Changes

The XO-1 is protected at the input by a 5V zener diode. ([http://wiki.laptop.org/go/File:XO-1_Schematics.pdf 1]) The allowable input is -0.5V to 5V. Inputs outside this range will cause excessive current and damage. Even a single 1.5V battery can cause damage if connected reverse polarity.

The XO-1 is protected at the input by a 5V zener diode. ([http://wiki.laptop.org/go/File:XO-1_Schematics.pdf 1]) The allowable input is -0.5V to 5V. Inputs outside this range will cause excessive current and damage. Even a single 1.5V battery can cause damage if connected reverse polarity.

The XO-1.5 and XO-1.75 (XO-3 and XO-4 presumably too) are protected by a resistor,(1/16W 470 ohm SMD0402) and a pair of diodes to ground and to +3.3V which should protect -6V to +9V continuously, and up to higher voltages for shorter periods of time.  

The XO-1.5 ( [http://wiki.laptop.org/go/File:XO-1.5_Schematics_rev_G.pdf 2] [http://wiki.laptop.org/go/File:XO-1.5_Schematics_rev_I.pdf 3] [http://wiki.laptop.org/go/File:XO-1.5_Schematics_rev_M.pdf 4] )and XO-1.75 (XO-3 and XO-4 presumably too) are protected by a resistor,(1/16W 470 ohm SMD0402) and a pair of diodes to ground and to +3.3V which should protect -6V to +9V continuously, and up to higher voltages for shorter periods of time.  

For the XO-1, the addition of a 150k ohm series resistor would (I expect, no guarantee) give a reduced sensitivity in voltage mode (0-4V) but allow inputs to +- 100V without damage. Input impedance in resistance, volume and pitch modes is much lower but a 1k ohm series resistor should still allow inputs to +-12V.  

For the XO-1, the addition of a 150k ohm series resistor would (I expect, no guarantee) give a reduced sensitivity in voltage mode (0-4V) but allow inputs to +- 100V without damage. Input impedance in resistance, volume and pitch modes is much lower but a 1k ohm series resistor should still allow inputs to +-12V.