How to shop for speakers?

HiFiTime & HiFiHvn, most tube amps (that have output transformers) are designed to provide approximately the same maximum amount of power when a 4 ohm speaker is connected to the 4 ohm tap, as when an 8 ohm speaker is connected to the 8 ohm tap. There are several things to watch out for, though:

1)If an 8 ohm speaker is connected to the 4 ohm tap, which is sometimes done to achieve tighter bass or other effects on sound quality, then there would be a 3db volume drop (compared to the 8 ohm speaker being connected to the 8 ohm tap).

2)If the speaker sensitivity is rated with respect to 2.83 volts, rather than with respect to 1 watt (2.83 volts corresponds to 1 watt into 8 ohms), and if the speaker's nominal impedance is 4 ohms, then a 90db/2.83V/1m sensitivity would be only 87db/1W/1m.

3)Fluctuations in the speaker's impedance vs. frequency curve will assume different significance for a 4 ohm speaker connected to the 4 ohm tap than for an 8 ohm speaker connected to the 8 ohm tap, if the fluctuations are not in the same proportion. For example, if the impedance of a nominally 4 ohm speaker varies from 2 ohms to 8 ohms over the frequency range, a greater impact on frequency response would result (with a tube amp) than in the case of an 8 ohm speaker whose actual impedance had the same tolerance (-2/+4 ohms, meaning impedance varying between 6 ohms and 12 ohms).

Along the lines of Ralph's comment the matchup of the speaker's variations of impedance as a function of frequency, and the amplifier's output impedance, should be consistent with the designer's intention.

Also, I second Tim's comments about the importance of the phase angle of the impedance, and its variations with frequency. A low impedance magnitude coupled with a highly capacitive phase angle is a worst case load.

Regards,
-- Al

Thanks guys.I guess I'll still keep using the 3 db for reference.Sometimes the 4ohm tap does sound better with some 8 ohm nominal rated speakers. That was just a guess.I had a couple of different Conrad Johnson amps,and from what I remember,they wired them all for 4 ohms out of their factory.They told me that they sound the best this way.I see an amp for sale now that has a 6 ohm,and 16 ohm output.Might be a compromise between the two.

Thanks Almarg and Timlub.Some odd things do happen sometimes.In the 80s I believe,a dealer friend tried two pairs of Rogers 15 ohm speakers on a tube amp,and it resulted in a volume drop that was enough to stop him from ordering a second pair for his home.This was on the 8 ohm tape.Being a tech,he was somewhat disappointed.He didn't bother going any further. It seems like we always are learning.I think there are some Magneplanar speakers that are a decent 4 ohm load.It would be nice to have Stereophile,or others do tests on a lot more speakers.That sure would help.To many speakers are rated 8 ohms that have characteristics that should put them in the 4 ohm category in my opinion.It makes it hard for people that starting home theater systems that can only handle 8 ohm loads.

If an 8 ohm speaker is connected to the 4 ohm tap, which is sometimes done to achieve tighter bass or other effects on sound quality, then there would be a 3db volume drop (compared to the 8 ohm speaker being connected to the 8 ohm tap).

Slight correction to this statement in my earlier post. The statement should say:

If an 8 ohm speaker is connected to the 4 ohm tap, which is sometimes done to achieve tighter bass or other effects on sound quality, then there would be a volume drop of up to 3db, the exact value depending on the amplifier's output impedance (compared to the 8 ohm speaker being connected to the 8 ohm tap).

Best regards,
-- Al

Al makes good points; all I want to do here is emphasize something that he mentions about speaker sensitivity:

Sensitivity and efficiency are different measurements! Sensitivity is 2.83 volts at 1 meter, efficiency is 1 watt at 1 meter. Now if you have an 8 ohm speaker, then they are the same. But if you have a 4 ohm speaker, watch what happens: 2.83 volts into 4 ohms is **2** watts. So if you have a 90 db sensitivity speaker that is 4 ohms, its efficiency is 3 db less, 87 db.

Conversely, if you have a 16 ohm speaker that is 90 db sensitivity, its efficiency is 93 db.

Tubes do not double power as you cut the impedance in half, and many transistor amps do. So if you are using tubes, you really have to pay attention to the significance of these figures else they will shoot you in the foot rather quickly!

Its my opinion that the sensitivity figure can be misleading, as it often creates the impression that a speaker is more efficient than it really is. Now whether we like it or not, the amount of power that the amp can make into the load is what we are talking about here- and not all transistor amps double power as the impedance is halved (and certainly no tube amp does...).

My room at home is about 17' x 22'; with 87 db the power levels for any serious dynamics gets a bit crazy- this is why I argue for efficiency if you can get it without compromising resolution and bandwidth, and quite often that is very possible. I make this point as there is a tube amp involved; if you really want to get the performance out of tubes, you owe it to yourself to use them with speakers that work easily with them. Maybe a poor analogy; but this is something like putting decent tires on a sports car that you intend to drive hard...