First your basic question: Can I hook up two 4 ohm speakers in parallel to an amp and get the amp to deliver its 2 ohm rated power, The answer is YES. Thats what we do when you hook up two 4 ohm subwoofer voicecoils in parallel to get 2 ohms.
Is that what you want to do? I am not sure it is.
Just because a component or coax speaker has two 4ohm drivers (mid and tweet) does not mean the amp sees 2 ohms. The crossover makes the amp see only one at a time.


The specification means this:
This amp will deliver 50 watts of power into a 4 ohm resistor load before total harmonic distortion goes over X% and
This amp will deliver 75watts of power into a 2 ohm resistor load before total harmonic distortion goes over Y%

this topic of how to calculate power delivered to speakers for a given impedace and sinewave rms voltage has been covered before. please search the forums

but basically:
Think of an amplifyer as voltage source that varies depending on the input signal. Say you put in a 1Khz sine wave of 1Vrms and set the amp gains so that the output of the amp reads 14.14 volts RMS on a 4 ohm resistor before it distorts or clips.
Using ohms law you get Power = V*V/r = 200/4 = 50watts.
At this power level the the current is using ohms law to calculate V=I*R or I=V/R or 14.14/4 =3.6 amps RMS

Now if you connect a 2 ohm resisitor instead of the 4 ohms and leave all the input voltage and gain settings the same you would think that you could now get 100 watts out of the amp because Power = V*V/r = 200/2 = 100watts, and the current is I=V/R or 14.14/2 =7.2 amps RMS
A very good amp would probably do this, and most Class D sub amps would do this too.
However if you mesure the voltage this time you only measure 12.25 volts RMS and therefore Power = V*V/r = 150/2 = 75 watts and current is I=V/R or 12.15/2 =6.12 amps RMS.
So what happened? Simply the amplifyer design did not allow the amp to continue providing 14.14 volts at the higher current draw that the 2 ohm resistor.

It is not the amp that somehow switches into a 2 ohm mode, it is the load of the speaker that draws more current and causes the amp to deliver more power into the load.

Besides the issue is really moot. From 50 to 75 watts the increase is only 1.7 db which you can hardly hear.

focus on the quality of your installation and not on 50watts vs 75 watts.

remember Ohms law is your friend! know it!

 

Thank you VERY MUCH!! for explaining all of that Zoomer. I have a much better understanding of it all now. I won't be bi-amping my setup as I enjoy what little hearing I have..lol.

I do want to address one off-topic thing you mentioned:

I will rarely, if ever have passengers in my back seat. It's pretty tiny back there. Since I haven't bought the components yet and in the interest of saving some $, would you recommend I bypass buying the SR5250's for the rear and just grab a pair of Polk MM691 coaxials? I only want rear speakers to give the interior more of a surround sound as opposed to just having mids and highs upfront and bass in the back.

I'm in another thread trying to iron out what I'm even putting in the trunk for lowend but I don't want the lowend overpowering the high end. I'll likely go with a pair of 10's running off a monoblock. Do you think that would drown out the rear deck coaxials or should it be fine?

Thanks for all of your input!!

-Ryan

 

Yea, very expensive speakers in back are a bit of a waste, spend it on something else. The cheaper polks MM or even dB are fine. As you can see what I did. I would have put some cheaper DLS coaxials but could not find any at a good price. Mind you the PolkMMs in the rear sound quite different from the DLS in front. I used to have a minivan with Polk MM650 Components up front diven by a Polk amp at 75watts per channel.. Ear splitting also and very very clear. I think my hearing was better then..6 years ago!

You may also wish to bridge your amp for the fronts if you can and drive the rears with your HU. I see fewer and fewer speakers in the rear deck. and fewer cars even with mounting holes there. Your choice but I would put them on the doors, that way passengers do get a forward sourced stereo effect and not something comming from the back.

Have fun!

 

I say that in your mind, you've already accepted spending the money on the rears... Sooo, ditch the rears and put the coinage into a better front stage. Bridge the 4 channel to deliver more power (not just for loudness, but for clarity especially at lower levels). 500wrms out back on 2 10's would keep pace just fine, but I would run a single 12 with the same power in a well built ported box.

 

Hey,

Thanks for the response RM, appreciate it. Yeah, I've decided to ditch the rear deck speakers. I hadn't really thought about it but given that its a small 2-door car I'd rather have the bass in back, mids/highs in front stereo imaging.

Do you really think 1x 12" would sound better than 2x 10" subs with similar power?

I went to an audio store and tested a JL W3, W6 and W7 up against the Polk Audio SR104DVC (10") and I really like the sound and power (700W rms) of the Polks a lot better so I've changed my setup quite significantly.

In the interest of saving space I am considering running 2x SR104DVC's with a JL XD600/1 on each sub.

However, one 12" would save even more space and I can throw one JL Slash 1000/1v2 (1000W rms) on it and that should be fine right?

But which setup would be better?

Also, I was going to get an Arc Audio FD4150 4-channel for the front speakers from a guy for pretty cheap but just found out its over 4 years old and well used.

I'm considering just grabbing a JL amp for the front speakers now. They are also about half the size or less of the Arc (21 inches wide).

Could I please get your opinion on something?

Would it be better to run the front speakers (100W rms) off of the:

JL Audio XD200/2 (60-75W rms x2 @4 ohms)

OR

JL Audio XD400/4 and bridge it (180-200W rms x2 @ 4ohms)

I've been reading that having an amp that puts out more than your speakers is good so its not always running at max power and over heating, so the XD400/4 wouldn't blow them right?

All thoughts are welcome on this. Thanks for your time!

-Ryan

P.S. I know I change my mind a lot and am probably over analyzing this but I'm learning tons and just want to get it as right as possible the first time around. Plus I enjoy discussing it.

 

2 10's and 1 12 will have close to the same cone area, and with the same power, you're SPL will be roughly the same. One driver and one enclosure is generally more error-proof, and it will save space

Yeah, totally. JL Slash with the Polk driver you like? Why not?

I'd choose the latter. The crossovers are going to limit the frequency range each driver "sees" so the average power handling will increase as a result. I can explain why if you need. The extra power will lend better clarity, especially at lower volumes and more SPL (loudness, if you will). If you're planning to have the power in the low end, the 150-200w you are looking at are going to give you the "oomph" you'll need from your front stage to keep the transition smooth between high-mid-low. If you over-power your low end, it will do exactly that to your response. Depending on how you like to listen to music, this effect may or may not be desired