Some preamp systems allow for the use of either 9 volts or 18 volts for power. Check the specifications for your preamp to make sure that it is recommended. Some known systems that DO allow for 9 or 18V are: Aguilar OBP2, OBP3; Bartolini NTBT 918, NTMB918, Delano (all); John East (all); ACG (all); Nordstrand (all); EMG BTS, BTC, BQS, BQC.
To make a 9 volt system into an 18volt system, connect black wire from first battery clip to red wire from second battery clip (solder together and tape off). This uses the (2) 9 volt batteries in series.
** Colors for hot and cold battery leads may differ from the common red and black used here. If that is the case, follow the connections for the positive and negative wires with your battery clip (or battery box).