Rockford vs. JL amplifiers

Hey all, I am confused as to what I have just heard in my car.
First, here is my system configuration:

Pioneer AVH-P7500DVD head unit
MB Quart QSD-213 (5.25 inch Q components) front speakers
stock rear speakers running off of deck power
Boston G5 12" sub in custom fiberglass box
all in an 06 Subaru STI

I have had a Fosgate Power 551x (4 channel) running in a Bi-Amp configuration (not sure if this is the right term, I am running the woofers from one set of outputs and the tweeters from the other set of outputs, which would usually be front/rear)

For the sub I have been using a fosgate Power 550S (2 channel, bridged)

Today I took the amps out of my car to paint them, and get them ready for the final install. I replaced them with some JL slash series amps, not expecting to hear any noticeable difference. Unfortunately there was a difference.

The amps that are now being used are JL's Slash 300/4 and 500/1

the highs are cleaner, the vocals are clearer, and the midbass is tighter. What gives? Why would these JL amps sound so much better than my Fosgates? I always thought that my Fosgates were tip top.

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Their old stuff was really great!!! Seems that the more 'commercialized' they become the more downhill their product slides. Ever since retailers have started carrying their stuff ... we have all been sad

Can you give us the specs on your speakers, and tell us which speakers are running from which amp? Maybe we can help you figure out what is going on!

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The fosgate amps are a few years old... one from 00 and one from 01 (not sure if this constitutes old). The MB Quart Qs are running off of either the 551x (rockford) or the 300/4 (JL) and the sub is running off of the 550S (rockford) or the 500/1 (JL).

the front speaker specs are:
Power Handling     
70 - 150 W
Impedance     
4 ohms
Sensitivity     
3,2 W (90 dB/1m)
Frequency Response     
50 - 32.000 Hz
Crossover Frequency     
2.400 Hz
Crossover Slope     
12/18 dB/octave


I'm not so worried about the sub. It hammers with either amp, and is quite clear and crisp.


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coppellstereo is right. I used to think very highly of Rockford. I still have some of their old subwoofer series that they no longer make. That was almost a decade ago and now I'm not too sure about RF's product line... It seems all right I guess, but nothing what it used to be. I agree they have been going down hill.

As for the JL Audio amplifier switch, that's pretty much a no-brainer. JL makes some very awesome amplifiers that are built to the highest quality. Their amplifiers have very low THD and are efficient and reliable when it comes to their power output. I love JL's amplifiers and if you can afford them, they are a very good amp to purchase for someone who wants their speakers to sound clean and loud. Keep in mind that many other companies can deliver this great performance too at a much more affordable price.

I do really like JL Audio, I just think they should come down on their prices a little. But hey, as long as their products are selling then why lower the price, right?

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Do whatever makes you happy in life without diminishing the happiness of others

Are you using a crossover for the mid's and tweeters?
Or are/were you just running them off of seperate channels of the amp? If you were, then you would be sending the same signal to both sets of speakers, this isnt a good idea. A crossover will seperate the signal into highs and lows and distribute to the speakers.

The only thing you changed was the amps? Did you change the way that you hooked them up to each component?


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The front and rear have seperate crossovers (active) that I have crossed seperately, also the components have passive crossovers crossed at 2,400Hz.

The amps were the only change, exact same wiring etc.

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whoops, I should have said that those active crossovers are built in to the amps. Seperate inputs (Front for woofers and rear for tweeters)

I think that answers it

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Yeah, I wouldn't really even consider JL amps and RF amps to be on the same level. JL produces a much higher quality product.

JL amps are a little more pricy but you get alot more technology then you would with another amplifier. Even an amplifier that is on the quality level as JL. JL amps are fully regulated. Which something that only a handful of companys do. Between 1.5 and 4 ohms its putting out full power, and between 11 volts and 14.4 volts your getting full power out of the amp. Any of the slash series amps are TERRIFIC amps, there are better out there but not much better, and for the amount of technology the price is justified.

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2007 Acura TSX
SQ setup in the works

I guess I'm just gonna leave the JLs in then. Wow, and to think I almost sold them, without even listening to them. Thanks for the info guys,

oh yeah, one more question:

The Boston G5 sub is a 4ohm sub (dual 2ohm voice coils, with 1 set of connecting posts)

I bought the 4 ohm model b/c I was using a 2 channel amp to run it (2 channels bridged to 4ohm speaker = 2ohm load at amp)

Now the JL is seeing a 4ohm load b/c it is a monoblock amp.

Am I understanding the literature correctly that it will still put out 500w RMS?
Are there any downsides to running it at 4ohms?

thanks for all of the help guys



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no, running at 4ohms is much better than running at 2 for your speakers (subwoofer you wont notice much difference). Most high end speakers at least would like to see 8ohms! It increases SQ

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Well, not increases, but minimizes noise. Isnt that right guys?

What loads are you amps seeing the way you have them setup right now for both the speakers and the sub?

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everything is seeing 4ohms all the way around currently.

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With the JL amps, this is optimal for your subwoofer for sure, since even at 2ohms they put out the same amount of power.

For your mids/tweets - would the amp put out the same amount of power at 8ohms? That might yield better SQ.

I dont have any experience with doing that, but it would be worth a try if you could!

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I would have to change my speakers to get 8 ohms right? Don't want to do that... spent way too much on the Quarts to get rid of them.

I don't think that there is any other way to wire them that would yield 8ohms.

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Oh yeah, sorry - wasnt thinking. Sounds like you are set!!

So how does it sound now? have any pics?

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with the JLs it wounds great! I don't have pics yet, building the amp rack as we speak. I'll shoot some pics out when I finish though. Finally got the perforrated (sp?) leather in a couple of days ago to match what I used on my fiberglass box.



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oh wow - sounds amazing! Cant wait to see!!

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don't get too excited... it is very basic (and light, since weight is an issue) a simple flat peice of hardboard covered in leather.

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Don't worry about trying to run your speakers at 8 ohms. They'll be totally fine at 4. I highly doubt you'll hear any difference other than they'd be a little bit quieter.

If you want an even bigger performance jump for the fronts switch the 300/4 for an Eclipse EA4000, you'll get the same power, much better sonics & save a few bucks.  IMO, of course.  Go listen to those two amps side-by-side on a soundboard somewhere & you'll see what I mean.

Running at higher ohms has it's pros and cons as with everything....

Just remember that ohms is the resistance of current flowing through your electrical circuit. The lower the ohms, the less resistance and more current is then allowed through. The higher the ohms the more resistance and less current is run through.

This to what it actually means as far as affecting the sound of your system:

The lower the ohms you run, the louder the speaker will sound since it is obviously running more current through to the speaker. Of course, the downside is that your speaker will be seeing more of a distorted signal from the amplifier. The higher the ohms, the less current is running to the speaker, so the speaker won't sound as loud, but it will be getting a much "cleaner" signal.

SPL is usually run with lower ohms, and SQ primarily uses higher ohms. Anywhere from 4 to 8 ohms is sufficient for your application. (assuming SQ is your goal) Since you already have a 4 ohm setup then you should just leave it alone and enjoy the music.

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Do whatever makes you happy in life without diminishing the happiness of others

Thanks a lot guys, I am very happy with the sound as is right now (and I don't have to shell out any more $$, always a good thing!) Gonna sell those rockford amps and hold onto the JLs. Now if I can just get a sunny day so that I can finish my amp rack and make it all pretty!

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