ROCKFORD FOSGATE PUNCH 200 Amp w/ 3 Ohm Sub

**hevnbnd** · 01-03-2009, 09:40 PM

Ok, I have been out of the car audio area for a while now. Time to get back in.

I just bought a JL AUDIO 13TW5-3 sub. It is 3 ohm. I am wanting to use my old school fosgate amp to power it. How can I do this. Its specs are as follows.

It is only recommended to be stable @ 4 ohms bridged and 2 ohms stereo.

So I guess I am little confused on if and how I can hook this up. Thanks for your time.

**PaulF** · 01-03-2009, 11:44 PM

You can wire a 1 ohm resistor in series with the subwoofer and run the amplifier in bridged mode.

**NSFW** · 01-04-2009, 12:51 AM

Where can you find a 200-watt 1-ohm resistor?
Wouldn't that get a little hot?

**PaulF** · 01-04-2009, 10:39 AM

Originally Posted by NSFW

Where can you find a 200-watt 1-ohm resistor?
Wouldn't that get a little hot?

That's the problem, finding one. If you can find one, then you're good. Otherwise, I believe there are some devices out there that can fix this problem. The issue is that I think they can get a little on the expensive side (around $100) so a new amp might be a better solution. I'll see if I can find one though.

**hevnbnd** · 01-04-2009, 11:26 AM

Ok found a 200w 1 ohm resister. So if this amp is briged would I not need a 400w?

**PaulF** · 01-04-2009, 12:03 PM

It should also be noted, that when you add more resistance to a circuit, you decrease the wattage in the circuit.
Ohm's law says that
I = V/R where I is current, V is voltage and R is resistance.
In a series setup, the voltage would drop by adding more resistance and in a parallel setup, the current would drop by adding more voltage. Wattage = voltage x current so either way you're losing power.

You really are better off with changing the subwoofer or the amplifier.

If you simply hook this up, assuming you keep the volume down it will work. If you go to loud something will blow, be it the amp or the subwoofer.

**trader007** · 01-04-2009, 12:04 PM

screw the resistor and just hook up the sub straight as bridged. youll have a 3-ohm load on a 4-ohm rated configuration, its pretty close. just watch your amp, if it gets too hot to touch just turn back the gain a little. being a fosgate amp i doubt it will have a problem running bridged at 3 ohm.

**PaulF** · 01-04-2009, 12:09 PM

Oh, you posted while I was typing that last response.
The resistor should be able to handle a wattage about as high as the peak wattage of the amplifier is.

Using a resistor, you're going to lose a fourth of the amplifier's power in this setup.

**trader007** · 01-04-2009, 01:04 PM

thats why its just better to skip the resistor and just watch amp temps... most fosgate amps are underrated as well, it might like 3ohm just fine...

**PaulF** · 01-04-2009, 01:25 PM

Yeah, just go it without the resistor at first and if the amp doesn't have any temperature problems, keep it that way. Only go down the resistor route if that doesn't work.