Why it works - External PSU with ground unattached?

[Robert M]

Hi Guys, 

I am working on a preamp based on many OPA1688IDR and I wanted to test if I can connect external power supply.  So I run the Entire preamp with one a standard power supplu that it has (+-12V)

Then I bent the + and - Power supply pins (4 and on the opamp so that they do not have connection to the main power supply. I connected them to the second +- 15 V power supply and since the opamp itself does not have the ground pin itself I did not connect the external PSU ground to the preamp ground.

I though it would not work correctly, but it did without any hum or issues. So my question is with +- 15V stabilised power supply with LM7815 and LM7915 i read that you have to connect the ground for proper operation.

So does the opamp + - Pins only requere the + and - difference form the PSU and basically work like the Vaccum Tubes heaters where all they need is the power ?

thank you for your insight since it's really intriguing to me   

[magic]

I doubt this could work. Current runs in closed loops, so how could the chip drive its output load?
Are you sure the ±15V PSU doesn't have a hidden connection with preamp ground, perhaps through mains?
What are the voltages on those supply pins with respect to preamp ground?