Active RIAA equalization means feedback equalization: the frequency response is tailored to fit that of the RIAA curve by varying the amounts of feedback returned to the input. The advantage of using feedback is consistency. Each channel will track the other to a very great degree in spite of aging parts or circuit wiring dissimilarities, as the feedback tends to iron everything out. The disadvantage is that, because it is active, the circuit can more readily suffer from input voltage overloads and the preamp must have voltage gain far in excess of nominal +40 dB usually specified, as the feedback uses the excess gain to force the output to conform to the desired curve. Another problem is potential instability, as each coupling capacitor and gain stage add some phase shift.
Since the bass frequencies must be amplified +20 dB higher than the 1-kHz center frequency, the +20 dB must be added to the +40 dB of gain, yielding +60 dB of total gain. On top of this +60 dB an additional 20 to 30 dB of gain might be added to feed the feedback mechanism. Thus, we need more gain, but we cannot risk adding more gain stages, as each coupling capacitor and Miller effect capacitance adds some phase shift, which can reverse the phase of the fed back signal, creating an oscillator, not an amplifier.
In the absence of this extra gain, the varying amount of frequency dependent feedback can result in looser bass reproduction because of smaller amount of feedback at low frequencies and possibly a pinched, compressed high frequency playback due to excessive feedback ratios at high frequencies since a 20-kHz signal is attenuated by -40 dB relative to a 20-Hz signal. In other words, a preamp with only +60 dB of open-loop gain will have zero feedback at 20-Hz and 40 dB of feedback at 20-kHz.