A question I often get is, "Why is my fuse blowing". There are many reasons this can happen and there are many fuses that can be blowing out. Some of these problems are outside of the amplifier and are relatively easy to hunt down. Others are inside the amplifier and cannot be seen while the amplifier is installed. However these internal problems usually show external symptoms and a knowledgeable installer or technician can make a good guess on what is wrong based on these symptoms.
For this article I've asked Perry Babin to explain what these common problems are for all of the fuses in a car audio amplifier installation. Perry is the webmaster of Basic Car Audio and Electronics and a professional amplifier repair technician with over twenty years experience. He's also created a huge tutorial on repairing car audio amplifiers and I've included a banner link at the end of his article that will take you directly there. You can also use the text link right below the banner. His tutorial is the only one I personally recommend for anyone wanting to truly learn hands-on, practical amplifier repair.
In the following images, the green indicators show whether voltage is present or not. If it's bright, there is voltage present. When dark, there is no voltage on that point in the circuit. When a fuse is blown, there is voltage on only one side of it. When the fuse isn't blown, the voltage is the same on both terminals of the fuse. In the image below, you can see that there is voltage on every point. This shows that there are no blown fuses. Click the images to open a higher resolution version in a new window.
If the battery fuse blows 'as soon as it's inserted into the fuse holder' (amplifiers off), the problem could be anywhere in the power distribution line. If the system is set up as shown below, and the fuses in the dblock are not blowing, the problem is likely a direct short to ground between the main fuse block and the dblock. It's not likely beyond the dblock because the fuses in the dblock are not blowing and they are rated to pass less current than the main fuse. If the problem were beyond the dblock, it's more likely that the fuses in the dblock would blow first.
If the main fuse isn't blowing but one of the fuses in the dblock is blowing, that tells you that there is a problem beyond the distribution block. It could be that the wire feeding the amplifier is shorted to ground or there is something wrong with the amplifier. If you can see the entire length of the wire, make sure the insulation is intact. If you cannot see all of the wire, disconnect it from the amp and see if the fuse blows when it's inserted into the fuse holder. If it does not blow, the wire is fine and there's probably something wrong with the amplifier. With more experience, you'll be able to test for a short with an ohm meter.
If the fuse in the amplifier is blowing, the problem is almost certainly in the amplifier. If the fuse is blowing as soon as power is applied to the amplifier (with no power applied to the remote turn-on terminal), the amplifier likely has power supply problems. Generally this means that the power supply FETs are shorted or the reverse protection diode is shorted. The FETs in the switching power supply generally fail when the amplifier is connected to an ohm load that's too low. The RP diode is generally damaged when the amplifier is connected with reverse polarity (positive/negative connected backwards).
If a fuse only blows AFTER the remote terminal has power applied, it's likely that the amplifier has shorted output transistors. The amplifier could also have problems like a shorted winding on the transformer or shorted rectifiers. If the fuse only blows after the amplifier is switched on AND the volume is turned up, there could be a problem with the speakers or the speaker wiring. The reason the amp doesn't blow fuses when the volume is all of the way down is because there is no voltage on the outputs and therefore no current through the short circuit.
When an amp blows a fuse (or fuses, for amps with multiple fuses), you should replace the blown fuse with a fuse rated for less current. Let's take an amp that has two 30 amp fuses. If the OEM recommended fuses blow, there is likely a serious problem. Fuses blow more easily if they're warm/hot. When the fuses blew in the amp, it had likely been driven hard and the fuses were hot. This meant that they blew relatively easily. When you replace the original fuses with cold fuses, it will take more current to blow them. Since it's likely that the amp has suffered some type of internal fault with the original fuses, you're likely to do more damage if you replace the fuses with identical fuses. When replacing the fuses, temporarily install fuses rated at ~1/2 the current of the originals. This will help protect the amp's power supply and circuit board if they were not damaged when the amp blew the original fuses. If the temporary replacement fuses blow when you switch the head unit on (volume set to the minimum), the amplifier probably has blown/shorted output transistors. If the amp powers up normally and plays at relatively low volume, the amplifier is likely in good working order and you can replace the temporary fuses with the OEM recommended fuses.
Blown output transistors are the most common failure in amplifiers and are also the most common cause of an amplifier going into protection mode. For most amplifiers, replacing the output transistors is relatively inexpensive. You can expect to pay between $60 and $90 for amplifiers up to ~150 watts/channel. Larger amplifiers use more transistors and the transistors are commonly more expensive so the repair will cost more.
If the power supply is blown (can often be avoided by following the previous suggestions), the repair can cost significantly more. Many times, when the power supply is blown the circuit board is damaged which can make it very difficult to repair the amplifier (sometimes, they are not repairable). If the power supply AND the output transistors are blown, expect the repair to cost at least twice as much as it would cost if the power supply had survived.
When you need to have an amplifier repaired and you don't know where to take it, call some of the better car audio shops in your area and ask them who they use for repairs. If several use the same shop, it's a good indicator that they do good work. If you have an amplifier manufactured by one of the large Japanese companies (Pioneer, Sony, Kenwood, etc.), contact the manufacturer to find the warranty repair station in your area. The warranty station is likely to have the parts and service literature to make the turnaround time relatively short.