8 ohm driver, 4 ohm woofer

[Adam Skookum]

Hi there. I just have a wiring/buying question (skip the following paragraph if you don't care about backround and specs)
A year ago I bought a jaton 5.1 av 318 surround sound system made in wherever china gets it's cheap crap from. It says it's 600 watts RMS, but that's bs. The current woofer is said to be 100 watts at 8 ohms, and has a nice dent kicked in the cone. It's never sounded that great, and I'm finally looking for a replacement driver. The catch is, it's a 6.5 inch woofer, 93 dB, and supposedly has a 35-300Hz range: I can't find a 6.5 inch woofer at 8 ohms anywhere.

I can find lots of 4 ohm 6.5 inchers out there, but no 8 ohmers till 10 inches.
So here's my question: the subwoofer is supposed to work at 100W "RMS" at 8 ohms- is that wattage low enough that I can risk putting in a 300 watt 6.5 inch sub at 4 ohms without blowing up my power supply? I know the resistance discrepancy is dangerous, but is 100 watts enough to worry about? Assuming that they are rms. The enclosure contains the power supply for the sub and additional 5 speakers, as well as a nice 1-2 inch port.

And if it is dangerous, does anyone know where I can get a 6.5 inch subby at 6 or 8 ohms with a decent range? Thanks for reading

 

[croseiv]

It's going to take more juice (current) to get that 4 ohm woofer playing well. Take the same sized woofer, but one is 8 ohm and the other is 4. The 4 ohm woofer will take twice as much current as the eight ohm woofer (in the same enclosure with the same sensitivity). You would actually need a 200 watt amp rated for 4 ohms to get you where the 100W 8ohm combo would.

 

[Djizasse]

Ok, this may be stupid, but can't you put a 4Ohm resistor in line with the driver?
That would raise the resistance to 8 Ohms.

(now, don't hit me please lol, just say why it would not work)

 

[mtrycrafts]

Ok, this may be stupid, but can't you put a 4Ohm resistor in line with the driver?
That would raise the resistance to 8 Ohms.

(now, don't hit me please lol, just say why it would not work)

Yes, with enough power dissipation in that resistor. However, you will get 1/2 the power into that driver only.

 

[TLS Guy]

Ok, this may be stupid, but can't you put a 4Ohm resistor in line with the driver?
That would raise the resistance to 8 Ohms.

(now, don't hit me please lol, just say why it would not work)

Yes, you deserve a smack for that one, or what my math master used to do when I committed a howler like that.

He would come up behind you, put his finger on the howler and tweak the hair of the back of your neck harder and harder, as he said "Say your wretched, say your wretched. The tweak got harder and harder until you said "I'm wretched sir! I'm wretched!"

Any how a four ohm resistor would consume half the power and get very hot. It would upset the Qes of the driver and upset bass loading.

The other problem with this whole issue, is that you can't substitute one driver for another. Every driver has a unique set of parameters called the Thiel/Small parameters. These determine the size of enclosure and its tuning. Also the crossovers are built for specific drivers as they all have unique electrical parameters and roll offs. So you have to replace with the same driver.

There are tons of 8 ohm drivers of that diameter by the way, but what you are planning is a non starter, quite apart from the parameters you mentioned.

 

[annunaki]

It's going to take more juice (current) to get that 4 ohm woofer playing well. Take the same sized woofer, but one is 8 ohm and the other is 4. The 4 ohm woofer will take twice as much current as the eight ohm woofer (in the same enclosure with the same sensitivity). You would actually need a 200 watt amp rated for 4 ohms to get you where the 100W 8ohm combo would.

If the drivers are of the same sensitivity, this is totally incorrect. They both need the same amount of wattage to get to the same level of output if the sensitivity is the same.

A 100 watt amplifier will have to output 20 volts rms to do 100 watts rms into a 4 ohm load. (V^2/I=W). For an 8 ohm load it will have to output 28.284 volts rms to do 100 watts rms.

(V/I=C) Voltage divided by impedance gives you current. The 4 ohm driver will require 5 amps of current while the 8 ohm driver will need just 3.5355 amps. The higher the voltage the lower the current draw. It is true a 100 watts into a 4 ohm load requires more current, but it has nothing to do with which driver will require more wattage. That is based upon the sensitivity and efficiency of the driver itself.

 

[croseiv]

10-4 on the wattage... Good math there.

 

[Adam Skookum]

Thanks for all the imput.
Annunaki, should I assume that the difference in required voltage between the 8 and 4 ohm drivers will be wasted by heat, make a 4 ohm speaker play harder, or blow up my amp?

Also, I'm hoping to get a driver with the generally same dimensions and sensitivity. I don't need a really perfect sound out of the bass, just enough to blast some tool when I'm angry. How noticable would it be if I did just stick a different driver in my enclosure?

And are there any good websites from whence to buy these illusive 6 inch, 8 ohm subbies?

And crosiev, does your comment mean that my amp would start to distort earlier? I thought that lower ohms meant that if one wanted to risk a fire, he could get more wattage out of a lower ohm driver than out of the one for which the amp is rated? Am I just a totalnubtard?

Thanks for all the posts, btw.

 

[TLS Guy]

Thanks for all the imput.
Annunaki, should I assume that the difference in required voltage between the 8 and 4 ohm drivers will be wasted by heat, make a 4 ohm speaker play harder, or blow up my amp?

Also, I'm hoping to get a driver with the generally same dimensions and sensitivity. I don't need a really perfect sound out of the bass, just enough to blast some tool when I'm angry. How noticable would it be if I did just stick a different driver in my enclosure?

And are there any good websites from whence to buy these illusive 6 inch, 8 ohm subbies?

And crosiev, does your comment mean that my amp would start to distort earlier? I thought that lower ohms meant that if one wanted to risk a fire, he could get more wattage out of a lower ohm driver than out of the one for which the amp is rated? Am I just a totalnubtard?

Thanks for all the posts, btw.

There are just numerous problems you have not considered. Quite apart from the Thiel Small parameters, which are vital to match, if you use a 4 ohm speaker the crossover frequency will be wrong. All the inductors in the low pass filter would have to halved in value and the capacitors values doubled.

Your only hope is to tell us what the internal volume of your enclosure is, and whether it is ported or sealed. Then we could try and work a driver backwards. This is actually quite a task.

This site has lots of drivers www.madisound.com Really it would be a lot easier to buy a new set of speakers or build some from scratch. You just can't buy a driver of the same size impedance of sensitivity and have it work properly. The misalignment that will almost certainly result, means the enclosure will have no bass or be boomey depending on the Thiel/Small parameters of the driver.

You need to do some reading to find out how a speaker really works.

 

[annunaki]

TLS guy is correct here. You really should just construct a new woofer. It can be done quite cheaply from parts express or other sites. You should be able to find a 6" 8 ohm woofer there, but I would hardly call that size woofer a sub. Figuring the volume and tuning frequency of the enclosure you currently have will be critical as well.

Thanks for all the imput.
Annunaki, should I assume that the difference in required voltage between the 8 and 4 ohm drivers will be wasted by heat, make a 4 ohm speaker play harder, or blow up my amp?

Also, I'm hoping to get a driver with the generally same dimensions and sensitivity. I don't need a really perfect sound out of the bass, just enough to blast some tool when I'm angry. How noticable would it be if I did just stick a different driver in my enclosure?

And are there any good websites from whence to buy these illusive 6 inch, 8 ohm subbies?

And crosiev, does your comment mean that my amp would start to distort earlier? I thought that lower ohms meant that if one wanted to risk a fire, he could get more wattage out of a lower ohm driver than out of the one for which the amp is rated? Am I just a totalnubtard?

Thanks for all the posts, btw.

 

[annunaki]

Thanks for all the imput.
Annunaki, should I assume that the difference in required voltage between the 8 and 4 ohm drivers will be wasted by heat, make a 4 ohm speaker play harder, or blow up my amp?


Thanks for all the posts, btw.

The amplifier is simply not designed to play a 4 ohm load. Depending upon how competently the amplifier was designed, it may just produce substantially more heat, to completely short circuiting and burning up.