I suppose I can run the Phono stage into a ADC, to get digital.
Then what?
- PEQs?
- or FIR based?
There is a finite number of those plots with 2nd and 3rd harmonic.
Let’s assume I have a good cartridge, and a good tonearm, and even a good TT.
Step 1 - have a good ADC - aim for 120db or better S/N - as you need to cater not so much for the signal/recording you want (which is most likely well within a 70db range) - but the occasional click/pop that messes with you - and which will in many cases clip the input of the ADC, causing a bunch of distortion...
Then you need a Phono stage with the output well matched to your ADC levels. The phono stage will typically also provide RIAA EQ in the analog side of things.... or you can seek out a "flat" phono stage and do RIAA digitally - more complicated... and doesn't really gain anything - but can be handy if you want the flexibility of other EQ's.
Preferably choose a phono stage that has highly flexible cartridge loading (unless you are going MC) - for MM you want a total (cables + phono stage) capacitance of around 100pf, and resistive loads you want a range that can go from around 20k to around 80k ohms. (my phono stage has been customised with built in 100k ohm, and I then use loading plugs to lower it to whatever I want... it also has no onboard additional capacitive load - again I provide that via the cables and via additional C plugs)
Next you want a good test record with which you can get Frequency response measurements - you can do it via sweeps, spot frequency, or pink noise. (I'm not up to date with the latest test records.... some test records have suspect test tracks.... so a little research is required)
Measure your F/R and adjust your loading to get a Frequency response from 1k to 20kHz that is as even as possible - you may well get peaks and troughs in there .... but if you will be digitally Eqing getting the overall response to be within +/- 5db would probably be fine... you want to avoid getting beyond the 10db out in amplitude from your 1khz reference starting point.
Once you have that F/R using your loading adjustments - then switch to digital - and apply filters to get it to a flat F/R.
If you used a pink noise track for your measurements, plenty of the digital tools will allow you to use a pink noise track for automated filter calculations/setting. Makes things easier.
If you have a choice of filter types, choose minimum phase rather than linear phase.
Digital PEQ's typically use FIR filters - and the FIR filters come in various phase versions - you can use either alternative - but life is easier if your software has pink noise base Auto EQ!
I probably should spend some time documenting all this at some point!