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You have probably blown the internal power supply fuses in your amp or the speaker fuse in the amp. The chances of blowing your sub are slim.
The internal fuses can sometimes be hard to find. They aren't usually intended for the user to change. DO NOT replace them with higher amperage fuses if they blow after you replace them as there has been severe damage caused to the circuitry. Also, don't replace them with higher amperage fuses because you think that this will let you get more power out of the amplifier. This won't work, and you will end up blowing your expensive output devices.
The fuse is blowing due to an overload. It's often and very likely caused by a transistor or ic that has gone short. The main culprits are the power transistors of the amp or whatever is on the big heat sink. Transistors will show the fault when you connect an ohmmeter to the terminals. It will be just like you have touched the test prods together! I don't think in your case it will be in the power supply. It's often caused by speaker cables that touch together.
Disconnect the speakers, all of them. If the fuse continues blowing off, you have a short circuit in the final stage of the device, called the amplification circuit. If the does not blow , then the short circuit in is one or more speakers.
The main power fuse for most amplifiers is a fast blow, 125 volt fuse for the US and a 250v fuse in countries with 220v. This unit is 125 watt so the main fuse should be 2 amps or so. The correct fuse size is written on the circuit board next to where the internal fuse was located.
Make sure that the fuse is the same rating with the old one.
If still symptom persist, I've suspected that your output IC is shorted.
You may hang this IC, then power up the unit.
Observe what will happen, if this cure the symptom then this is a clear indication that this IC is shorted and made the fuse blown while it is in the circuit board.
Hope I helped you.
Have a nice day!
Thanks for using Fixya.
Have you the remains of the fuse, or was it just not in place when you got it?
The reason I ask is that the amp's fuse doesn't have much to do with the voltage of the mains supply (in your case a 250V fuse will cover it). It goes on the load in Amps or Milli-amps. Shown as A or MA. If you have the old fuse you might be able to see it on the case of the fuse. It might say 1.5A or 500ma etc. Also there are two main types of fuse, known as fast and slow blow fuses. Again if you have the old fuse the remains of a coil inside will mean it's a slow type. The other type is just a wire. However you can fit a fast blow for a slow one. If the fuse blows again it will be a fault that causes it.
If you have no fuse left at all you will either need to find someone who has the same amp and to tell you what it is, or the circuit diagram for the amp. Sometimes it's even printed on the PC board!
Looks like a electrical problem. Ensure your plug's earth is properly connected to the building earth. Amps like 640A uses sensitive circuits and the fuse will definitely blow, when there are issues, spikes or returns in the power supply.
Try shifting the amp to a different wall plug with some proper ground earth, and check it. If the fuse still blows, then the power cable in the amp may have some problems.
if the fuse blowes when you swtich the power on its the power supply circuit. if the fuse blown after a short while that the Amp is on its possible that protection circuit is short
if you want to fix the Amp let me know