It seemed strange to me that I would need 2x9v batteries.
Now in this tutorial it says that you need 2 9-volt batteries in series (the middle to ground) to generate the positive and negative 9 volts for an op amp......and negative 9 volts for an op amp. ...and +/- 4.5 volt for an op amp.
Now the same author have made a tutorial for an audio input for the arduino (offsetting the signal so there is no negative signal going into the arduino)
My problem is I really don't want to use 3 of my power-suppy outtakes to power a single pedal.
You can do similar things with an op-amp to run it from a single supply.
If you've got something like this, the outputs probably have common grounds so you can't get -9V or connect the outputs in series.
What difference do you think it would make if the signal were inverted?
There is so much out there on the internet and youTube.