RIAA cannot be disabled without modifications.
We do not recommend DSP/Digital EQ due to the very large amount of EQ needed (40dB) using digital EQ degrades the ADC output by almost 7 Bits.
RIAA EQ was designed for analogue systems and is best handled in the analogue domain. The goal is to play 78 records that pre-date the use of EQ during cutting, this is a very specialised application and several small manufacturers offer dedicated solutions for 78 RPM mono records.
