Audiophiles, I need help!

[ravegoo35]

OK, I have a decent amount of know-how in car audio, but this problem is stumping me.

I drive a 2003 Infiniti Sports Coupe, and am trying to install one MTX Thunder 8000 12" sub, powered by a Kenwood KAC-7252 amp. The amp is rated at 570 watts RMS, 1000 peak.

The Infiniti comes with a factory Bose HU that has no RCA outs. It does however, send signal to an external Bose amp and that amp then powers the speakers. What I did for signal was quick tap into the rear deck outputs, BEFORE the Bose amp. This way I am not running a strong signal into my amp. I then take the signal and splice it onto the speaker level input harness. So that takes care of my signal. Oh, I am running the sub bridged.

To power the amp, I am running 8 gauge power and ground wires, with an inline 40amp fuse on the power line. This was all part of a Scosche kit. I am getting remote turn-on from the same place as my signal, from the Hu outputs, pre-Bose amp. This 8 gauge kit said that it could handle the kind of load that a 4 gauge could, that's why I bought it. In my last car, I had about 900 watts RMS between two amps running on 8 gauge with no problems at all, so this is why I went 8 gauge. That takes care of power.

Now for the problem. So, the amp is receiveing power and turning on and off at appropriate times. That seems OK. When the volume is low, then he sub seems to be responding correctly, no phasing issues. What happens is that any time I turn up the volume, gain, or input level (on the amp) then the amp cuts off. It gets very hot very quickly and then cuts off. I can get the sub to hit at maybe 1/5 of the power that I know is there, but then it just shuts off.

Any help is greatly appreciated, Iv'e been trying to overcome this problem for weeks! This is the third brand new amp I've tried on this setup, with no changes. I even hooked up my friends sub to my amp and it ran fine, and then hooked up my sub to his amp and that worked fine! There is some problem between my sub and my amp! Once again thanks guys/girls.

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[jrz126 10+ year member]

how high do you have the gain set? and what ohm load are you running to the amp? sounds like you might be running too much power through it and the thermal protection is kicking in.

 

[RandyJ 10+ year member]

I would say there is a gain problem or you are running too low of a load with that amp. Most amps when bridged are only 4ohm stable.

 

[ravegoo35]

Well, if I keep the gain low, then the sub hardly hits at all, and as soon as I turn it up the problem arises again. For example: the input level on the amp is at 3/4 of its' max, the volume on the HU is at 5 of 35 and the gain is all the way down. In this setup I could turn the HU all they way up and it would not cut off ( I also would not hear the sub), but as soon as I turn up the gain, the amp cuts off again.

As far as the ohms INTO the amp, I have no idea. How could I get this info, and how would this be affecting the performance?

 

[RandyJ 10+ year member]

A lot to learn young grasshopper.

Give us some specs on yoru sub and amp. Specifically an online manual for the amp and coil specs for the sub. Then I will go into the physics of it.

 

[ravegoo35]

I can't find any online manuals, sorry. Crutchfield's website says that the amp's minimum impedance bridged is 4 ohms, and unbridged is 2. Cardomain.com's specs on the MTX Thunder 8000 12" says it's impedance is 4 ohms. This should be the correct setup right?

Oh, N00blet, yes this grasshopper has a lot to learn.....

 

[RandyJ 10+ year member]

lol.

Well, if those subs are svc subs then there should be no problem with resistance because 4ohms is all you can show the amp.

Im confused now. You say you hooked up your friends amp and it didnt shut down? But as soon as you did it with your amp it did. Hmmmmm, im sure the answer is really simple, im just not catching it.

 

[ravegoo35]

I've heard that if I am grounding to the car's body, then the ground to the negative battery terminal is only maybe 12 gauge, and this impedes the circuit, causing the amp to run harder than it should. Have you heard anything about this? I can't verify it's accuracy.

 

[60ndown 10+ year member]

take a dmm (digital multi meter) to the speakers / level coverter, see what you get at lower volumes vrs higher volumes? perhaps even tho you have tried to bypass boses' install you are geting a problem with output / input voltages?

 

[SSS 18734 5,000+ posts]

take a dmm (digital multi meter) to the speakers / level coverter, see what you get at lower volumes vrs higher volumes? perhaps even tho you have tried to bypass boses' install you are geting a problem with output / input voltages?

^ I dont think that is your problem. Im running 1000+ watts of a factory ground with no problems

 

[RandyJ 10+ year member]

You are only using 12awg for a ground? That may not be a good thing...

 

[ravegoo35]

No, I'm running 8 gauge, I heard that the battery itself is grounded by 12 gauge, and that is the bottleneck.

Where would I need to test with the DMM, on the speaker level input harness?

Also, if this is helpful at all, I hooked up the sub to the left channel (unbridged) with no change in performance.

 

[Goldenknob 10+ year member]

have you tried your amp in another vehicle to check and see if it works properly or acts up again?

 

[ravegoo35]

That's actually my next step. I have my old car where the same MTX sub was working properly, so I will hook up this system back in that car. The waires are still run there so it should not be difficult to re-install.

Besides my old car having RCA's, and my new car using speaker-level inputs, what could be the difference between the two? I've even tried a speaker level to RCA converter in my new car to no effect. I have no idea what it could be.

 

[ravegoo35]

OK, I just read that my Bose speakers are at 2 ohms. The sub will be running at 4 ohms. Does this present a problem?