turn off sonos port audio line output

The amp has output lights and they turn off properly. The only thing that doesn’t happen is that the amp doesn’t go into low power mode. 

What happens if you mute the PORT’s?

If I hit mute for each of the PORTs, there is no change.  It looks like there is always power to the analog output of the PORT.  Not enough power to turn the amplifier on so that the output channels are active, but some power so it doesn’t go to low power mode. 

Things will become more difficult to diagnose from here on. Likely you have what is known as a “Ground Loop”. These can become quite a problem in large installations, such as recording studios and radio/TV stations. There are specialized consultants that earn a nice living resolving these large issues.

The “shield” in audio signal cables is a knitted wire mesh that encloses a center conductor carrying the signal. The shield attempts to keep out stray electric fields. The idea is that an electric field will be intercepted by the shield and conducted away from the signal wire inside. This works well as long as the electric field is not too intense. 

You are also familiar with electric generators where a moving magnetic field induces current that can be channeled for lighting and mechanical functions. A magnetic field near our audio signal wire can induce a small current in our signal wire -- but our signal is also a small current. This induced signal can swamp our audio signal. Sorry, but the magnetic field will not be stopped by the shield.

I’m sure that at some point you’ve touched two appliances and received a shock. This is due to the (usually metal) appliance shells being at different potentials and, as you touch both, current flows through your body. Appliances should be “grounded” in attempt to keep all the shells at the same potential and eliminate this shock. Unfortunately, all “grounds” might not be at an equal potential (why? This is where the consultants earn their big money) and you receive a shock anyway.

With respect to electronics, it is common that the outside shield of an audio jack is connected to the equipment shell, which is often connected to the building electric ground. If the grounds are not equal, current will flow in the audio cable’s shield. We call this a “Ground Loop”. Part of the physics is that this current creates a magnetic field. Just as in our power generator, this magnetic field will induce current flow in our audio wire -- competing with the audio signal.

As a diagnostic:

If you don’t have a couple “shorting plugs”, create a pair by cutting into a discarded RCA cable and connecting the central conductor to the shield. Keep the wire short. First insert your shorting plugs into the amplifier inputs (power down before doing this) and verify that the amplifier remains in standby. This eliminates the possibility of a ground loop between PORT and the amplifier.

Next connect PORT as usual and install the shorting plugs into Line-In. If PORT is muted, the amplifier should shut down and stay shut down. Even if you unmute PORT and play its Line-In, its output should be near zero because of the shorting plugs. Other than the wire connection to the amplifier, PORT’s  own power cord, and the shorting plugs, there should be no wire connections to PORT.

If the amplifier-PORT combo goes into standby when shorting plugs are inserted into Line-In, we’ve eliminated the possibility of a ground loop between PORT and the amplifier and need to start casting a larger net for ground loops. We’ve also eliminated the possibility of some low level, stray output from PORT.

You may find that simply reversing and re-inserting a power cord in its socket will create or improve a ground loop. And, you’d need to explore every possible combination of power plug orientation.

I understand what you are after.  I am starting by unplugging (from the power supply) both sonos port.  The result was that the amplifier did go into low power mode.  I then plugged in 1 of the ports and it stayed in low power mode.  When I plugged in the second port, the amplifier came out of low power mode.  I have to wait for 20 minutes to see if it will go back into low power mode.

It appears that it is one of the two ports that make the amplifier stay out of low power mode. That amplifier has no inputs.  It only outputs items from other inputs that it can select. I tried to use the digital output to the amplifier and same issue.