The input RCA filter is here:
Low Pass Filter - Passive RC Filter Tutorial
Calculation is:
fc = 1/[2?RC]
One has to assume at least one of the above variables. For example, if we assume fc of about 200 kHz, and let's say we have a capacitor of 100 pF, then we can calculate value of R. Typically R is between 3.3K to 10K. Using a standard R value of 6.8K, we get about 225 kHz.
This value is not sacred. What input RC filter does is cut off induced RF energy which could be any frequency. Without proper test tools it's not possible to determine whether there actually is induced RF frequency at some frequency, say, 400 kHz. Every installation is different and the interfering frequency, if any, will be unique to that installation. We're, at best, taking a shot in the dark in the absence of proper test tools. One may ask why bother with such a high frequency? We should consider the wide bandwidths of modern line level preamps, or even amplifiers which stretch to MHz range. Now if there is an unwanted signal at say 400 kHz, it is going to do two things - first, it will consume some of the energy of the preamp/amp. We want to amplify only the audio signal and not waste energy in amplifying some spurious frequency. Second, it's going to produce an infinite series of harmonics below and above 400 kHz, and some of those harmonics will fall in the audio band, causing interference to the actual audio signal. The interaction between the undesired harmonic and actual audio frequency will produce it's own set of harmonics. This is inter modulation distortion. The amplitude of the harmonic that falls in the audio band is probably quite small already, and the corresponding IM distortion will also be quite small, but our ears are very sensitive to any sort of distortion, so it's best to kick out that source of distortion.
But this filter may not help reduce hiss in your analog chain if the noise is electrically induced. The analog chain, starting with tonearm wiring and grounding, needs to be carefully studied. In my limited experience, even well shielded tonearm cable from tonearm base to phono preamp can be very prone to electrically induced noise, especially if the cable passes near power cables or other electronics. One easy way to check if this is the source of your noise is to move the cable as far away as possible from nearby power cables and electronic gears. Have another person listen close to the speaker drivers while you move the cables. No need to play record to check. We're interested in the noise when nothing is playing. If the hiss level is high, you will hear drastic drop in hiss level as you move it away from the source of interference. As far as I recall, this type of induced noise does not change its volume when volume is turned up. It's just there irrespective of volume level. Personally I accept that phono preamp output will not be as silent as a line level output like CD or DAC. I think it's realistic to accept some amount of noise from most phono preamps. If it's dead quiet, then that's very nice, but probably not attainable unless one takes some extreme steps.
PS: shielded wire highly recommended for phono applications.
PS2: 22/24 AWG shouldn't make a world of difference. Most pro cables are either 22 or 24. 24 is easier to use when you have to insert a wire to a hole in a circuit board.
