View Full Version : I need a new amp -- please help!

05-21-2007, 01:56 PM

My JL Audio 2 channel amp died this weekend :(

It was running my tower speakers, a pair of Skylon Rubicon 450s. The amp was only capable of 150w bridged, and the 450s are rated at 450w.

I want to upgrade to a more powerful amp, dedicated to the tower speakers. Something in the 400-500w per channel range. I haven't been able to find any affordable marine amps in this range.

All my wiring is in place, so it should be an easy swap. My system is wired and fused (4awg).

05-21-2007, 02:24 PM
What's the nominal impedance on the 450s?

05-21-2007, 02:29 PM
What's the nominal impedance on the 450s?

Nominal Impedance - 3 ohm

Specs can be found here as well:




05-21-2007, 02:39 PM
Ah now I see why the JL is toast.

It's going to be tough to find a multichannel amp that will safely deliver the power you are requiring at the specified impedance.

I would go with a very large 2 channel amp which is safe a 2ohm stereo. The Kicker SX900.2 or ZX850.2 would both fit the bill.




05-21-2007, 02:47 PM
Thanks so much Diesel.

The SX900.2 looks like a perfect match. I like the fact that it is fan cooled.

Can you explain to me (in short) why the JL audio amp died? I am no electrical engineer. (I may have to shoot the Mastercraft dealer that recommended and installed that JL amp -- j/k)


05-21-2007, 03:28 PM
Not too sure how they are wired but it sounds like the amp was running at 1.5ohms bridged mono. The JL amps can only handle a 4ohm mono load.

If it is not bridged and is running at 3ohms stereo it's just too small an amp for those speakers. It most likely overheated.

If you go with either of the kickers you will need to up your power cables to 0. Also make sure you have room the kickers are both over 2' in length.

05-22-2007, 12:09 AM
Not too sure how they are wired but it sounds like the amp was running at 1.5ohms bridged mono. The JL amps can only handle a 4ohm mono load.

So is anyone suggesting that there is such an amplifier that will handle a 1.5 ohm load bridged? That is the same as the amplifier seeing a .75 ohm load in stereo operation and I have never heard of an amp being stable below 1 ohm.

According to their website, there are 6 drivers in a pair...your best bet could be to go fully active. It would be an expensive move, but you would be very impressed.

05-22-2007, 12:39 AM
shurgot-First of all, 4 AWG is too small for a 450-500W amp unless it's not going very far. Running it hard with a low impedance load will compound the problem. Watch your loads and keep them where the amps are happy. You probably showed a load that cooked the amp and I wouldn't be surprised to find that your tweeters are toast. The fact that you are using a 150W amp on speakers that are rated at 450W and think there's no reason for problems tells me to recommend that you do some reading about audio, electronics and system design. Low or inadequate power kills speakers faster than high power and driving an amp to the point of death can also be caused by improperly set input sensitivity controls. They aren't gain controls- that's determined by the op-amps at the input and the output devices. The input controls limit the voltage going to the first gain stage of the amp. If you heard a lot of hissing and whine when the volume was turned down, the sensitivity controls were set too high.

If I knew the system was going to be run hard and put away wet, I wouldn't use less than 2AWG or 1/0 power cable and a second high-reserve battery, with a high output alternator. No stiffening cap, a second battery. If the amp(s)' positive and ground cables don't go directly to the battery negative, full power will never be developed, either.

05-22-2007, 12:49 AM
shurgot- there's nothing wrong with the amp, it just wasn't compatible with the way you want to use it. Did you tell the dealer exactly how you listen? If not, don't expect him to know how to put an audio system together as well as a good car audio shop, or how to tell what the customer really wants. For that matter, why didn't you just go to a car audio shop? Boat dealers sell audio because they want to sell things, not because they necessarily have someone on staff who really knows how to design a system or correctly install it. If I had a dollar for every customer who told me that "I don't listen THAT loud", then come back in a couple of days with blown speakers and fuses, I'd be typing this form a desert island somewhere. It was your job to tell him how you listen and it was his job to find something that would do twice what you told him you needed. Some are in the position of being lucky enough to have someone who really knows audio systems well, most aren't, but the customer has to ask specific questions in order to find out what the dealer can or can't do and what they do or don't know.

If you ask 1000 people just how loud is "loud enough", you aren't going to have a lot of agreement. The dealer may have thought it was loud enough, based on his desire to retain his hearing or maybe he's just tired of hearing really loud systems.

05-22-2007, 01:11 AM

Thanks for taking the time to respond.. Lots of good info.

True, I guess I didn't do a good enough job telling the dealer what I wanted. Yes, I want a loud system. I want it to sound good and hit hard 70' out at 20mph (while wakeboarding).

The speakers are still good. I tested them out on another amp at low volumes and all checked out OK -- phew!

I currently have two optima deep cycle batteries dedicated to accessories and the stereo. I have a third optima battery that is for starting. All three are combined when a charging source is detected (done by a Blue Sea battery combiner system).

The amps are directly wired to the batteries with 4awg wire. I will upgrade the wire on the replacement amp to 1/0 (or 2awg). I am going to order the Kicker SX900.2 that Diesel recommended. I called Skylon and they recommended the same amp.

05-22-2007, 01:25 AM

Per your advice I searched the internet for some info on audio, system design, etc. I found a good article I would like to share:


This explains why my amp died. Thanks again! I am going to keep reading.

05-22-2007, 09:13 AM
If you look at speaker specs, they're usually rated as XX db @ 1 Meter (or .5 Meter) with 2.83V output. 2.83V is the reference output level and with an 8 ohm load, it constitutes 1 Watt. 1 Watt @ 1 Meter has been the standard for decades and in an effort to show a larger number, some companies started using .5 Meters for their testing, which is BS since there's a 6db increase with halving the distance and is the same as multiplying the power output by four times.

That said, if you look at the speaker sensitivity for your speakers and take note of the distance used to determine the output level with a 2.83V signal. Now, double that distance until you get to 64', which is close enough to 70' that the volume at either position won't be perceived as different. If 1 Meter is the measurement distance, there will be an 30 db drop in volume level at 64' and if they used .5 M, it'll be 33 db. Assuming they used 2.83 V at 1 Meter (1 Watt), it takes 1000 Watts to be perceived as the same loudness (intensity) from 64 (or 70) feet and if they used .5 Meter it will need 2000 Watts. You're not able to send the sound like a focused beam of light, it's trying to fill a sphere and the rate of decrease follows the inverse square rule, with means that the power decreases to 1/4 as the distance doubles. Increasing or decreasing the power by 1/2 equates to a 3 db change and cutting it to 1/4 (halving it again) knocks off another 3 db, for a total change of 6 db.

What this means is that if your speakers' output is 93 db @ 1 Meter, 2.83 V output, the volume level you'll hear at 64' will be approximately 93 db. The sound of your motor will also mask some of the sound, meaning that below a certain level, your music won't be audible because the sound of the motor is louder.

This effect is just like taking a boxed speaker outside where there aren't any hard surfaces nearby, like a field. You know they get really loud inside but once they go outside, the first thing asked is usually, "Why are they so quiet?".

It's not going to hit hard at 70' without more or larger speakers and more power.

05-22-2007, 09:23 AM
The amps are directly wired to the batteries with 4awg wire.
Please tell me that you don't mean what you wrote here and that you do have the proper fuse in line...:confused:

05-22-2007, 09:27 AM
MYMC- thanks for mentioning that. SAFETY FIRST!!!

Fuse or circuit breaker within 12" of the positive battery terminal.

05-22-2007, 10:19 AM
MYMC- thanks for mentioning that. SAFETY FIRST!!!

Fuse or circuit breaker within 12" of the positive battery terminal.

Yes, of course. Both amps are fused.