Replacing dash stock speakers - resistor question

[osprey92]

I have a stock 1500 Tradesman Crew. I'm replacing the current 3" unit with an Android head unit. While at it, I've decided to replace the speakers as the upgrade is relatively inexpensive. Door units will be Infinity
PR9603IS 6x9. I've read several threads regarding the dash speakers over powering the front 6x9s if they are less than 8 ohms. If 4ohms to use resistors to balance the power. I'm undecided on the dash speakers but leaning in the 4ohm range as there is a larger selection. Question is how do you know to use a 10W 4ohm resistor vs a 5w or 20w resistor for dash speakers? What is the function of the watts - power reduction or power limiter?

 

[TomT]

Are you also adding an external amplifier or are you just using the power from the head unit?

 

[ImNickJames]

The 5W, 10W or 20W power rating of the resistor is the amount of power the resistor can dissipate, typically at 25C ambient, before it reaches its thermal limit (typically 125C or 150C). Note this is defined with some amount of airflow (varies by manufacturer) and you stuffing them in your dash surely will not have - hence you shouldn't design your system to dissipate the full amount of the resistor's rated power.

P = I^2*R

With the resistor in series with the speaker, the current I is the same. So for a 4 ohm speaker and 4 ohm resistor, the power dissipated in each will be the same. In this scenario, if your headunit is rated for 20W into an 8ohm load, you will dissipate 10W in the resistor and 10W in the speaker.

 

[osprey92]

Are you also adding an external amplifier or are you just using the power from the head unit?

Just the head unit for starters. I don't need loud, just need better audio.

 

[osprey92]

In this scenario, if your headunit is rated for 20W into an 8ohm load, you will dissipate 10W in the resistor and 10W in the speaker.

Perfect. Thanks.

 

[TomT]

Just be aware that the doors and dash are wired together so you want to be sure the head unit is ok with the final resistance.

 

[WilliamS]

Just be aware that the doors and dash are wired together so you want to be sure the head unit is ok with the final resistance.

Thats why hes adding the resistor. Keeping the headunit load the same with aftermarket 4ohm speakers.

 

[PowrRam]

Thats why hes adding the resistor. Keeping the headunit load the same with aftermarket 4ohm speakers.

Right. But is that what his aftermarket head unit wants to 'see'?

 

[WilliamS]

Right. But is that what his aftermarket head unit wants to 'see'?

The load would be the same as the stock speaker setup so why wouldn’t it? Most headunits likely would be super nice with a 2ohm load if he didn’t use resistors.

 

[PowrRam]

The load would be the same as the stock speaker setup so why wouldn’t it? Most headunits likely would be super nice with a 2ohm load if he didn’t use resistors.

I'm probably overthinking it but I was wondering if the Android head unit had a different rated output impedance for the front channel compared to the stock unit. Don't most aftermarket units have 4 ohms out on all four channels? Is this the same as the stock unit?

 

[TomT]

The load would be the same as the stock speaker setup so why wouldn’t it? Most headunits likely would be super nice with a 2ohm load if he didn’t use resistors.

“Most likely” being the key words here. I’m just suggesting that he take a measurement of the speaker wires and then make sure the aftermarket head unit supports it. Better to be safe than to find out the hard way.

 

[WilliamS]

“Most likely” being the key words here. I’m just suggesting that he take a measurement of the speaker wires and then make sure the aftermarket head unit supports it. Better to be safe than to find out the hard way.

Again I think your thinking backwards, the resistors lower the load on the head unit not raise it. If he doesn’t use resistors in parallel it would lower the ohm load making the head unit work harder.

 

[TomT]

I'm not overthinking anything. With or without the resistors, having the two speakers wired together will change the load presented to the head unit's internal amp. I'm suggesting that before he hooks it up he should measure the final impedance so he is not just assuming it will be fine.

 

[BrazenHaven]

Aren’t the front and dash wired in parallel? If the door is 4ohms and the dash is 8 then the factory head sees 2.7ohms. If the door is 8 as well then the factory head sees 4. So if he puts a 4 ohm in the dash and in the door the head will see 2 ohms. He will still need to see if the head unit he is replacing will support a 2 ohm load.


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