Speakers/power

Now that the rain has stopped, here's what i'm thinking of doing:

-connecting the amp to the rca output on the head unit that can be controlled separately. The radio instructions say that when that connection (display indicates "subwoofer on or off") is on, amp will be used, when it´s off, speakers will be used without the amp.

My doubt is:

1-What would happen if i connected the amp not to rca's on the HU, but where my rear speakers are, replacing the speaker with the amp and then connecting the speaker to the amp. The signal that used to go to the speakers from the HU would go to the amp and then to the speakers. Would that be different from connecting it to rca's?

2-Since the HU is 4x 45 watts, by replacing the power that feeds my rear speakers (because it's a 2 channel amp) with the power from the amp that is 2x 20 watts, isn't it correct to think that the speakers are going to have less power to them?

Thank you

Actually, that is incorrect.  You keep quoting 4x45.  That is MAX power, not RMS.  Speak in terms of RMS.  MAX power means nothing, it is simply a marketing tool.  Your HU probably does 20x4 RMS AT MOST.  Most HU manufacturers totally overrate their power specs.  (i.e. they cheat by only taking the power rating with one speaker being driven and pushing some specific frequency for a very short period of time)  In other words your REAL RMS power from a head unit across the entire frequency range with all channels being driven for a long period of time may be much less than the head unit's stated rating.  Most standalone amplifiers, even if rated similarly to a head unit, will totally blow head unit power away.  They can usually use much higher quality parts, will run cooler, have more available headroom, have much lower distortion, much higher signal to noise ratio, and the list goes on and on.  Unless your other amp is total crap it will almost always be better and more powerful than head unit power.  There simply isn't space in head units for quality amps. 

Thank you kfr01. That makes sense. My doubt number 2 is no longer a doubt.

I´d still like to know the answer to doubt number 1 if it's not too much to ask you guys.

Thank you.

I´m thinking that my rear speakers will get two sources getting to them, the mini amp from the HU and the amp (rca's).

Even knowing that when rca's are activated, the mini amp from HU will be deactivated (as i read in the service manual), isn't it odd that the rear speakers will have 4 wires in common and the signal from the amp will also enter the HU because the wires touch?

I'd like to keep the wires from the HU to the speakers directly AND the rca's so that i could choose on the HU at any time if i wanted amplified signal or direct signal to rear speakers.

It doesn't feel right. What do you think?

Ok, now I have an idea of what you are trying to do.  Don't do it!  You'll harm the head unit if you do.  Only have one amplifier hooked up to each speaker.  If you really want to switch back and forth I'm sure you can pick up a home audio speaker selector and try it that way.  I don't see any reason to leave the head unit speaker level output hooked up if you have rca's to the amp. 

Now THAT was USEFUL. I was about to make the connection my way.

Thank you kfr01.

My amp, even as simple as it it, has a yellow wire and a blue/white wire. I assume that the blue/white one is the remote turn on, but about the yellow wire, if i connect it to the yellow one on the HU, isn't it correct that there will always be power heading for the amp? even with the ignition off?

This my last doubt before connecting the amp. I´ve got a lot of wires hanging in my car. :)

Thank you.

I couldn't tell you what the yellow wire is coming out of the amplifier.  Anyone else?