I am extremely new to car audio, so please bare with me.
I have a JVC cd receiver and an Alpine 4 channel amp (360 w) which is bridged to a Pyramid amp (800 w) which is connected to a 10" sub (600w I think).
Everything was all fine for a few months. Then the 2nd amp and sub quit.

The deck, and Alpine amp work fine, the Pyr. amp lights up but the rear power and protection lights do not light. I noticed the fuse on the Pyr amp had blown. I went to change it and it blew again on contact. I checked all connections seem fine and the ground is fine. I've opened up the amp and it seems all good.
The fuses by the battery connections are good. I can't see any place on the power cable that is damaged or shorting out anywhere.
Any ideas what the problem could be?

I hope those two amps aren't connected.

What size fuses are you putting into it?

It sounds like the output is shorted/blown. Try disconnecting the subs/speakers from that amp and replace those fuse again and turn the system on. If it continues to go, then it would be an internal output of the amp that possibly causing it which unfortunately, you'll have to send it in for service.

Pyramid?

__________________
Alpine CDA-9887, TUA-T500HD, CHA-S634, CHA-1204 & KCA-410C
Pioneer TS-A6981, 6 x GM-5100T
Kicker 2 x CVR15,
MBQ RCE-216
PG MX2
Soundgate LanStar1
PAC SWI-ALP

I did as you suggested, I disconnected the sub and went to replace the fuse. It blew again on contact and this time the amp was smoking.
(The fuse (#15) is located on the amp, I do not mean the fuse on the power cable from the battery).
I opened up the amp and saw that a small piece labeled "IRFZ44N" had burned. It is located on the board on "Q35" Is this the internal output that you mentioned?
I'm really concerned as to what caused this. Bad connections? ground? faulty amp part? Is is possible that my power cable is too big? (4 gauge)

One blown fet.... usually a good bet that all the fets on that channel are blown too.....

Its possible that you had a faulty component.. but a shorted a connection is the most common cause of blown fets aside from pushing a load the amp can't handle or putting the amp in a position where it can't cool properly....

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SIC Audio

As suspected, an IRFZ44 is commonly used for either power supply or on the output stage. It's quite deficault to conclude what may have really cause it to go but most mosfet components are really sensitive when it comes to power surge or static (such as booosting the vehicle's battery) as just one of the possibilities. Using a bigger gauge wire doesn't harm it at all (in fact, it makes it better). Sometimes, we've seen an output driver resistors failure before the actual amplifier stage component goes, when the subs are shorted could cause it too depends on an amp. In other words, we could not be more specific unless we diagnose that amp on our bench. Please feel free to contact us if you require to arrange a pickup to get that amp service.

You're not supposed to let the smoke out )

__________________
Alpine CDA-9887, TUA-T500HD, CHA-S634, CHA-1204 & KCA-410C
Pioneer TS-A6981, 6 x GM-5100T
Kicker 2 x CVR15,
MBQ RCE-216
PG MX2
Soundgate LanStar1
PAC SWI-ALP