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Old 10th May 2002, 08:38 PM   #1
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Question Why would anyone want 8ohms?

i am still new to the diy scene and i was wondering.....
why anyone would want a higher Impedance.....
in all cases i have seen the higher impedance the lower wattage an amp can put out..... their for less sound....lower spl.....
right?

for example:
a partsexpress subamp puts out 250w @ 4ohms and only 180w @ 8ohms.......
what is the benifit of 8ohms over 4?
it looks to me like it is a bad decision to go with 8ohms.....


i am not sure so that is why i am asking.....

please respond....
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Old 10th May 2002, 08:56 PM   #2
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Slice,

Many amps dont like four ohm loads very much. its true that most sub amps love 4 ohm loads, but many hifi amps do not. Tube amps especially dont like low impedance loads. Another reason is that the impedance of speaker systems changes in relation to the frequency, so with your eight ohm nominal impedance speaker, the impedance could dip much lower at certain points, and if this where a nominal four ohm load, then you could see maybe one ohm at certain frequencies. sorry if this isnt very clear.
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Old 10th May 2002, 09:22 PM   #3
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so their is absolutly no benefit to using different ohm loads....?
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Old 10th May 2002, 09:35 PM   #4
Bill F. is offline Bill F.  United States
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There are much higher caliber amp gurus about this forum, but I'll take an awkward swing.

If you test the THD of a given amp at the lower end of its impedence envelope, and again at the higher end, you'll generally see distortion drop as impedence rises.

My understanding of this phenomenon is that amplifiers are typically voltage-controlled, that is, the output is basically a boosted voltage image of the line-level input, with a (hopefully) sufficient current supply to chase the voltage through the target resistance/impedence. A small voltage swing drives a lot of current through a low impedence. A high impedence must be driven with a large voltage swing.

For lack of a better analogy, the larger voltage swing necessary for a higher impedence equates to a higher resolution image of the input signal, more tightly controlling the current output. Since magnetic transducers are current-driven devices, this added control is a good thing.

For this reason, I've dreamed of someday winding a nice 1Kohm voice coil and seeing what kind of magic it could make with the right amplifier.

Bill
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Old 10th May 2002, 09:38 PM   #5
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One must keep in mind to, that, in general*, if you halve the load, you double the distortion. Lower impedances also tend to put more strain on the amp decreasing their lifespan.

*with all the caveats that come with an "in general" ie Doug Self's load invariant amps (recent audioXpresses) where he gets the ratio much closer to 1

dave
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Old 10th May 2002, 09:42 PM   #6
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You are correct however in your initial post, Slice, in saying that lowering speaker impedance increases SPL. This is generally true, and you will often see subs that are four ohms, or less.
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Old 11th May 2002, 02:01 AM   #7
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Slicemaster:

Figured someone was going to come up with this question sooner or later.

The home hi fi industry started about fifty years ago, and picked up steam in the 1960's with the arrival of transistor amps. There were 8 ohm and 16 ohm speakers at first, but the industry gradually settled on 8 ohms. That way, anybody could hook almost any speaker up to almost any amp or receiver. The 8 ohm standard was adopted by PA and prodfessional systems as well.

Cars always had speakers in them, but car speakers, as well as electronics, were strictly lo-fi. No resemblance to a hi-fi speaker at all, and car radios put out maybe 3 watts, and even cut off the bass below 70 Hz! In many cars, there was a single speaker located behind the front dash. Car audio and home audio existed in two completely different worlds, and car audio was lousy.

Remember, back then the best sound possible was from vinyl records, which you obviously cannot have in a car. Actually, I am told that Chrsler had a special vinyl record player in the early fifties-you had to buy special records for it-but it never took off.

With the advent of 8 track tape players and later Dolby cassette players in cars, people actually were able to have something resembling high class sound in their autos. So aftermarket car audio companies gradually started supplying amps of 10 watts or more with distortion under 1%. You might laugh now at a cassette player played through 15 watts now as being hifi, but really, it only takes about 10 watts to reach fairly loud levels in a room, let alone a car. The difference from the tinny sounding systems that came with the car was dramatic.

Things have progressed to where many people have a better system in their car than their home. Why not-you can really let loose with the volume in a moving car on the highway more than you can at home. [Caution-please do not turn your car stereo up for extended periods. Ear damage is epidemic among young people]. The market for subwoofers is greater for cars than it is for home players, and so many people wanting a good subwoofer for a good price end up selecting a model marketed for cars.

With the merging of the car and home speaker markets, amplifier manufacturers started manufacturing amps that could handle, if not actually thrive, on 4 ohm loads. In fact, many home amps actually do deliver more power into 4 ohms than 8 ohm speakers now.

This is a complete turnaround from the 1970's. A friend of mine had a Phase Linear amp-the first superamp that went over 200 watts-and the thing had trouble with 4 ohm loads. In fact, it delivered most power into 12 ohm loads! Stupid? No. Back in the early seventies, there were few really good 4 ohm speakers to drive. 4 ohm speakers were mostly lousy car units which no self respecting stereo fan would allow in his home. Obviously, that has changed now.

You are quite right, Slice, to wonder why 8 ohm speakers are being made. A cyber-friend of mine in Cork, Ireland, is a DJ and he says most professional amps are 4 ohm amps nowadays as well. However, the 8 ohm standard has been around for so long that it will take time to completely convert to 4 ohm speakers. Adding to the longevity of 8 ohm speakers is the fact that it is not so far away from 4 ohms, and it is relatively easy to make an amp that plays 8 ohm speakers almost as loud as 4 ohm speakers. I do think that 4 ohm is the direction we are going, however.
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Old 12th May 2002, 03:00 PM   #8
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>I do think that 4 ohm is the direction we are going, however.

More the pity. And so it will go as long a marketing can keep power handling above efficiency in the minds of consumers.

Oh, well. Raising speaker current is one way to sell fat speaker cables, too.

Bill
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Old 12th May 2002, 03:42 PM   #9
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4 ohms means double current. Double current for the same amp means trouble.
More distortion, more work for the power supply, more losses on the speaker cables, more losses on the crossovers, bigger coils in the crossovers for the woofers and so on. 4 Ohms also means that high class amps like tube amps or single ended amps also don´t function perfect for high power.
Many problems.
Also more power doesn´t mean more spl. The same speaker can have the same spl in 8 ohm or 4 ohm.
The ideal for high end is 16 Ohm.
Why do the use 4 ohm in cars. in the car you dont have a problem with current, you have a problem with voltage. To get higher voltage you need dc/dc converters which at the end also need more current from the battery. The older years 4 and 2 ohm speakers where ideal for cars but today with the higher tech dc/dc converters even high end 8 ohm speakers can be used in cars.
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Old 13th May 2002, 08:10 AM   #10
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As I read through the post, some post are right and some are wrong. Amplifiers can handle any load if it is well ventillated, though the wire have to be thick in order to handle the current. Second there is no difference in loudness from a 4 ohm speaker driver to a 8 ohm speaker driver. The enclosure will change the effect of loadness of either type of load. Car amplifiers by now can handle 4 ohms or even 2 ohms because of efficiency wise and Class A/B designs. Class A/B amplifiers can handle the wattage better at 4 ohms than 8 ohms. Using 4 ohms speaker drivers won't hurt in either a 2-way or 3-way loudspeakers because the resistance of the crossover components and the wire used. Using 8 ohm speaker drivers is great for multiple speaker drivers connected in parallel.
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