Ohms Law?

I have a 2000 Pontiac Grand Prix with a Bose system. My question has to do with the sub I just bought to put in it. It is a JL 12W6, dual voice coil @ 6 Ohms, powerhandling = 300w RMS according to the man. specs. I plan to put it in a sealed box in the trunk.

I get the basics of what needs to happen, but I am not sure how to size an amp to the 6 Ohms. I would like to know what would be the best amp to get if I were to look at the specs where it shows the following... [ RMS x 1 @ 4 Ohms ] for 14.4 volts, or for12.2 volts.

So, I have been told that the Rock Fosg 351M (175W x 1 @ 4 Ohms) will work fine, the 500bd (300W x 1 @ 4 Ohms) would be good, and then someone said instead of those the RF 700s (700W x 1 @ 4 Ohms)would be the best. So now I am confused because of the apparent wide ranges they suggest.

If I could have someone explain 1) what RMS, 2) number of channels, 3) number of Ohms, and 4) what volt input that rating is from, then I can start looking specifically for the best amp without being conned into paying for an amp that will just help out someones daily sales. I would appreciate any feedback on this.

Thanks.

Replies (4)

schlsux on 04/22/2003 20:00:58
Pro's correct me if i'm wrong

RMS - constant power that you amp will put out. ex a 1200 watt will put out a continuous 600 watts. so that 300 x 1 may only put out 150 watts RMS. your sub can handle 300 rms. Same goes for subs, read carefully. A 1200 watt infinity reference can only handle 300 rms.

Number of Channels - This is for what kind of speaker wirring you want to do. If you are hooking up one sub a mono amp may be the way for you to go. One channel one speaker. Or a 2 channel amp you can put the + from one side and the - from the other and it bridges the amp to give it the same effect. You have to make sure the amp is 2 ohms stable. At 3 ohms you should be safe doing this if it is a last resort.

Ohms - The lower the ohms, the less resistance. This is mainly for the amp. 300x1 @ 4 ohms will put out 600 watts @ 2 ohms. You amp can be run at either 3 or 12 ohms depending on series or parallel wiring.

Input ratings - the lower the voltage the amp is tested at the more wattage your amp will put out. Most car batteries will only put out 13.5 volts or around there. So an amp rated at 14.4 volts, you wont even see all of the watts that amp can put out. On the other hand an amp tested at 12.2 volts w\ a battery running 13.5 volts, you will get more power out of the amp than what it is advertised as.

Everyone raves about the JBL 600.1 amp on this site. @ 3 ohms this amp will kick your sub perfect. It puts out 300 rms @ 4 ohms. So at 3 you will get a little more than that, and i'm sure a JL audio sub can handle that.

Anyone feel free to clarify or correct me if i'm wrong on anything. Hope i could help.

Not to sound like an advertisment but i am selling an audiobahn amp which @ 3 ohms would put out 300 watts rms. 200 bucks. Hopefully if they put a classifieds section up on this site it could help out people like you and me get good deals. ;) admin, still waiting for it. lol

ttocs on 04/24/2003 01:13:32
a 6 ohm sub is really made to be used with more then one sub to wire in parallel to get more pwr out of the amp. You would really be better getting a normal 4 ohm sub...

Swez on 04/24/2003 08:42:35
Asking some very good questions... but try not to get bogged down in the details and become paralyzed by information over load.
Allow me to simplify the task a bit.

1. You have a good sub... it can be wired in several ways (as ttocs) mentioned. this sub was designed for applications where 3 subs are used.

2. Wiring this sub in parallel, you will have a 3 ohms load. The owners manual should have that detail or see the JL web site under tuitorials for additional details and diagrams.

3. As for amps to consider, a MONO amp (1 channel) that is able to handle 2 ohm loads, would be a safe bet.

Since this sub is rated at 300 RMS, an amp that puts out ~300 @ 2 ohms, will net ~225 RMS @ 3 ohms. Not a bad place to be. You will not hear much difference in audio output SPL between 225-300 watts RMS. (less than 1.5dB)

There are several good Mono and 2 channel amps out there in this range to choose from:

JL: 300/2 http://www.jlaudio.com/amps/3002.html

JBL: BP-300.1 http://www.jbl.com/car/product.asp?prod=BP300.1&ser=POS&cat=AMP

MTX: Thunder 421D http://www.crutchfield.com/cgi-bin/S-z7poMZBTj4Q/ProdView.asp?s=0&cc=01&c=3&g=130&I=236T421D++

Alpine MRP-M350: http://www.crutchfield.com/cgi-bin/S-dQyT2y8thp9/ProdView.asp?s=0&cc=01&c=3&g=130&I=500MRPM350

Kenwood KAC-8151D: http://www.crutchfield.com/cgi-bin/S-dQyT2y8thp9/ProdView.asp?s=0&cc=01&c=3&g=130&I=113KAC8151

Kicker KX400.1: http://www.crutchfield.com/cgi-bin/S-dQyT2y8thp9/ProdView.asp?s=0&cc=01&c=3&g=130&I=206KX4001+

Rockford Fosgate Power 351M: http://www.crutchfield.com/cgi-bin/S-dQyT2y8thp9/ProdView.asp?s=0&cc=01&c=3&g=130&I=575351M+++

There are more out there, but this list is plenty to chew on for now. The JL, MTX, Kenwood and Rockford amps are very good products, but you'll have to shop hard for a good prices. Major brands demand higher prices, but not always superior amps.

The Alpine, JBL and Kicker amps are also major brands, but careful shopping will net better prices if you hit Ebay or other discount E-tailers.

A word of caution... avoid large retail chain stores when buying your gear. Go to look and listen to products if you wish... but leave your wallet in the car. Their goal is to figure out how much $$ you have to
burn, shove a few boxes under your arm and... Ohhhh YEAH, WE SELL JUNK... SO YOU'D BETTER BUY THE EXTENDED WARRANTY PLAN.

Hope that helps,

Swez

PS Crutchfield is one of the best E-tailers on the web for a wide variety of choices and carry good products. They stand behind their products well, but there is a higher price to pay for that service and overhead. Other sites are more lean and sell at lower prices.

Swez on 04/24/2003 22:20:26
Have looked at many amps and sub configurations over the past 3 years and there are many fine amps and subs out there to choose from.

In your specific application, I would ask you to consider what your most important goal or priority is. Obviously, you'll want to "invest" in a quality amp to power your sub. The other considerations, are the features, power and price.

Since you have a 3/6/12 ohm sub, are working an oddball combination here. The JL and RF amps are likely to be the most expensive choice on the list. However, both make good products. Solid amps, dependable and durable within thier operating range.

The JL is most suited to your sub, as it puts out 300 RMS watts in a wide range of ohmic loads. However, this is probably the most expensive 300 watt amp mentioned. RF mentioned is a good option as well and also pretty expensive too.

The Kicker and JBL amps make the most sense to me here. The Kicker will give you a bit more watts, but not much more SPL than the JBL 300.1. You can get a JBL 300.1 off the web for under $200.00. Am not sure what the lowest price on Kicker KX400.1 might be. My guess, about $225-250.00 A few more features here, but still a good value if you can get one close to $250.00

Can get a JBL 600.1 for ~$220.00. At 3 ohms, you get ~400 RMS watts @ 3 ohms, which is higher than that sub can take alone. However, you can always add a 2nd sub later, and take full advantage of this amps power, if you really want that much bass. However, you can always dial back on the amp gain to protect your sub from excess power using this amp.

Personally, I use a 300 RMS 2 channel version JBL and a pair of 10" DVC's. There is more bass than the stock HU can keep up with, but will be adding a 4 channel amp for the interior speakers to offset the difference in SPL.... FULL RANGE.

I get ~130dB of bass right now and 100 dB interior now. Am shooting for a clean sound that has plenty of SPL... 110dB is my target, full range.

Since you have the Bose upgrade now, not sure you need much more SPL from the interior speaks... just enough bass to balance out the spectrum. In that case, the 300.1 and a single W6 will do the job. If you really want lots of bass SPL, then the 600.1 will be your best choice and mabey add a 2nd sub if you want more bass.

Hope that covers the options adequately. If not, ask a few more questions before you take the plunge.

Swez