hum problem, please help

I encounter hum problem now, that is my normal connection, no hum

CD analog(RCA) -＞ pre-amp(CD) -＞ poweramp
CD digital(COX) -＞ DAC -＞ pre-amp(Tuner) -＞ power-amp

after I add one more digital out from CD to same DAC

CD digital(XLR) -＞ DAC -＞ pre-amp(DAT) -＞ power-amp

Then I got hum nosie, the only solution is remove CD analog connection

my CD has three digital out (TOS/COX/XLR) and two analog out (XLR/RCA)