I'm using shielded wire to connect all of my inputs to my switch, with the shield grounded. Unfortunately, they way I've designed my unit has my input lines going directly over the SMPS (SMPS500R from Connexelectronic) on the way to the switch. I was considering adding a faraday cage to the SMPS with aluminum foil around the sides to keep the EMI to a minimum, but this is a lot of work. Will it be beneficial to add a cage if I already have shielded wire? Or will this shielded wire be enough? It is 18GA 100%foil shield. I'm not able to test the amount of noise with the new shielded cable since my amp and SMPS are being fixed, but I can build a cage if it would help while I'm waiting.
You might try 22ga mic line. It has a nice tight twist for noise rejection. You might try grounding the shield at on end and floating the other end. Make some measurements on each way and let us know what works the best.
A foil shield works best at radio frequencies. At lower frequencies a braid shield works better. You might think about using a steel plate or tube as a shield.
My cable has a 100% foil shield, but I'm considering adding a cage for extra protection. According to a wavelength calculator I found online, even 20kHz is only a .678" wave, so a .25" mesh from Home Depot will work fine as the spacing is less than half the wavelength. These combined, I should be able to contain most if not all of the EMI from the SMPS.
You are mixing acoustical wavelength with electrical wavelength. The 20kHz wavelength is about a mile.
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