Tubes and the LS50

Hi Doug,

KEF rates the LS50 at 84dB. Can they be run well with a 40Wpc PrimaLuna Dialogue Five EL34 power amp?


A speaker’s sensitivity doesn’t tell you anything about the sound quality, but it is a useful indicator for how much amplifier power you’ll need. Our measurements of the LS50 show the sensitivity to be 84.25dB (2.83V/1m), which is below average, so the 40Wpc that the Dialogue Five delivers (their website indicates 42Wpc in ultralinear mode, but close enough) won’t go as far with it as with a speaker that’s more sensitive. (Over the years, I’ve found 87dB to be about average. While 3dB doesn’t sound like much, it’s important to realize that every 3dB increase in sound pressure level requires a doubling of amplifier power!)

A secondary concern is the impedance. Generally, the LS50’s impedance stays around 8 ohms or above, but it does dip to about 4 ohms around 200Hz. That won’t give most solid-state amplifiers grief, but you never know with tube designs.

All told, a low-powered tube amp probably isn’t the best match for a pair of LS50s, but it might work well enough providing you’re not using the speakers in a large room and you don’t need to play them super-loud. . . . Doug Schneider