Amplifier Power Question and Install Question

[imnfni]

When I was in college I used to know this stuff like the back of my hand as I got academically suspended from school because all I messed with was my car audio system, but any way......


If I have a 4 channel amp, lets say 50 x 4; when in 2 ohm mode it's 125w per channel. I plan on running my sub off of the rear 2 channels in bridged mode and the front to channels for all 4 6.5's which will be replaced. My questions is do you divide the power rating among the 2 speakers for each side (left & right) when matching up which speakers to use. I don't want to overpower them. Do I take the 125w and assume 62.5w are going to go to each 6.5? Hopefully the question is clear.


Next question, the amp I am considering has high level inputs. I was intending on running front channel from factory HU into amp and sub output from factory HU in to amp; yes I know I will lose front to back fading, but I think I would rather have same sub control from factory HU since I think I'll use it more that fade. I have 2 questions regarding this....I assume the factory HU has built in crossover, what frequency is th crossover point? My second question is would it be ok to run the single sub output from factory HU to the rear channels, both left and right, on the high inputs on the amp. In essence, split the sub output wire to go to both left and right on the high input on the amp.

I hope the questions are clear and I appreciate your time and response.

[hofffam]

If you bridge two channels of a 4 channel amp - you have two remaining channels left. You cannot power two pairs of door speakers unless you do not want fader control of front to rear. You also have the risk of pairing two 4 ohm speakers which will be 2 ohms.

[imnfni]

Thanks for you quick response.....I am willing to lose front to back fading and I already checked with Fosgate about remaining to channels running in 2 ohm mode and they said it would be fine. It's the 501x amp.

[hofffam]

imnfi - I'm sorry about my short response earlier. I was walking out the door and didn't answer all your questions.

1. factory hu sub crossover frequency - I don't think anyone knows for sure. No specs are available and I don't think anyone has measured it.
2. factory hu sub input to your amp - you need to look at the amp manual, but usually a bridged connection requires a mono signal to both left and right inputs. So you will likely connect the factory sub output to both left and right inputs of the rear channel.

I want to add that I think you are making an unnecesary compromise by powering both door speakers from your new amp. Losing the fader is one thing - but you have no way to adjust the volume of the front vs. rear door speakers. You need to do that at least once - when you calibrate the system. What if the speaker efficiency of front vs. rear is different? I suggest you consider two alternatives:

- buy a six or five channel amp, or a separate amp for the sub so front and rear have their own amplification. Your hu fader will work.
- use the head unit amp to power the rear speakers. Your hu fader will work.

[imnfni]

Hofffam,

Thanks for the response, I do agree about your concern regarding ability to adjust fade; however, I think I do have that problem solved I just didn't mention other equipment that I will be using. When I sold all of my stuff years ago, I did hold on to a rare piece called the Polk Audio Mobile SDA (stereo dimensional array). I used this in my last system, which was also basically a 3 channel system except before I used it with 2 Orion HCCA 225 amps. I used one amp (bridged) for sub and the other 2 channel for mids and highs. The Polk SDA passively uses the rear speakers and modifies the sound and actually makes the front two speakers sound 10 feet farther away from you and at the same time gives you a rear fill effect. In my last set up, it was really nice.

You did answer my question about the sub signal into the amp. What about how much power is calculated going to each speaker when run in parallel? Is it half? I cant remember.

Thanks for your time.....

[hofffam]

A perfect amplifier will produce exactly double its power when the load impedance is halved. That means an amp that produces 50W into 4 ohms should produce 100W into 2 ohms. But that doesn't happen in real life because the amplifier power supplies don't have enough current capability. So it varies by amp. Your amp should have this on the spec sheet, if it is rated for 2 ohms use.