Anyone know how low the voltage would have to go to blow an amp?
I'll take a stab at this.
First you need to determine if your amp has a regulated power supply or not. Look in the owners manual. If it shows the amp will produce more power at higher system voltages (for example 500W at 13.5VDC and 600W at 14.4VDC) then its power supply is NOT regulated. Running this type of amp at a lower voltage will not damage it. It will just deliver less power to the load. A dangerous scenario with this type of amp is when you run too high of a voltage.
If the amps manual does not list increasing power levels at various system voltages then chances are its internal power supply IS regulated. What does this mean? It means it will try to maintain a constant power level delivered to its load (speaker) regardless of input voltage. I'm going to ignore losses and the fact that an amp will not continuously deliver full rated power to a load while listening to music just to simplify things. If the amp is rated to deliver 500W into a 4 ohm load it will need to draw about 35A from your vehicles charging system at 14.4VDC to do so. It will need to draw about 42A at 12VDC. And finally it must draw 50A at 10VDC to deliver that 500W to your speaker. Accounting for losses increases the amount of current the amp needs to draw. Accounting for the fact that listening to music instead of sine tones reduces the amount of AVERAGE current the amp needs to draw.
Its the added current pulled by the amp that eventually kills it. High levels of current stress the semiconductors inside. A lot of heat is dissipated by them. If they get too hot, they get damaged or fry.
Why did your amp die? Well, one could only speculate. Chances are that at some point, the MOSFETS residing in its power supply could not keep cool enough. They may have been damaged during some period where you were working the amp REAL hard. This could have weakened them. Another possibility is that one or more died and left the rest tied in parallel to them to do all the work. This causes the remaining ones to work much harder than normal. They would eventually fry.
Another possibility, the MOSFETS did not have a good thermal transfer medium to the heat sink. Thermal paste that dries up or breaks down will do this. Maybe the screws or clamps holding the MOSFETS to the heat sink came loose. If the tab to the FETs do not press hard against the heat sink the FETs cannot keep properly cooled.
Another possibility is that something in the output stage failed. Shorted output transistors, output transistors that are stuck in the on state due to a failure in a prior stage, etc... This would cause the power supply to overload and fry prematurely.
This is VERY important. When you install the amp, make sure it has plenty of ventilation. Don't throw your gym bag or dirty socks on top of it and then work it hard. If the air mass surrounding the amp heats up, so does the amp. Also, DON'T mount your amp to a sub enclosure. The vibration will eventually beat the amp hard enough to cause premature failure.
A number of other candidates also exist but I will not go into them now.
The fact of the matter is amps can die prematurely. The only way to prevent this is to use them within their rated limits and to perform maintenance to them on occasion. Always start with a clean install and ensure the amp can vent properly. I do other things that are above and beyond what people consider typical. For instance, I pull the board out of my amps and place fresh thermal paste (or a new sil pad) on the power transistors about once every year or two. I also make sure the clamping mechanisms are good and tight. My amps have fans inside for forced air cooling. DO what you would do with your computer equipment. Blown the dust bunnies out once in a while.
