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The XO1 is protected at the input by a 5V zener diode. 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 XO1 is protected at the input by a 5V zener diode. 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.
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The XO1.5 is protected by a resistor,(1/16W) which should protect up to 9V continuously, and up to higher voltages for shorter periods of time. Similar protection is planned for the XO1.75.
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The XO1.5 is protected by a resistor,(1/16W 470 ohm SMD0402) and a pair of diodes to ground and to +3.3V which should protect up to 9V continuously, and up to higher voltages for shorter periods of time. Similar protection is planned for the XO1.75.
    
For the XO1, 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 XO1, 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.  
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<i> 680 ohm series resistor on a 3.5mm audio plug</i>
 
<i> 680 ohm series resistor on a 3.5mm audio plug</i>
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Test leads with a zener diode and series resistor would expand the safe range for the XO1.5 and XO1.75.
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Damage can also be caused by applying voltages between the grounds of any of the external sockets.
    
==Voltage Mode==
 
==Voltage Mode==
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