Best Tube Based Options For B&W 803S Speakers

You will need to be very careful about impedance compatibility when pairing a tube preamp with your Anthem A2. The A2 has a specified input impedance of only 10K unbalanced and 15K balanced.

The output impedance of the BAT VK-3ix is described in John Atkinson's measurements as follows:

The unbalanced source impedance varied from 7k ohms at 20Hz to 1460 ohms at 1kHz and 2650 ohms at 20kHz, while the balanced figures ranged from a very high 14k ohms at 20Hz to 1850 ohms in the midband and above.

If the output impedance of the VK-3i that was suggested above is similar to that of the VK-3ix, it would be a completely unsuitable pairing, which would result in severe deep bass rolloff.

Ideally the output impedance of the preamp you choose to use with the A2 should be no higher than 1.5K balanced or 1K unbalanced, AT ALL AUDIBLE FREQUENCIES. If only a nominal output impedance is known, which would presumably be at a mid-range frequency such as 1 kHz, to be safe I would suggest an upper limit of around 200 or 300 ohms.

Regards,
-- Al

The input impedance of the Halo A21 is spec'd at 33K unbalanced and 66K balanced, which would be a suitable match for most tube preamps. I'd still be hesitant to pair it with the BAT preamp, however, assuming the output impedance of the VK-3i is similar to that of the VK-3ix. Although with recordings which don't have significant content in or near the bottom octave you may not notice any issues.

Regarding use of a tube power amp with your speakers, I couldn't find an impedance vs. frequency curve for the 803S. However, impedance curves for several other B&W models, including some that seem to have a degree of commonality with the design of the 803S, all show a rise to very high impedance values (well above 20 ohms) in the upper midrange/lower treble area, around 2 to 4 kHz or so.

The interaction of that kind of impedance variation with the relatively high output impedance of a tube amp will result in greater emphasis of those particular frequencies, relative to other frequencies, in comparison with the frequency response that would occur with a solid state amp. That would seem to be the opposite of what you are trying to accomplish, namely softening the highs.

As others have indicated above, when it comes to power amplification B&W's tend not to be tube-friendly.

Regards,
-- Al

Good input from Lewinski. I would add, though, that the output impedance of the MC275 is much lower than that of most tube amps. I had said earlier in the thread that:

Impedance curves for several other B&W models, including some that seem to have a degree of commonality with the design of the 803S, all show a rise to very high impedance values (well above 20 ohms) in the upper midrange/lower treble area, around 2 to 4 kHz or so.

The interaction of that kind of impedance variation with the relatively high output impedance of a tube amp will result in greater emphasis of those particular frequencies, relative to other frequencies, in comparison with the frequency response that would occur with a solid state amp. That would seem to be the opposite of what you are trying to accomplish, namely softening the highs.

The current Mark VI version of the MC275 has a specified damping factor of 22, which is much higher than that of most tube amps, and theoretically corresponds to unusually low output impedances (for a tube amp) of 8/22 = 0.36 ohms for the 8 ohm tap, and 4/22 = 0.18 ohms for the 4 ohm tap. John Atkinson's measurements of the Mark V version are only slightly higher than those values.

So the tonal balance effects of the interaction between the MC275's output impedance and the speaker's impedance vs. frequency characteristics will be considerably closer to those of a solid state amp than will be the case with most tube amps.

Regards,
-- Al