The preamp dates from 1979. The THD was incredibly low, It used a mix of bipolar discrete transistors as input devices followed by and op-amps. The op-amps were of the best at that time the LM301. These were used in the inverting mode for a fast slew rate.
Despite the age of the PCB it can be made extremely capable by performing the following updates:- a) faster op-amps b) close tolerance values for the RIAA disc section c) J-fet input devices in the line stage d) ESD hardening to protect the j-fet muting at the output e) new electrolytic caps with low ESR f) removal of the intrusive “rumble” filter (-3db 17Hz) d) all new electrolytic capacitors
This next file has details for the technical details of the updates.