I just reinstalled an amp in my truck - a Kenwood KAC-745

Its a 4 channel, and I have 4 speakers running off it. All are 4 ohms. For some reason, the amp is getting EXTREMELY hot after 3 minutes of playing (even at low volume). Amp failure?

 

Are you running one speaker on each channel or did you put the front speakers bridged over 2 channels and the back bridged over the other 2 channels?

If this is what you did then it's because your Kenwood amp is NOT a mono stable amp and when you add your 2 - 4ohm speakers together it divides the ohm load in half, therefore:

Front: 4ohm + 4ohm = 4ohm / 2 speakers = 2 ohm load (mono)
Back: 4ohm + 4ohm = 4ohm / 2 speakers = 2 ohm load (mono)

If this is not what you did then one of the speakers might have a blown coil and it's causing the amp to overheat instead of going into protect mode for some reason. There's a few other possibilities, but check those 2 first.

-Kyle-

 

Thanks
I know how ohms work and I have everything wired correctly. Been doing this for over a decade at least. Just seemed puzzled one day, when one of the 745s, which pushes 2 ohms bridged easy, what almost dead cold, while the other 745, running 4 channel 4 ohms is burning the hell up.

 

I thought the more impedance would cause more heat. Its like resistance right? Restricting the flow which the energy has to go somwhere so its transmitted into heat?

 

That is odd for a 745. There good amps but gets some weird problems. Try running the amp with no load (all of the speaker disconnected). If it's still getting hot when it's on with no speakers it's the amp. Probably a bias problem.

Also check your speakers with an ohm meter, it's not that unusual for a bad speaker to sound "OK"and have a low resistance. You may fined that you have a speaker that's only 1 ohm.

 

I've read and heard that it's a really bad idea to run an amp with the speakers disconnected and the RCAS hooked up...

 

Most of the time you would be right, but THAT amp will be fine, some wouldn't be.

 

what causes other amps to have problems with that?

 

It's possible for an amp with no load to developed standing waves on the output. This is "like" a free running oscillator on the output transistor and would damage them. That amp and others are designed to keep this from happening.

 

AWesome, thanks for the help. Didnt think of testing the ohms of the speakers. Will do when I get home.

 

I'm always checking my speakers these days. It's good practice too incase something gets pinched and you won't assume it's safe as you connect your equipment to it.

A typical reading is usually 3.8 to 4.1 ohms for 4 ohm spekaers, at least for me anyways.

 

Latest: I still cant find my Ohms meter, so I just unplugged each speaker, one at a time, and cranked it.
It happened to be the front door speaker (6.5 inch and 1 inch tweeter). Not sure which of the two, since i only disconnected going into the crossover.

But nevertheless, the amp runs so much cooler now.

I did notice, when I hooked it back up to just play around with it, that it picks up alternator noise BIG TIME. Cant believe I never heard it before. It also sounds kind of scratchy and nasty. All the other speakers are fine, and I guess since it is the one furthest from me, I just never noticed.

 

I'd still like to see some confirmation with an ohm meter or by switching the speakers. There still a small chance that it could be the amp. Just disconnecting the speaker don't tell use if there's a bias problem on that channel of the amp or not. It's not real likely but if that is the problem you would be dropping money on the speakers for nothing. When you check the speaker, also check the output of the channel of the amp for DC. Don't think this is you're problem but it's easy to check.

 

Ok well FIGURE THIS OUT. I cant

Checked all speakers. 4 ohms from all, so all have good resistance.

Once again, I am only having the problem with the front passenger speaker set (which is a 6.5inch polk and 1 inch polk tweeter - both hooked up to their own crossover.

Anyhow, I tested each speaker. 4 ohms, good. Hooked them to the crossover, tested the input to the crossover for ohms. 4 ohms, good. Hooked amp back up to the cross over. Amp overheated and some serious alternator whine in the speakers.

Get this - if I unplug the positive input to the crossover, IT STILL PLAYS MUSIC. I was like WTF.

So, I pluged the positive back into the crossover, and just disconnected the tweeter (running only the woofer). Amp did not over heat and sound was good. I hooked ONLY the negative output of the tweeter to the crossover - and sound came out AGAIN.

Something is seriously up with that tweeter, even though it has good resistance.

Figure that one out.

 

no. impedence and resistance are totally different (impedence is frequency specific, for one). the reason an amp works harder into steeper loads has to do with the work asked of the power supply and output stage. the harder they work, the hotter they get.

esp mosfet design class a/b amps...they run hot by design.

bmoney

 

WTF! You live to faraway, I've got to see this. Are the speakers and X-over still mounted when you where disconnecting wires and still getting sound? If so maybe theres a pinched wire hiding somewhere. Try un-mounting ( ) the speakers and X-over. Just let the X-over, mid, and tweet sit on the carpet and try again. Keep the wires separate from one anther, don't let any thing hit anything! If there is a pinched wire, or even a wire that was pinched and now the insulation ripped, and now the wire is hitting ground, the WOULD make the amp heat up as well.
Post back what happens.

 

You don't have frayed wires hanging off of the screw terminals do you?

 

Nope, everything is nicely secure.
Ill root up all the wiring underneath the carpet again tonight though.

 

Rule one, there is no random.
There has to be something shorted to ground. It's the only thing I can think of that could cause this. If you are disconnecting one wire in a circuit that has only two wires (like to your speakers) there can be no current flow. So logically there MUST be anther path for the current flow, most likely a wire pinched to ground. I'd look over the speaker wire to the tweeter fist. Look for ANY defect in the insulation. If this is indeed the problem this would cause the amp to heat up.

 

Thanks, it turned out to be a pinched wire along the trim. Problem solved, although I need to rewire now.

 

a quick test to see if a wire is going to ground is to test for continuity to ground. you end up finding LOTS of mess this way....and then you at least have something to look for.

bmoney

 

Dont know why I didnt think of that in the beginning.
I have been running the amp (testing) and its not getting as hot as it used to.
Still getting up there, but not as bad. I assume it might have done some permanent damage? Im serious - that amp got HOT HOT HOT. Cook breakfast on it, I dont think the thermal protection auto-shutoff device works on it.

 

could very well be something wrong with the amp.

bmoney

 

Vary likely there was permanent damage to the amp if the amp got that hot. An engineer I knew always said, "There's only two ways to kill a transistor, heat and a hammer. Over voltage counts as heat...".
How's it sound? Can you check for any DC on the outputs of the amp?