I am getting the nine volts from the power supply by using the +12V and +3.3V wires.
I seem to recall reading somewhere that it was unwise to do that.--Phil.
QuoteI seem to recall reading somewhere that it was unwise to do that.--Phil.Do you remember what the reasons were for that? Like I said above....I am very new to all of this.
the computer will supply currents different than what was supposed to work.
connecting a +3.3v as a "relative gnd" and in some point of the circuit you can forget you did this, and connect the real gnd to the same point, creating a short-circuit on +3.3 power supply
That is totally wrong. There is nothing odd or different about the current sourced this way.
because the current will flow in opposite direction on the 3.3 pinout[\quote]Yes I understand what you are saying but I disagree. A power supply has a low output impedance and will not be affected by the current flow what ever the direction.If there is already current being sourced by the 3.3 volt rail then the extra load will go to supply that load. The regulation will then take up the slack and throttle back the current from the load. The only way I can see difficulty is if the 3V3 rail is not regulated but relies on being a proportion of the higher voltage. Even then I can't see it being much trouble at the currents we are talking about. Powering op-amp circuits like this has been going on for years and I have done it many times myself.