Turn Table Cable Length

Preferable to have a shorter length from turntable to amp and longer interconnects or speaker cables. If tt is your only source than move your amp together.
G

Your cartridge is MM/MI or MC? 

If you have a moving magnet cartridge this is a really bad idea as the added cable capacitance is going to really screw up the high frequencies.
You can get an inexpensive phono stage, very small and put it next to the table then as mentioned above run long interconnects from it to your amp. It is still not ideal but a bunch better then running long turntable cables.

If you are talking about a distance between the tonearm and the phono stage of 20 feet, that is a very bad idea regardless of what type of cartridge you will use, but it is an especially bad idea for a low output moving coil cartridge. Can you figure out some way to place the phono stage close to the turntable, within 3 ft ideally?And then it would be more acceptable to run a long cable from the output of the phono stage to your line stage. This is not just audiophile fussiness. A 20 foot phono cable will seriously degrade your listening pleasure.

@smccarthy  This is a really Bad Idea! Here's why:

The cartridge has an inductance. The tonearm cable has a capacitance. They are in parallel; when this is so an inductance and capacitance produce an electrical resonance. If its a MM cartridge the resonance will be in the audio band (the response will be a long way from flat). If a LOMC the resonance is slightly higher Q value (more peaked, due to the nature of a moving coil) and will be about 1/3rd the frequency that it would normally be (which might be from 100KHz to 2-3MHz)


In the case of the former obviously you want this outside of the audio band. In the case of the latter, that peak, if energized, can and does overload the input of many phono preamp designs (resulting in ticks and pops). The lower you make that frequency, the more likely the phono section will have bandwidth; the easier it will be to overload it.


If you really want to go that length, you'll need, at the turntable, either a preamp with a balanced output or a transformer that is used to run a balanced line to your phono section where it it converted back to single ended with another transformer, if you don't have a balanced input on the phono preamp.