Browsing around for a 300/2 to power my new Hertz HSK165 components since my 300/4 isn't gonna be enough, I found on the JL site that it's possible to power 4 speakers with the 300/2. Giving the rear fill 25watts with a resistor in place.


it says theres two 8ohm 25w resistors, is that seperate things that ih ave to buy to make it work? here's the pic


googled it, is it something like this that i'd put in between the positive speaker speaker wire goign to the rears?
http://www.tedss.com/2021008383/

Now since the 300/2 is underrated I assume that the fronts will still recieve roughly 140-150 watts per channel for the fronts, and 25watts to the rear.
However, I'm not sure if the resistor will split the power of the channels, therefore taking 75watts pre-resistor, and onyl taking 25watts from that, giving me 50 watts to the rears and leaving only 75watts for the fronts, or does it only take 25watts out of the 150, leaving me with 125watts for the fronts
Does it affect sound quality?

If anything I can just power the hertz without rear fill,until I have the money to trade the 300/2 for a 450/4, that way i'd get 150 to the fronts and 75 to the rears which would be near-perfect.

but if I could get that lil bit of rear fill without affecting SQ and taking too much power from the fronts i'd like to do that since I have rear passengers often

 

I don't quite understand why you would use a 2 channel amp for a 4 channel job? That pdf even says to use a 300/4 for the satellites aka: interiors. The reason for the resistors is essentially to reduce the amplification of the rears to keep the sound stage closer to the front since the 300/2 will not have the capability to adjust front/rear gains. The resistors won't necessarily suck up juice as much as they will impede it. I am assuming those VR650's are 4ohms. With those resistors installed you will be at a 5.33ohm load to the amp which alone is sapping some of your potential juice. The most output for your setup is going to be from a 300/4 in my opinion since you will have a 4ohm load per channel (excusing any resistance from wiring for both setups), as well as retaining your ability to fade in any direction if you so choose. Either way, it's JL and will likely sound great

 

why not get a 4-ch amp?

you'll have much more flexibility for adjustments/sound tuning, plus the added power and less hassle.

 

i have a 300/4 right now and its only bout 90watts to the fronts, i need about 150watts for my new hertz hsk components, which is why im trading for a 300/2.
then i saw that powering 4sp. with 2ch amp on jl site so i thought i could do that, if it doesnt take too much juice from the fronts (i wanted to just have some kind of power going to the rears for the passengers, but if its too hard i dont really care).

sounds like its gonna be too hard... guess ill jsut run the fronts until i can afford a 450/4 and then sell the 300/2

 

Tehvine,

How'd you do your math?

By my calcs, the amp would be seeing a 3 ohm load.

1 / ( (1/4) + (1/(4+8)) ) = 3

wong,

I don't know if I can explain it, but you're not thinking about it quite right. Basically, the 8ohm resistor added to the rears will create a 12ohm (8 + 4 in series) load for the rears, and they will see 1/(12/4) = 1/3 the power of the fronts. I'm pretty rusty if any EE's would like to chime in here to explain better?

@cUr@-TL,

So you have no frame of reference here, Donny. You're like a child who wanders into the middle of a movie and wants to know...

OP, if you already have the 300/2, I say go for it. If not, the 450/4 will save you some hassle and offer additional adjustment.

 

thanks for the info,

seems to be more negatives than positives of doing this..

just gonna keep the 300/2 until i have enough for a 450/4 and then sell the 300

 

Yeah I think I was half asleep and counted the resistor as a 3rd driver for some reason. But you're right, (12*4)= 48, (12+4)= 16, 48/16 = 3Ohms.

 

Really? Sweet! I was hoping someone could double check, cause I don't have too much faith in my own calcs after the grades I got in EE class.

But really, OP should be fine with the 300/2, right? A couple resistors will be much cheaper than a new amp.

Really, the only functionality you're losing is the fader, w/ the rears always getting 1/3 the power of the fronts (is that right?). If you wanted to change that attenuation, you could pick a different resistor value.

 

Some Math...

300/2=> 150W per channel

Imedance per channel = 1/(1/4+1/(4+8)) = 3 Ohms


Power =I^2*Z=>150W=I^2*3ohms
->I^2=50W/ohms->I~=7Amps

Front speaker amps = Total amps* (Rear Ohms)/(Front+Rear Ohms)=
7*(4+8)/(4 + 4+8)=5.25 Amps

Rear Speaker amps and Resistor amps (Same current path) = total amps - front speaker amps = 7-5.25= 1.75 Amps

Front Speaker Power = Front Speaker Amps^2 * Impedance = 5.25A^2*4ohms ~=110W

Rear Speaker Power = Rear Speaker Amps^2 * Impedance = 1.75A^2*4ohms
~= 12W

Resistor Power = Resistor Amps^2 * Impedance = 1.75A^2*8ohms
~= 24W

This is why they require a 25 W resistor. If the resistor value changes so will the wattage requirement assuming you exercise the full wattage output of the amplifier.

There is a 10:1 power ratio roughly of front to back amplification. There will be rear fill, but only fill.

For some basic adjustability, a 4 resistor and an L-pad could give you some range of adjustment.

 

woot, thanks again Castles!

 

so another drawback is that the resistors are indeed actually "wasting" some power