What happened to 8ohm speakers?

[herbu]

Back in the day, (1970s), speakers were 8 ohms. Period. Now they're 4, 6 or 8. Most of the comments and reviews I see extol the advantages of 8 ohms. Why are so many speakers, even very high end, not 8 ohms now?

 

[Irvrobinson]

Most of the comments and reviews I see extol the advantages of 8 ohms.

Which comments and reviews are these? I've never seen one, unless it's a comment from a tube amplifier advocate.

 

[ski2xblack]

Back in the day, speakers had impedances that varied with frequency, just like now.

 

[markw]

Most speakers are still 8 ohms and a few are 4 ohms, but 4 ohms were always with us from as far back as the fifties. AR3 anyone?

An occasional 6 ohm may pop up though.

 

[j_garcia]

Back in the day, (1970s), speakers were 8 ohms. Period. Now they're 4, 6 or 8. Most of the comments and reviews I see extol the advantages of 8 ohms. Why are so many speakers, even very high end, not 8 ohms now?

There is no true "advantage" to an 8 Ohm speaker. Not to mention that "back in the day" there were plenty of speakers that weren't 8 Ohm. What actually matters is that you choose the right speaker for you, which means you can pick one that is 8 Ohm if you like.

 

[3db]

Most 2.5 way towers are 6 Ohms even though a lot of manufactures claim 8. The impedance varies with frequency so its all over the map

 

[ImcLoud]

seems a lot of bookshelfs and 2 ways are 8 ohms and towers are 6, but there isnt a rhyme or reason to it, its what ever the manufacturers engineer wants to put together....

 

[j_garcia]

Rhyme or reason would be most bookshelf speakers are 2 way and end up being much simpler, where many towers are 2.5 or 3 way with multi-driver arrays that may result in lower impedance. In the end though, I don't think most designers of "high end" speakers are as concerned with the impedance while designing as mass market brands might be.

 

[Philip Bamberg]

Simple question, not so short an answer.

"Back in the day" there were also stronger Alnico magnets, so there was good efficiency to be had even if the voice coil impedance was 8ohms. But when Alnico suddenly increased by 20x and climbing, we went to old fashioned ferrite magnets. No more easy sensitivity.

Tweeters tend to be more sensitive ("efficient") than woofers and they don't need baffle rise compensation as woofers do. The baffle rise effect requires woofers to be tilted down by 4-6dB until they sound/measure flat throughout the midrange.

All else being equal, dropping a driver's impedance from (nominal) 8ohm to 4ohm yields +3dB sensitivity. Paralleling two 8ohm woofers yields 4ohm load and +6dB sensitivity, and then finally the pair of woofers may challenge a tweeter to keep up.

Over the past several years, I have seen tweeters go from 8ohm to 6ohm to 4ohm (3ohm minimum), and I favor the 4ohm tweeters. They have sufficient sensitivity to keep up with dual-woofer designs, and they have a slightly better extended top end response. Any extra sensitivity can be padded down in the network, thus raising the impedance back up. I use two techniques (or a combo) to nicely control the minimum impedance at both the lower midrange and treble bands. Kind of a "balancing" act.

Yes, the higher impedance is easier to drive by the amplifier (less current demand), and for the bass range there is a damping factor improvement to be gained at the expense of mass-loaded sensitivity. But since "watts are cheap" and there is the "bragging rights" factor of powerful amplifiers ... again higher-end speakers at 4ohms is nothing to become alarmed about.

I agree with the earlier statement that "high end" speaker designers are not concerned by a design that ends up at 4ohms, although I personally "always keep one eye on the minimum impedance".

Finally, about the "no rhyme or reason" and "all over the map" comments ... yes they can be all over the map, but in general there are logical reasons for the impedance figures.

I have been tasked to raise the minimum impedance on a speaker by one client, and I have been asked to falsely state the nominal impedance by another (so that a true 4ohm speaker would thought to be compatible with cheap lightweight receivers).

 

[JerryLove]

Actually: I've seen 16ohm as well.

Not having been around in the real-old-days; my understanding is that the issue is related to the amplifier used. Tube amps seem to work better with high-sensitivity (because they are low power) and high resistance. I know I read why at one point, but don't recall.

 

[herbu]

Which comments and reviews are these?

1. Uh oh... I was afraid someone would call me on that. Can't remember them all, but went back thru various threads and found a few...
> For ht with an avr I would lean towards 8
> I would go with the 8 ohm version because the receiver will be happier with it
> As your Pioneer is not rated for a 4 ohm speaker I would recommend the (8 ohm)
> The 4 ohms load which stresses the power amp is a different thing , which they avoid to discuss.

2. I will admit that "back in the day", like now, I was no expert... but I was an Electronics Technician in the US Navy, and knew what an ohm was. And I don't remember giving any consideration to the resistance or impedance of home stereo speakers.

3. Mr Bamburg, my hat is off to you. I don't understand a lot of what you said, but enough to understand there really has been an increasing diversity in speaker resistance, and there are legitimate reasons for it.

4. Thanks to all for your time and answers.
I think my practical takeaways are:
> Get a good AVR and/or amp
> Use Audyssey (or similar) to help balance
> Don't worry about it

Thanks!

 

[Irvrobinson]

Actually: I've seen 16ohm as well.

Not having been around in the real-old-days; my understanding is that the issue is related to the amplifier used. Tube amps seem to work better with high-sensitivity (because they are low power) and high resistance. I know I read why at one point, but don't recall.

Tube amplifiers, at least the kind with output transformers, don't really care about the impedance of the speakers they drive, so long as the output transformer has a tap matched to the target speaker's impedance. In fact, is the inclusion of an output transformer that really defines the impedance sensitivity of the design, not the use of tube or solid state devices, because:

- The constant load as seen by the output stage of the amplifier, because all that it sees is a winding of the output transformer, means the power output of the amplifier will be the same regardless of the speaker's impedance, and...

- The actual power output of the amplifier will be determined in part by the efficiency of the matching between the selected output tap of the output transformer and the speaker.

Solid state amplifiers with differential output stages typically have an output impedance that is very low relative to the impedance of the speaker, so an output transformer is not necessary as with a tube amplifier. This also means that while an amplifier with an output transformer is a constant power device, a solid state amplifier without an output transformer is a constant voltage device, at least as far as the power supply and the output stage allows. A constant power device means that the power output is the same regardless of load, while a constant voltage device means that as impedance drops power output will increase. As we all know this increase can't be perfectly linear, but if it was it would mean power output would double as impedance halves. This means that, in theory at least, a robustly designed solid state amp without an output transformer "likes" a lower impedance speaker because it allows the amplifier to deliver more power output. Remember, a solid state amplifier without an output transformer puts out half the power into a 16 ohm load as it does an 8 ohm load; the power transfer function works both ways.

(I probably should mention McIntosh solid state amplifiers at this point, because, strangely, McIntosh uses output transformers on them. So for the purposes of this discussion McIntosh solid state amps act like tube amplifiers. Don't get me started about why McIntosh does this, but their amps are normally very powerful, and they work great regardless of the strangeness of their topology.)

Amplifiers with output transformers like speakers with a relatively constant impedance curve, and with an impedance level that closely corresponds to one of the rated taps on the output transformer. Since most output transformers have taps for 4, 8, or 16 ohms, historically most tube amps "like" 8 ohm speakers with crossovers that make the impedance curve nice and linear. Tube amplifiers "like" sensitive speakers because most tube amps have a low power output relative to solid state amps.