Voltage Limiter help!

[DJ Higgumz]

building a sub box for a friend, and i want to limit output from the amp to 19 volts. don't get on me about miscalculation of volts, this sub is very efficient, at like 110db per 1 watt, that is the correct voltage . anyways what I want to achieve is to have something to limit the voltage once it gets to that point. even if it is a led that turns on at 19 volts that would work. simplicity and affordability are key! new user so be nice