one or two

[ade_beer]

Well, after lot of research and reading I think I have decided on SI BM's for my F-250 crew cab. I have very limited space, and am building a box and amp rack to go along the back of the cab behind the rear seat. I was only planning on doing one, but read somewhere Nick was recommending two be used in most situations. I listen to mostly rock, and am by no means a basshead. One would be getting 500W and the fronts will be massive audio CK6's. Is there any reason to try and find the space for two?

[Electrodynamic]

Originally Posted by ade_beer

Well, after lot of research and reading I think I have decided on SI BM's for my F-250 crew cab. I have very limited space, and am building a box and amp rack to go along the back of the cab behind the rear seat. I was only planning on doing one, but read somewhere Nick was recommending two be used in most situations. I listen to mostly rock, and am by no means a basshead. One would be getting 500W and the fronts will be massive audio CK6's. Is there any reason to try and find the space for two?

I would advise two on that much power. Power handling is not a power requirement and 500 watts is too much power for a single driver. Either that or wire it to 8 Ohms and run with ~175 watts or so on a single driver.

[ade_beer]

The amp makes 250W @ 4 ohm, so I originally assumed 500W @ 2 ohm, but on another board I was told it would probably be only about 380W or so. According to my understanding the same would be only 125W wired in series for 8 ohm. Does this make sense and would you still recommend the 8 ohm config?

(The amp in question is a Phoenix Gold ZX475ti)

[Electrodynamic]

That amplifier is only stable down to 4 Ohms when bridged. See below:

Into 4 ohms Stereo @ 14.4 Vdc 75 x 4
Into 2 ohms Stereo @ 14.4 Vdc 150 x 4
Into 4 ohms Bridged @ 14.4 Vdc 250 x 2

(a little bit of Google searching came up with a PDF of the amp here http://dl.owneriq.net/d/d6be7899-80b...a67982c6cc.pdf)

[ade_beer]

Thanks. I do have the manual. It is stable at 2 ohms bridged.

From the manual:
Minimum bridged load is 2 ohms.
Minimum load per channel is 1 ohm.

and:

When designing the ZXti amps we wanted a design that made its best power at normal loads. By that we mean 4 ohms bridged. Now we know that no matter what we say or how loud we say it people will still hook the amp up to lower impedances trying to get more power. So our number one goal is that the amp continue to operate at lower impedances. The only way to do that is to limit the power output of the amp when it's hooked up to lower impedances. But it really isn't that big of a deal. Here's why:

Wattage is equal to voltage times current. Running the amp at 2 ohms bridged means more output current. Current makes heat. We can't allow the amp to make more heat than the fan can deal with. The fan can deal with a bit more heat than what's generated from a 4 ohm bridged load. So there's some room for more power. Here's how we limit the power increase to what's manageable:

The ZXti amps have a circuit that detects the amount of current going out of the speaker terminals. If the current is high enough AND last long enough (longer than 50 milliseconds), the circuit limits the amount of voltage that the power supply can produce. This in turn limits the power output of the amp. So continuous power output remains about the same with the power composed of more current and less voltage.

But there is a bright side to all this. Remember I said that the excess current demand had to last longer than 50ms. The dynamic peaks in most music last less than 20ms. So the circuit never has a chance to effect the power supply voltage for musical peaks. All this means that the amp IS allowed to make more power with musical peaks while continuous power remains about the same. You could say that the "headroom" of the amp is increased.

If you want to run the amp 2 ohms bridged, go for it. Just understand that it will run slightly hotter and will pull more current from your car to make those musical peaks (add more capacitors and use thick gauge power/ground wire). I wouldn't recommend running it 1 ohm bridged. The amp will shut down (red LED) if the output current gets too high (looks like a shorted speaker).

[Electrodynamic]

If you want to run the amp down to 2 Ohms that is your decision. Current, not voltage, is usually the major culprit of voice coil deaths. And the BM mkIII's are rated up to 450 watts RMS. Power handling is not a power requirement. You do not NEED full RMS power on a BM mkIII to get it to perform at full potential.

PS: Adding more (any) capacitors [see the suggestion in the manual] won't help anything. Doing so is like putting a tiny bandaid on your arm that just got sawed off. Not going to help a lot.

[ade_beer]

Agreed, thanks for the input. I made the decision to use a different amp, will send 300W to a single MkIV when they're out.