ayeomans You would need to buy one and then solder it into the circuit board where USB power comes in. Maybe add a switch.

ayeomans You don't need to switch. It depends on the load. The converter will supply constant voltage at lower currents. If you try to draw more current, it will limit at the constant current value by reducing the voltage.

ayeomans You will need a separate pre-amp to boost and equalise the turntable signal. (Unless that pre-amp is already built in to the turntable).

ayeomans Yes - Sim.

ayeomans Use mode select button to setting "bLUE". On other device (computer or phone) select Bluetooth settings and scan for new devices. The amplifier appears as a headphone device called "APPT". Select to connect and you are done. Note the amplifier can only connect to one device at a time, you will need to disconnect from any other devices before connecting to a new device.

ayeomans It turns the DC output on and off, but the unit and displays remain on. They remain at the set voltage, as you ask. You would need a separate switch to turn the converter itself off. You can set a default voltage which will be remembered for next time the whole unit is turned on. A nice unit! But do read the descriptions to see all the functions. I wish there were a separate manual rather than web pages.