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....
 
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.
 

Bill F.

Member
2001-11-15 5:25 pm
SW MI
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
 
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
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
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.
 

Bill F.

Member
2001-11-15 5:25 pm
SW MI
>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
 
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.
 

Electro

Member
2002-04-12 4:41 am
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.
 
Reading through the posts, I noticed that nobody really touched the deepest issue in all this. Close came the notion that car amplifiers used to have problem with voltage and home amplifiers had problem with current.

But the real question still is, why aren't we debating 40 and 80 Ohms instead of 4 and 8? Or maybe we should look into 0.4 and 0.8 Ohms and decide which one is best?

And my answer is, the laws of physics. Very big voltages require parts that are rated for big voltages. For example, electrolytic capacitors are best available at 100VDC and below. And currents in excess of 20A or 40A require special connectors and leads, parts and designs to be feasible.

One could make a dynamic driver with a 5 kOhm voice coil and hook it up to a transformerless vacuum tube amplifier, but the voice coil lead would have to be so thin as to make this impractical. Then, every part handling voltages ought to stand 0.75kV or something and yet be safe.

So my point is, we stand at the crossroads of voltage and current. It's the laws of physics that will some day guide us to the ultimate speaker impedance. Until then, I am happy trying to cover 4 and 8 Ohms and scratch my head when I see a subwoofer driver with two 2 or 1 Ohm coils.

I once had a big disappointment when I included to my simulations and calculations the effect the impedance of 545VA transformer's secondaries have on my Leach amplifier's output power into 2 Ohms. With perfect transformer, I could get more than 400W RMS, but in reality, I just get enough current to make it near 300W RMS before the voltage is too low.

Now I also understand why Mr. Pass has his transformers rated twice the maximum output power into 4 Ohms, or four times the output power into 8 Ohms.

-Kimmo S.
 
All,

For the sake of this discussion that answers the spirit of the original question, assume that more power equals more volume or is "louder".

In terms of speakers, the 16 ohms, 8 ohms, 4 ohms, the ohms refers to impedence. The big advantage of MATCHING impedences (as in an 8 ohm amplifier driving an 8 ohm speaker) is maximum power transfer.

Any given source can output a given level of volts. This level is usually determinded by the power supply rails. The same amplifier can output a maximum amount of current. The dominate factor in this amount is typically the transformer. Yes, you could select components to screw this up, I will assume a complete, reasonable, and thoughtful design.

A simplified version of the math:

Power amp outputs 8 Volts RMS (given)
It can output max 1 Amp RMS (given)
The optimum load (for maximum power) is 8 Ohms (derived 8 volts / 1 Amp)
This amp is an "8 Watt" amplifier, into 8 ohms. (derived 8 Volts * 1 Amp)

If you to change the impedence, higher or lower, you will not get any number greater than 8 watts. Lower the impedance, you "run out" of current, raise the impedence, you "run out" of voltage.

The typical fix: design an amplifier that can deliver more current than the theoretical amount needed. If you doubled the current CAPABILITY on the above amplfier to 2 Amps, an 8 volt source into a 4 ohm load = 16 watts. Now you are buying a bigger (and more expensive) transformer.

So 4 Ohm speakers are not more powerfull (and there for louder) than 8 Ohm speakers. In theory, an 800 Ohm speaker could be just as loud as an 8 Ohm speaker. To do that you would need to raise the voltage, but you can lower the current.

Regards,

Aud_Mot
 
If a given amplifier has an output impedance of say 0.1 ohms and we apply a 4 ohm speaker then the damping factor will be 40. 8 ohms will be 80. 16 ohms will be 160. So higher impedance is not all bad.

A tube amplifier will drive any impedance you like provided the o/p tranny has a suitable number of turns on the secondary winding. With 5000 turns on the primary and a single turn of flat copper bar like in a spot-welder transformer for the secondary it will match 10,000 ohms plate-to-plate to 0.0004 ohms. That's 10,000 4 ohm speakers in parallel.

When Britain was the most powerful and influential nation in the whole world they had their share of movers and shakers in the loudspeaker industry as well. That being the case I'm surprised they didn't settle on 12 ohms, it being England's favourite number.

They had some funny ideas about speakers back then. Fritz Langford-Smith in his "Radiotron Designers Handbook" quotes Gilbert Briggs of Wharfedale Wireless Works fame as saying 'speaker boxes are like cabbages, the bigger the better, but too big and they go to seed'. :p

GP.
 

AndrewJ

Member
2001-09-05 6:37 pm
Circlotron, I think you flatter us too much in implying we were still so powerful at the time speakers were introduced. I think in fact we had pretty much lost it by then!
Also, 12 was the number that we based our monetry system on because it has the most number of common divisers, 1, 2, 3, 4 and 6. Very useful when using up small coinage.
Musher, I agree with your statements. The choice of nominal impedance is governed by practical limits on voltage and current capability optimization of the components used in amplifiers, go too low and we have to supply high currents, go too high and we have to supply high voltages. High voltages are dangerous and so would require special precautions for connections between amp and speaker and cause difficulty especially in choice of transistors, they don't work well at high voltage. High current requires very low resistance cables, thick copper tracks on the pcb, thick lead out braids on the spaker drive unit(prone to fracture)....there are lots of other reasons that need not be catalogued here.
Promitheus, in cars, low impedances are popular because as you said the voltage from the battery is limited, so unless one uses dc-dc converters, the only way to supply enough power is to work at low impedance. However there is a reason that 2 and even 1 ohm speakers have become popular, even with the use of dc-dc converters. This is due to CHEATING!!. In SPL soun-offs, the category at one time was rated by how many watts were available from the amplifier, but this was the 8 or 4 ohm rated power. By designing a high current capable amp, whilst only supplying say 50W into 8 ohms, could supply 100 into 4, 200 into 2 and 400W into 1ohm. Wind a 1 ohm speaker and you now have 400w available from an amp that is supposedly only 50W!!!!
I would also point out that speaker efficiency is independant of impedance level. A 16 ohm speaker is just as efficient as a 4 ohm speaker and will go just as loud with a given power level.
Cheers
Andrew
 
Practical Reasons ?

Hi all, maybe the origin of 4, 8 and 16 ohm speakers is just practicality.
Maybe using practical diameter wire, practical diameter voice coil former, and practical length voice coil, and practical amplifier output transformer, 16 ohms or so just settled out as being practical to make in quantity.
Solid state techniques made it practical to deliver higher currents and then 8 and 4 ohms settled out as being practical values.
If vaguish memory serves correctly this is given in The Radiotron DHB, but mine is on loan - don't lend yours if you get one.
Bose make 1 ohm drivers wired in series, and stage can be 16 or 8 ohm in parallel.
Driver parasitic capacitance is one the problems.
Anybody have a definative historical reason ?.

Regards, Eric.