Isn't it strange, that with your AVR and DSP off, and the ADA to DSP cable connected, the hum is still there? But when you remove the ADA to DSP cable, the hum subsides.
I have a theory and the explanation is technical but I will try to explain as simple as possible. This is just a theory which has not been tested. There is a 60Hz noise source in the ADA. It is between a signal input line and ground. In technical jargon, this is called a common mode noise source. The source of this noise within the ADA is not known but it is parasitic and it is related to the 60Hz power fed into the ADA.
Why do I suspect this? When you disconnect the ADA to DSP cable at the DSP, the only thing "across" the cable circuit is the capacitance of the ADA to DSP cable. This capacitance is tiny - on the order of 150pF. This small capacitance cannot pass the 60Hz current from the noise source or passes very, very little. Therefore the voltage across the cable capacitance is tiny. This voltage is right across the ADA signal input. Therefore you hear very little buzz if any at all. I don't hear anything BTW when this cable is disconnected.
When the cable is connected to the DSP, the DSP has a finite output impedance. That impedance is right across the cable capacitance. It allows 60Hz current from the noise source to pass. Hence, you hear the buzz.
The theory therefore is a parasitic, common mode noise source in the ADA that arises from the ADA's input power and is somehow impressed on one or both of the ADA's input signal lines.
If this theory is proven true, there is nothing you personally can do about it.
