1's base terminal.