Series/Parallel Ohm, Watts Problem

[Blazemore 10+ year member]

I want to hook up three 3.5" speakers in my dash to the center channel. I looked around on the net but still having problems understanding what the final ohm would be and the wattage each speaker will get. Pic of problem.

To find total ohm the amp will see?

Zt = (Z3 x Z 1, 2) / (Z3 + Z 1, 2)

Zt = (4x8) / (4+8)

Zt = 32 / 12

Zt = 2.667

^is this right?

The SS amp center channel

4ohm 65watts

2ohm 100 watts

I found this formula example used for finding how many watts each speaker will get. But this was for 2 speakers not three. Don't know the true formula I need

Pab = Po x (Zt / Zab)

Pab = 100 x (4 / 8)

Pab = 100 x 0.5

Pab = 50 watts

Anybody help out?

Click to read more...

 

[maylar 10+ year member]

Impedance calculation is correct.

The amp will do somewhere between 65 and 100 watts with 2.66 ohms.

P = (E^2)/R

therefore E = sq rt of (P*R)

For 100 watts @ 2 ohms E = 14.14 volts

For 65 watts @ 4 ohms, E = 16.12 volts

Therefore assuming the amp would output 15 volts at 2.66 ohms:

P = (15^2)/4 = 56 watts for speaker #3

P = (15^2)/8 = 28 watts for the 8 ohm combo of speakers 1 & 2. Each would get half.

 

[Blazemore 10+ year member]

Do you know of a better way to wire the 3.5's to the center channel? SS manual gives 4 ohm 12.6vdc @65w and 2ohm 14.4vdc @100

 

[maylar 10+ year member]

With 3 speakers that's about your only option. I'd probably only use 2 of them, wired in parallel for 2 ohms.

No 3.5's will handle that kind of power full range BTW. I've blown out Kappa's and Boston's with just HU power. You'll need to hipass them at 150 Hz or so.

 

[Blazemore 10+ year member]

I was trying to think of other ways to wire them after seeing the 56watts to speaker 3. Dumb question (I know) buts there's no way to add wire 4ohms in series with speaker 3 without adding another speaker?

 

[theCybe 5,000+ posts]

Sure, add a resistor.

 

[Blazemore 10+ year member]

So just add a 4 ohm resistor like this:

+______Speaker 3 4ohm_____resistor 4ohm____ _

Is there any down side of doing this.

 

[RandyJ 10+ year member]

Not sure about adding resistors in car audio, it may add some unwanted side affect but I doubt it. The only thing I can think of is make sure the resistor can handle the power you will be running through it.

 

[Blazemore 10+ year member]

Found my answer in this post:

http://forums.caraudio.com/vb/showthread.php?t=76669&highlight=adding+resistor

Pretty much using resistors is a waste of time.

 

[maylar 10+ year member]

Actually... in your case it might be the correct solution, if you really want to run 3 speakers. A 4 ohm resistor in series with speaker 3 would make two 8-ohm pairs in parallel, giving 4 ohms total. The 65 watts would be divided 4 ways equally (including the resistor). 16 watts to 3.5's is just about right, and all 3 would be at the same volume.

Normally I recommend against resistors for impedance matching but in your case it kinda makes sense. So you waste 16 watts in heat. Oh well.

Make sure you get at least a 50 watt 4 ohm resistor and heatsink it to some metal.

 

[Blazemore 10+ year member]

The dash has cutouts for three 3.5's, one on each side and one in the middle. When the middle is taken out you can hear a difference, like a hole in the middle. At low volume the missing middle is not noticeable but at higher volumes it makes a big difference.

Would something like this work? http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=019-015

looks like it comes with its own heatsink and I can just mount it under the dash (clear of anything under there) out of the way.