Ok, i have a 850 with a dual 6.5 multimedia indash center unit, had a kenwood kac729s, one 12, Mtx THUNDER in slotted box. First off, the kac amp seized a powerbass sub, so i puchased the mtx, single voice coil, 400rms, 700 peak. The pair played fine, but over a matter of two days the amp seemed to only produce clean bass at med- to high volume levels, it would go in and out, like the amp was losing voltage. today it took its last drop. the amp power light is on, current going thru the rca, and speaker leads. checked wires for damage, none, fuses still good. i tested all the outputs on the headunit, good, u can still listen to the sound in all speakers, just no action from the amp back.
One strange thing i noticed was a pulse going thru the sub. like with the power on, with no volume, you hear and see the sub pulse like a bass note. its not a thump just a faint pulse every 15 seconds. none of the other speakers do this, i had another amp put in its place with a sound meter hooked up and no speaker present, you can see the meter bounce to the pulse.
i conclusion, the sub cone moves freely, no smells, just no sound, at all at the sub, but current going to the sub, if you touch both red and black connections at the sub you feel the current.
WHAT THE HELL IS Going on!!!!!!! this is a clean setup, head unit+rca to amp+ subwoofer. THATS IT. Amp wired to the battery via 4gauge, 100amp fuse.
HELP!!!!!!

Sounds like you blew up your amp. Your 729 seems to be sending DC power to your speakers causing them to go bad. Verify the HU isn't sending a pulse through the RCA's by unplugging at the amp to see if pulse disappears, but more than likely it is inside the amp. Meter the resistance of the speaker to see if it is damaged. Conclusion, buy a larger amp. BTW a 100 amp fuse is a bit high with your current set up.

yea, it literaly shocked me when i touched the connections on the box. the amp was slowly geting warm like it was running corectly.
the 100amp fuse came with the install kit, i have smaller ones. thought it was a little much, but never had an issue priar. if the pulse is coming from the head unit, cant you filter it? inline rca filter, and grounded rca's?
Well, got a Pheonix gold 1000x2 500x1 on call, will this one be large enoughf?
i have a 5farad cap bank i could install if that helps with voltage. i spent too much time in the fab and paint booth, i can hook up anything, but sometimes things get crazy. :haha:

I doubt you can do anything with the RCA signal if it is the culprit, barring a head unit repair or change. Problem is probably in the amplifier anyways. Phoenix Gold makes nice equipment, it's probably 2 steps above the Kenwood in power and quality. The fuses are for protection so the 100 amp fuse is unlikely to blow before damage occurs to the amplifier. The amp under normal operation sends an AC current to the sub, if it is sending DC then it is bad. I am not a big fan of caps, but they do serve a purpose with certain install setups. Caps provide a bank of capacitance for high demand, but will probably drop your voltage a smidge in real world applications.

Test then buy a new amp.

tested, the output went out to the point the level is to low to play the sub. Its a Mtx thunder sub wit 400rms real time power, 800 peak.... i found a MTX Thunder 1800 Watt 1 Channel.
Model: 8001

1800 Watts Max Power

600 Watts RMS x 1 Channel @ 2 Ohm

300 RMS Watts x 1 Channel @ 4 Ohm

Class D (Patented Adaptive Class D)

Low Level Input

Hi-Level Input w/ Smart Engage Auto Turn-On

Frequency Response: 20Hz-200Hz

Signal to Noise Ratio: 75dB

Maximum Input Signal: 2.5V

Thunder EQ: Selectable Bass Boost (0, 6, 12dB) centered at 40Hz

Crossover: 18dB/Octave, 85Hz, Low Pass

Low-Pass Crossover: 40Hz-200Hz, 24dB

THD+Noise (Distortion): <0.25%

Subsonic Filter: 30Hz, 12dB/oct

Maximum Input Signal: 5V

Maximum Sensitivity: 100mV

Dimensions: 9.75" L x 8” W x 2" H

Thunder EQ: 0-18dB, Freq. 40Hz
this is the match for the sub. should be trouble free. or do you still feel the PG is the better choice?

Not to butt in here, but did you figure out the cause of the original problem?
Was it the amp that went bad or something else? If the amp, was it just the amp or something that caused the amp to go bad?

Out of curiosity, when the previous amp when bad, had you been listening to music very loud? If so, was there any clipping going on? Some amp will have clipping protection, some wont, and some will only have warning lights when it is occuring. This can be damaging to an amplifier.

Next...where is the amp located? Does it have good ventillation? I've seen many amps die due to overheating, and trunk mounting can help lead to this, especially during hard usage.

Next....your RCA cables...when you hooked them up at the head unit end did you tape up the exposed ends? (the remaining outer metal surface of the femal end is usually still exposed when you put the wires together). This can cause problems if that is ever allowed to touch a metal surface. Cover them up if you haven't.

As for the amps you mention, I am personally more partial to Phoenix Gold than MTX, but again it's a personal preference thing. I think the PGs are a little more efficient also, but probably a little more pricey.

More perspectives are welcomed. Swed indicated that his sub amp was sending DC current at the outputs. Haven't been a fan of the D series thunder amps. Your setup Swede is 300 watts RMS at 4 ohms.

if its sending a strong enough dc current to shock you then the thermal overload protection system is probably knackered. Its a new amp, if its under warranty get them to sort it.

Are you sure there is no shorts on the wires, HAVE YOU GOT A GOOD ENOUGH GROUND ON THE AMP?

A poor ground can often signify this as the amp will try and ground itself through its chassis and the sub, and just build up current.

Dont attach the ground to a light fitting or something like this, use a clean metal mount, such as seat belt bolt where the metal hasnt been sprayed with paint. This is often 90% of problems when installing amps and troubleshooting.

If that doesnt solve the problem, ask a friend to try one of his amps on your sub, or your amp on his sub, this will also help to determine.

with your sub and idea for a new amp, try to match the subs specs, say for example

400w RMS at 4 ohms (800w peak) is the sub, look for an amp between 300w rms and 600w rms at 4 ohms. The lower the ohm the harder the sub is going to be to drive so more power will be produced.

once again check all your grounds and connections, make sure they are solid and safe (not shorting) and you will probably only need half the ampage on that fuse, say 50.

i run a 100amp fuse on my split charge kit in my boot as its high power stuff.

remember voltage x current = power so 12v x 100a (your fuse) is 1200watts, your running half that so 50 will be good :)

Matt