[Kirium]

anyone got any ideas as to why one impedence on a speaker would be favoured over another.....

i.e.

like my B&W's are 8Ohms, the speakers on my old Denon DM-50 were 6ohms (built by Mission). apart from being easier to drive a speaker at lower impedence at a given output from an amp, but is their any other advantage for speakers to be (a majority from what i see..) rated at 8 ohms...

cheers

 

[crazychile]

Well, some receivers (cheap ones) will only safely drive an 8 ohm speaker. But almost all quality amplification will safely drive a 4 ohm load or higher.

This is only part of the story because the impedance rating is an average of the impedances at all frequencies. You can have a speaker that is rated at 4 ohms, but it may in fact vary greatly from that and require a beast of an amp toproperly drive them.

I can remember when I first got into Hi-Fi in the mid 80's the Infinity Kappa series of speakers (especially Kappa 9) were rated at 4 ohms, but they dipped down to around 1/4 ohm at some frequencies. That made for a real amp killer. I went to CES one year and the Kappa 9 was the hot speaker to use for Amp manufacturers that wanted to show that their amp could drive about anything.

In the early(er) days of hi-fi it seems as if most speakers were rated at 8 or 16 ohms. Now it seems to be pretty evenly split between 8 and 4 ohm.

An easier loadis generally "favoured", but due to the selection of components and the complexity/simplicity of the xover, this is not always possible. Plus like I mentioned, most amps will drive lower impedance loads pretty well so an 8 ohm load isn't as crucial as it used to be.

 

[gerG]

My own observations: 8 ohms evolved to be the standard of the industry long ago. Most receivers and amps were designed for 2 pairs of stereo speakers, so could drive a single pair of 4 ohm speakers safely. I don't know why 8 ohms was the de facto, but a standard was important so that both sides (amps and speakers) were broadly compatible with each other.

This all got upset when car amplifiers became a reality. With limited supply voltage it is very advantageous to drive the lowest impedence at which the amplifier will remain stable. This drove the auto amp/speaker industry down below 4 ohms. Most car amps can drive 2 ohm loads now. This has stirred up some confusion in the home audio industry, where 8 ohms is still the norm. I don't know of any real advantage in deviating from 8 ohm. The exception is in the subwoofer realm, where the desire to put big drivers into little boxes requires big power out of small packages. All kinds of combinations out there right now.


gerG

 

[princeclassic]

if you have a nice amplifier that puts out say...50 watts into 8 ohms...but can put out 80 watts at 4 ohms....it might be nice to use some 4 ohm speakers and take advantage of the extra headroom that the amp offers for the 4 ohm load

so if you had to choose between two pairs of speakers with exactly the same sound....but one was 4 ohms and one was 8....itd be a good idea to go with the 4 if you have the nice amp

 

[wallijonn]

ohm's law. and the law of energy conservation.

for a smaller resistance, a speaker can draw more current at the same amp voltage output. the amplifier's output voltage is directly tied to the power supply bias and rail voltages (as least as espoused by ICs). the transistor devices themselves must be able to pass more current at lower impedances without shorting the devices as the output load impedances approached ground, or 0 (a short).

in the case of 16/8/4, 8 was chosen as being a balance between the 4's high current to the 16's low current. the 8's were right in the middle, and therefore more likely to drive more devices, and were less likely to tax the transistor devices. remember, these impedances evolved from tubes and transformers. computer chips have changed the triggering voltages necessary to forward drive a transistor. newer transistor manufacturing methods have therefore produced devices which, for a given power supply voltage, produces more current at lower resistances. it is the current which "pushes" and "pulls" a speaker cone. And do not forget that the speaker itself has evolved - lighter cone materials to more quickly respond to signal changes, thereby giving a faster transient response. Voice coils have evolved from alnico aluminium bobbins to some truly exotic, if not esoteric, materials. And with them, the magnets themselves have evolved.

just do a search for "impedance". you'll find hundreds of articles which explain it. my favourite is the K1000 tube impedance transformer output discussion.

 

[Kirium]

I see said the blind man...

many thanks to those for their answers....