Replacing my Krell FPB 600 amp with a tube amp

A point to keep in mind regarding passively biamping the FPB600 with a tube amp, in addition to the gain matching concern that was mentioned, and in addition to the possible effects on coherence of driving the speaker with two very different amps, relates to the fact that in a passive biamp configuration (i.e., without an electronic crossover "ahead" of the amps) both amps must put out voltages corresponding to the full frequency range of the signal.

Your speakers have impedances in the vicinity of 4 ohms in much of the bass region. The FPB600 is rated to provide up to 1200 watts into 4 ohms. If you were to passively biamp it with say the ARC Ref 150 you would be reducing the usable power capability of the FPB600 from 1200 watts to not much more than 150 watts, because if you were to turn the volume control up high enough to utilize more of its power capability the lower powered amp would be driven into clipping.

Good luck. Regards,
-- Al

The impedance curve for the Matrix 800 that is shown in Figure 1 of Stereophile’s measurements indicates an impedance in the vicinity of 4 ohms in much of the bass region, as I mentioned previously, and more than 10 ohms at almost all frequencies above 1 kHz. That strongly suggests that the speaker is intended for use with amplifiers having low output impedances, and correspondingly having relatively high damping factors. Which disqualifies many tube amps as being suitable matches, regardless of how powerful they are. B&W’s recommended range of amplifier power, 150 to 800 watts, further reinforces a conclusion that the speakers were designed with solid state amplification in mind. (Solid state amps almost always have very low output impedances and relatively high damping factors, those two terms being inversely proportional).

The likely result of driving speakers having this kind of impedance characteristic with an amp having a highish output impedance/low damping factor would be weak bass, precisely what the OP wants to avoid.

While as many here realize I have been a great supporter of Ralph/Atmasphere and his products and his many contributions to this forum, the 2.3 ohm output impedance of the Atma-Sphere MA-1 makes them a non-starter for use with this particular speaker, IMO. Certainly without using the amps in conjunction with a pair of Zero autoformers, and very possibly even with them. And I’m doubtful about the more powerful MA-2 as well, with its 1.75 ohm output impedance.

I’m pretty certain that the CAT JL2 that was suggested, which I see has a specified output impedance of 0.7 ohms, would be a suitable match. As would some or all of the larger ARC amps that have been referred to. Not sure about VTL or CJ.

Regards,
-- Al

Thanks for the nice words, Dentdog. I found that information about the CAT JL2 in the measurements section of this review of the Signature Mk2 version of the amp, in SoundStage. The data provided near the top of the measurements page states "output impedance at 50Hz: 0.7 ohms." And the damping factor measurements shown in Chart 4 indicate a damping factor of about 11.5 across nearly all of the audible spectrum, with a slight decrease to about 11 at 20 Hz and 20 kHz.

I’m not sure what output tap or taps the amp provides, but a damping factor of 11.5 corresponds to an output impedance for an 8 ohm tap of 8/11.5 = 0.696 ohms, and less than that for a 4 ohm tap.

Best regards,
-- Al