When using a tube amplifier, most know to look for speakers with high efficiency ratings. What is sometimes overlooked is impedance. This is important too because it may affect the response curve in some cases. I'm not talking about its (8 or 16 ohm) "rated impedance," although you certainly want this to match whatever your amp expects to drive.The thing to be concerned about is impedance fluxuation. Peaks and dips in the response curve will translate into peaks and dips in the amount of power available to move the voice coil, if the amplifier's output impedance is high. This, in turn, causes peaks and dips in amplitude response. So when buying a speaker for use on a tube amplifier, look at the impedance curve of a loudspeaker in addition to its sensitivity. Be careful of a speaker with high Zmax, because it may not sound right on a tube amp.
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