It is very hard to measure riaa accuracy accurately as the riaa curve covers a 40dB
not that tricky.
Just change your function generator amplitude over the range. And measure with an oscilloscope Dual trace is easier or you can use single channel and go back and fourth to measure difference between function generator and output.
I start at 1khz.
Then check the preamp gain by converting the Volt difference between input and output and converting to dbv
remember each double in voltage difference is 6dbv difference.
1 khz sine wave input 1mv peak = output at 0.1V peak = 100 times amplification = 40db gain. (useless info is that I measured my d3a based one at aroun 38db, which is I suppose good enough)
So for 20hz to 20khz you want to see 60db gain to 20db gain from your input.
For that 1khz baseline you want your 20hz to be around 20db higher or at least 19db.
So for the same 1mv peak and 20hz you want to see close to 1V peak at the output. 40db gain + another 20db at 20hz = 60db = 1000 times gain.
20khz is different.
You are going to want to see only 20db gain. ( 40 db (at 1khz) - 20db)
so for a 20khz 0.1V function generator peak input you are going to want to see 1V peaks on the output.
If you prefer only testing down to 50hz your are going to want to see a 17db or 7X gain from your 1khz measurement.
I'm hoping i didn't rape math too much in above post.....
nice list of attenuation curves vs frequencyhttp://www.beigebag.com/case_riaa_1.htm
Gain/Loss vs db calculator:http://www.sengpielaudio.com/calculator-gainloss.htm