dual voltage with 4 batteries 9V, please help

[DIP8]

hi guys;
I know this wiring to connect 2 9v batteries to have a dual voltage +9v , 0, and -9

how i can to connect 4 batteries? to obtain +18 0 -18?
Thank you very much if someone can help me 🙂

 

[Mooly]

Just connect another 9 volt battery in series with the ones you have.

 

[DIP8]

Mooly...thank you very very much 😱

 

[motronix]

or find converter from 1 9V to 2X18V

 

[mchambin]

I have seen -18v 0 + 18v and +45v for a ultra low noise portable microphone preamp.
With seven 9v rechargeable batteries.
What does exist doing this with SMPS, with ultra low noise.