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I have a Sony MEX-XB100BT head unit which has a [40w RMS @ 4 ohm] internal amplifier hooked up to an Alpine KTP-445u [45w X 4 RMS @ 4 ohm] running Alpine SPR-68s [100w RMS @4ohm] - I am under the assumption that I am feeding my speakers 95w RMS @ 4 ohm.
Is this wrong?
You can bridge the alpine powerpack amplifier so that it puts out 90 watts RMS into two channels at 4 Ohms. You have to select the two channel mode on the Alpine amplifier to enable this. Not sure if you can bridge the Sony HU. If you're asking if the power from the Sony HU and Alpine power pack somehow combine together then the answer is no chance ever. You could however by-amp your speakers by sending one channel just to one tweeter and the other channel just to a woofer for each side using four channels total with two teeters and two woofers. I've owned that Sony HU and the alpine powerpack and they are very comparable in output to each other.
I should clarify that I cut off the RCA's and spliced them in-line with the speaker outs from the head unit.
Don't know if that makes a difference or?
The ultimate goal is to put a 4 channel 400w rms amplifier to feed the speakers - this would go in the preamp RCA's in the back of that head unit and bypass the Sony's onboard amp altogether right?
Not sure why you would have cut off the low level RCA inputs when you could have used the low level RCA outputs from that Sony HU unless you wanted to save on a set of RCA interconnects. It doesn't make a difference though as long as you have the switch selected for high level input up to 10 volts. With either situation it still only puts out it's rated power of 45 watts RMS x4 and the internal Sony amplifier of 40 watts RMS x4 is bypassed.
The answer to your second question is yes it's bypassed with low level RCA conections too. I'm assuming you mean 100 watts RMS x 4 for a total of 400 watts RMS. That is a lot of power and should get plenty loud.
Again, not sure why you are even using the power pack as so, you're gaining little in terms of output. On top of this, the HU is capable of feeding the power pack more voltage than it's input is rated for, so you'll want to be careful so as to not overload the input and push the amp into clipping. Set the input switch to speaker level 0.5V-10V and leave the gain at minimum on the power pack.
I'm not sure what volume level the Sony can reach before it's own internal amplifier clips, but chances are good it'll be able to produce 10V before clipping.
Really, you are better off switching the power pack to 2-channel/bridged mode and powering your front speakers with upwards of 90ish RMS and letting the rear speakers be powered off the Sony's 40RMS internal amp.
If the rear speakers seem louder than the fronts at this point, then dial up the gain on the power pack slightly until the rear speakers are no longer the focal point.
I'm merely running a set of Alpine SPS-610C's in my cavalier powered off a Rockford PBR300X4, it's able to get plenty loud for most I've had in the car. Well, I have a sub as well, which usually stays dialed down quite low so as to blend better.
Unless your ride is quite loud (road noise, exhaust, etc.), then those speakers should get plenty loud for most with what you have available.
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