Tube amps and speaker matching


Recently I read somewhere that tube amps are not good at driving speakers which have a wide impedance curve, say if the impedance swings from 3-20 Ohms across the audio range. On the other hand speakers with a narrow swing like 3 to 9 Ohms who be a good match for the tube amps. Is this correct? What is really the ballpark figure to make a judgment if the speaker and tube amp combination will be a good match.

Long time ago a fellow audiophile told me that as a rule of thumb the speaker sensitivity should be in the following range for a good match:

Below 90 dB/spl Solid State only
90-100 dB/spl Solid state or tubes
Over 100 dB tubes only

As for my personal experience I have heard a 12 watter drive the Dynaudio Contour 5.4 and was very listenable, similarly I have also heard a Cary amp with just five watts drive a pair of Maggies. I personally have driven a Quad 15 watt vintage tube amp with the Maggies. All these combinations to my ears sounded pretty listenable albeit one could not drive them hard as the amps were very low powered.

Can anyone enlighten me with their opinion?

Thanks
128x128quadophile

Showing 1 response by atmasphere

Tubes are expensive power. With efficiencies less than 89 db or so, you start to need a lot of power to really make the speaker play, although there is a lot of variance due to rooms and taste. So the power rule of thumb thing is generally OK but should not be regarded as cast in concrete.

As far as impedance swings go, that all has to do with the type of speaker and the intention of the designer. For example Sound Labs and older Quads have wide impedance swings but tubes do quite well with them.

With regards to intention of the designer see:
http://www.atma-sphere.com/papers/paradigm_paper2.html

As for general guideline/rule of thumb things I can add another one:

"If you are investing in a tube amplifier, your tube amplifier dollar will be best served by a speaker that is at least 8 ohms or more, all other things being equal."

Of course 'all other things' are *never* equal, what this comment is speaking to is that all tube amplifiers whether they have a 4 ohm tap or not will have more power (although not by all that much) lower distortion and possibly wider bandwidth when driving higher impedances.

Bottom line is YMMV, so audition is always the best policy!