SET amplifier recommendation

...  its not enough power on speakers like that unless the intention is to have the music mostly in background or serious listening at lower levels.

I would add to Ralph's comment that the dynamic range of the kinds of recordings that are listened to (i.e., the **difference** in volume between the loudest notes and the softest notes) will dramatically affect how much power is required. For example, many well engineered minimally compressed classical symphonic recordings require **many** thousands of times as much power to reproduce brief dynamic peaks as to reproduce their softest notes. While many and probably most pop and rock recordings are dynamically compressed such that they require no more than around ten times as much power to do likewise.

Regards,
-- Al

@lewinskih01

Thanks for the mention!

Regarding your experience with the MC275, it’s worth noting that it has a considerably lower output impedance (and correspondingly a considerably higher damping factor) than most other high quality tube amps. So the likelihood of adverse impedance interactions with the speaker’s impedance variations over the frequency range, that I referred to for the 805 in an earlier post, would be minimized with that amp compared to most other tube amps.

For example, Stereophile measured the output impedance of the MC275 version 5 at most frequencies as being only 0.33 ohms for the 4 ohm tap and 0.57 ohms for the 8 ohm tap. And version 6 is specified at the McIntosh website as having a damping factor of 22, which theoretically corresponds to an output impedance of 4/22 = 0.18 ohms for the 4 ohm tap, and 8/22 = 0.36 ohms for the 8 ohm tap.

In contrast, the damping factors of the majority of high quality tube amps are in the single digits, and some SETs have damping factors in the vicinity of 1 or even less. With the corresponding output impedances being much higher than those of the MC275.

So in terms of the tonal effects resulting from impedance interactions with the speaker those numbers suggest that the MC275’s behavior will approach solid state territory, rather than being representative of the behavior of most high quality tube amps.

All of that is not to say, though, that no tube amp having a relatively low damping factor/high output impedance can work well with these speakers. For example Ralph’s amps fall into that category, but I don’t doubt for a second that the reports of good customer experiences he has cited involving pairings of his amps with these speakers are accurate. But generally speaking, obtaining good results pairing a tube amp with these speakers figures to be less predictable and a good deal more doubtful than with a solid state amp.

Best regards,
-- Al

The issue of bandwidth becoming more limited as the total output power is increased happens with any amplifier with an output transformer. In SETs this issue is exacerbated.

Thanks, Ralph. That would say that in the case of tube amps having output transformers published frequency response specs should be taken with grains of salt, unless the power level they correspond to is clearly stated (with that basis preferably being full power). (And also, as with the frequency response or bandwidth specs of any amplifier, grains of salt should be applied if the amount of rolloff the stated response corresponds to is not indicated).

And if I may say so, the specs provided at your site are exemplary in those (and other) respects.

Best regards,
-- Al

With SETs, the bigger the amp the less bandwidth. 300b is about as powerful as an SET can get and still be considered ’hifi’ as far as bandwidth goes.

That’s an interesting point, Ralph, which I presume derives mainly from the bandwidth limitations imposed by output transformers when they are used in SET configurations.

Would the bandwidth limitations you refer to apply to the small signal bandwidth as well as to the bandwidth at or near full power? Or just to the latter?

Thanks. Best regards,
-- Al

The B&W speakers are voiced for SS amplification and are never going to sound their best with tube amplification; possibly with the exception of a pair of Atma-Sphere OTL monoblocks. My recommendation is to stay with SS or, if the OP really wants to do SET, change the speakers to something more appropriate.

+1. (Although Ralph/Atmasphere is of course much better positioned than I am to comment on the suitability of his amps for this particular speaker).

And it’s not just a matter of having enough power. Given that the speaker is presumably voiced with solid state amplifiers in mind, which almost always have a near zero output impedance, the interaction of the very wide impedance variations of this particular speaker (see Figure 1 of JA’s measurements) with the relatively high output impedance of nearly all tube amps will result in a tonal balance that was not intended by the speaker’s designers.

Regards,
-- Al