I have an amplifier for my car, the car voltage is peaking at about 15 volts, problem is the amp has a cutout circuit above 14-5 volts.
How can I reduce the input voltage without replaceing the alternator, can I do it with resistors etc remembering that it will probably pull about 25 amp

You can't regulate voltage to your amp do this with simple resistors, as the actual current your amp draws is not constant. Hence anything you tried to cobble together with simple resistors would not work. You should look into getting a Voltage Control Module, something like this: XS Power Voltage Control Module - SSA Car Audio Forum (http://www.soundsolutionsaudio.com/forum/XS-Power-Voltage-Control-Module-t35339.html)

Why not just do the right thing and replace the parts to get it right??
Your alternator/regulator should NOT be putting out 15 volts.

Have to disagree Stan, alternator in a 12 volt system should be about 14 volts.
http://autorepair.about.com/od/glossary/a/howitworks_alternator.htm

Dwec,
Voltage can be reduced with the use of a Zener diode, resistor , heat sink and a little box to put it all in. All available at Radio Shack. Sorry can't tell you the number of the diode or the value of the resistor needed. Had some one build one for me to reduce voltage in plane so I could use 9 volt head set.

Undervoltage lockout is more common. I would be interested in knowing what the voltage regulator chip is in the AMP.

13.8 V is considered normal. I'd also like to know what AC voltage you have in the electrical system.

15 V does sound a little high and could provoke other electrical problems down the road. I would urge you to get the electrical system, evaluated. I think you may have a blown diode in the alternator.

Now back to our originally scheduled program:

You can use a bridge recitifier such as this one: Amazon.com: 400V 25A Bridge Rectifier: Electronics (http://www.amazon.com/Parts-Express-400V-Bridge-Rectifier/dp/B0002KRDPI) and connect the positive terminal to power in of the amp and the (-) terminal to the AMP. It will reduce the voltage about 2 diode drops or 1.2 Volts.

Th bridge probably will require heat sinking. Mount to metal with thermal grease.

Radio-Shack may have something too.

You can also try connecting the amp to one of the (~) terminals (The one that works). It will reduce the voltage to about 0.6 V or so.

Bear in mind that this is a very non-standard application and someone might look at you funny.

Is that amplifier designed for automotive purposes? Seem a bit odd that it would have a cut out at the same voltage an alternator produces. Any adjustment on that cut out?

Voltage can be reduced with the use of a Zener diode, resistor , heat sink and a little box to put it all in. All available at Radio Shack. Sorry can't tell you the number of the diode or the value of the resistor needed. Had some one build one for me to reduce voltage in plane so I could use 9 volt head set.

A simple voltage regulator circuit like this is fine for small electronic devices that draw miiliamps of current, but not for use with an amplifier that will be pulling 25 amps. To drop the 15V to 12V for use with a 1/2 ohm load means using 12-volt zener diode and 1/4 ohm (or less) resistor - each of which must handle 60 amps! Like I said - the OP needs a more advanced solution. Or else figure out how to drop the alternator output to 14V .

Or else figure out how to drop the alternator output to 14V .
The answer is super easy.
Replace the darn alternator!

WHY put a band-aid fix on this that will only mask other problems??

For the most part, alternators are designed to put out 14.2 to 14.4 volts without any load. It has become more critical over the years since so much if not all of the cars circuitry relys on computers. Have the alternator tested; it's very likely it's beginning to fail.