This amplifier is ONLY a RIAA amplifier, it is not a preamp with a RIAA circuitry. It was built as a contrast to my tube McIntosh/LJ preamp, which also includes a RIAA input. Here, focus is on precision, when it comes to RIAA accuracy. Actually, it is quite hard to be precise, because you have to get your time constants right, otherwise the RIAA correction will deviate from the standard curve. There are lots of information about LP recording standards on the internet. Shortly, component values has to be precise for the resistor / capacitor network, 0.1 % or selected/measured components, and circuit measurements and comparison to the RIAA standard curve. Knowing your measurement equipment is mandatory.
Here, I went for an all passive solution, makes heavier demands on noise suppression, but is easier with component selection, since you can disregard amplifier transfer function when the setup is proper. All IC circuitry, gain in the first and second stage, RIAA correction in between. No volume, fixed gain. You could include a trimmer parallel to R10, value maybe 100 K ohms, to allow for gain adjustment, to compensate/adjust for a pickup with different L and R channel output.
Another danger is overload, you have to be sure of input level, gain level and subsequent stress on the second amplifier. At the moment all IC:s are OPA 134, but this could change. OPA 134 is excellent when it comes to distortion and slew rate, you can find op-amps with lower noise figures, but this is not a MC head amp. DIP package, socket mounted, easy to change!
In all small signal amplifiers (read all amplifiers!!), PCB layout is important. Today, I am more careful than before when it comes to ground paths, separating supply ground from signal ground, there should be only ONE central ground point, I try to locate this as close to input ground as possible. Good wire shielding is also important, and avoiding ground loops. It sounds easy, but one has to be careful, one incorrect wire shield connection, and the loop is there.
