a subwoofer - 4 ohm or 8 ohm, what's the difference?

[Anonymous]

same subwoofer, comes in (at least) 2 flavours, 4 ohm and 8 ohm
same price, i havent a clue, what's the difference please?

thanks

[mph]

same subwoofer, comes in (at least) 2 flavours, 4 ohm and 8 ohm
same price, i havent a clue, what's the difference please?

4ohms   s:wink: :wink: s:wink:  

  s:oops: :oops: s:oops:  well someone had to say it.. Sorry no idea, all to do with impedance and what your amplifier is expecting; someone will be along shortly to give a slightly more helpful answer.

[Tem]

Basically, less resistance (ohms) means more sound level. Car stuff usually uses 4 ohms, home stereos 8 ohms, maybe the sub is sold for both applications?

Just make sure your amp can handle the speakers/sub resistance you get  s8) s8)

[Anonymous]

Martin, I would've been veeeerrrry unhappy if i didn't receive such a reply    s:wink: :wink: s:wink:    s:D s:D  

Thanks Tem

[GSB]

Basically it works like this, if you want lots of power, use a 2ohm sub and and an amp capable of driving a 2ohm load. this is imporatant as mismatched impedances between amps and speakers is bad, both for sound and for the hardware.  The upside of 2ohm installs is more power, the downside is less sensitivity, i.e. it wont sound as nice. Ok for dickheads in Novas, but personally I like to "hear" my music...

Different impedances are however very useful for multiple driver installs. i.e. If you want to drive into a 4 ohm load to get the best compromise of power vs. quality, but need to run 2 subs from one amp channel then depending upon the way you wire your 2 typical (i.e. 4 ohm) subs, you can present the amp with a load of 2 or 8 ohms. This is bad... The option of different impedance speakers means that you can wire 2 2ohm subs in series, which gives the amp a 4 ohm load, or you can wire 2 8 ohm subs in parralel, which also gives the amp a 4 ohm load...

Useful stuff... Most Mr2 owners are happy with / only have the space for one sub, so 4 ohms is the usual standard.  s:wink: :wink: s:wink:

[Anonymous]

cheers Grant.
I will be buying bits piecemeal, due to monetaries, 1 item a month say, till I've got all I need, then get it installed.
So, in your opinion, it would be ok for me to get the sub now (JL 6W0), the 4 ohm, then will it be easy to find a great small amp to compliment this? No idea which amp, or where to put it, but dont want to compromise on space, so any hints/tips on which amp to go for would be gratefully received.
The other thing is, I dont even know what stereo I'll be going for, got stock unit at the mo, but won't need to install the sub/amp until get a replacement for that, so, last question, if i choose and amp/sub combo, will it be ok to choose practially any of all the new-ish stereos? (bearing in mind it will be the 4ohm 6W0 and suitable amp) they'll all be ok? or is that another factor to worry about when choosing the system.
Christ i hope not!! too much to think about!
thanks in advance
Steve

[GSB]

cheers Grant.
I will be buying bits piecemeal, due to monetaries, 1 item a month say, till I've got all I need, then get it installed.
So, in your opinion, it would be ok for me to get the sub now (JL 6W0), the 4 ohm, then will it be easy to find a great small amp to compliment this? No idea which amp, or where to put it, but dont want to compromise on space, so any hints/tips on which amp to go for would be gratefully received.
The other thing is, I dont even know what stereo I'll be going for, got stock unit at the mo, but won't need to install the sub/amp until get a replacement for that, so, last question, if i choose and amp/sub combo, will it be ok to choose practially any of all the new-ish stereos? (bearing in mind it will be the 4ohm 6W0 and suitable amp) they'll all be ok? or is that another factor to worry about when choosing the system.
Christ i hope not!! too much to think about!
thanks in advance
Steve

Just about any current stereo will be fine. Neary all aftermarket head units come with high impedance pre-outs (the phono plugs on the back) that are connectable to all amplifiers. The difference is usually how many pre-outs are offered, and how high the voltage is on them. The voltage bit you dont need to worry about. Alpines 4volt pre-outs are supposed to offer more protection against signal loss, but frankly you dont need to worry about it. The number of pre-outs has a greater effect though, as it can make your system more controllable. For instance my sat nave system has 2 pre-outs. That is it has 2 pairs of plugs coming out of the back that offer me front and rear stereo channels. So I can send seperate signals to 4 speakers if I so desire. This meant thatI had to derive a specific low frequency channel for my sub though. Which isnt hard, but its does mean tha if I want to change any crossover settings or adjust the ouput level of the sub relative to the rest of the speakers, that I have to start farting around in the back of the car. Compare that to the Alpine in my last MR2 which had the same 2 pre-outs for front and rear speakers, PLUS a single dedicated phono plug that supplied a subwoofer signal. The beauty of this was that I could control the sub direct from the headunit. I could adjust its level, change the crossover points, change its phasing, or even turn it off entirely, all from the Alpine Fascia.

In my opinion this sort of functionality not only makes for an easier installation, it also makes adjustment easier (Because its going to rquire a long drawn out session of twiddling pots on the amp in the back of the car, I still haven't adjusted the sound on my set up, and I put it in in November!), and its far nicer to live with.  Its definitely worth looking for.


Now as for amps that can be fitted without losing storage space, well, theres only a few, and most of them have been discontinued. I will be replacing my amp fairly soon with one of these:

 m http://www.bluespot.co.uk/stock/amplifier_4.asp m


Which, according to my calculations, should fit under the plastic floor of the passenger side luggage compartment. This of course will mean that my trusty Kenwood amp that is mounted with the top flushed into the passenger luggage floor will be for sale at some point, as it will be surplus to requirements.

[Anonymous]

great stuff, thanks for the info G, much appreciated

[heathstimpson]

And I can confirm I have just squeezed the BlueSpot amp under the passenger cubby hole plastics. The felt and webs have to be removed and its a tight squeeze getting the plastics mounting bolts back in but it goes  s:? :? s:?

[GSB]

Thought it would Heath, for reference the TSA 460 shown above is actually slightly smaller than Heaths amp.

[Anonymous]

Here is my understanding of this issue.

Speakers have impedance (resistance). This is measured in ohms.

n.b. Opposition to current in a direct current circuit is called resistance and measured in ohms, opposition to current in an alternating current system is called impedance, and also measured in ohms

The lower the ohms in a circuit the nearerer you get to a short circuit (sparks etc)

Amps are designed to push current around a circuit of a particular ohmage....hence a 4ohm stable amp can safely play into a 4 ohm speaker.

A 4ohm stable amp CANNOT play into a 2ohm resistance as it will be pushing too much current, get hot, and eventually knacker the amp.

A 4ohm stable amp CAN play into a 8ohm resistance, as it is pushing less current so keeps cool. Unfortunately as there is 2x the impedance it will be 1/2 as powerful.

If you bridge two channels of a 4ohm stable amp, then each channel 'sees' half the resistance of the speaker. so it is seeing only 2ohms, gets hot, smokes etc etc

So manufactures give ratings,

e.g.

4 ohm stable - safely play a 4 ohm speaker
4 ohm bridged - safely play bridged into a 4 ohm speaker
2ohm stable - safely play into a two ohm speaker(usually twice the power of a 4ohm)


Right, onto speakers:

When two 4 ohm speakers are in a cirtuit in series, one after another, they present an 8ohm load to the amp (4ohm + 4ohm = 8ohm)

When two 4ohm speakers are in a cirtuit in parrallel, splits to the positives and joins again after the negatives, they present a 2 ohm load to the amp (4ohm / 2 = 2ohm)

e.g.

If you have an amp that is 4ohm stable bridged, then two channels can be safely bridged to a 4ohm sub.

In order to bridge into two 4ohm subs, these can either be wired to show a 2ohm load...not a good idea....or an 8ohm load....only half the power

So to get round this we can now get dual voice coil subs (DVC). Normal speakers have a single voice coil.

DVC's have two voice coils, often of 4ohms, these can then be wired in series or parrellel to be 2 or 8 ohms.

So if we take two dvc subs, configered to show 8ohms each, and wire them to the amp discussed above in parrellel, we will present a 4ohm load to the amp, which it will play happily.

However, the same sub which takes 1000 watts RMS at 1 Ohm will output the exactly the same SPL as the same sub which is 1000 watts RMS at 4 Ohm. Ohms doesn't make anything louder. It's all power v's resistance.

Also, if you had one amp with 1000 watts at 1 ohm and another amp with 1000 watts at 4 ohm, they will produce the same amount of heat (assuming they are the same amp class)

Then there is the class of amplifier, Class A/B are approx 50% efficient. So for every watt of sound, there is a watt of heat. So 1000 watts of heat.

Class D are approx 80% efficent, so for every watt, there is .25 watts of heat. Therefore 300 watts of heat.

Hope this helps

Stu

[kanujunkie]

nice write up Stu and well described technically, heres a fly in the ointment that can help with the heat, some manufactuerers now get around the problem by cooling there speakers and amps with water, my DVC sub and monobloc has the facility to have cooling built in them with a plug in the core of the speaker but i've not fitted it yet, too many problems to iron out first  s:? :? s:?