Pass XA30.5 enough power for Wilson Sophia 2?

According to this list of specifications at the Wilson site the Sophia 2 has a sensitivity of 86db/W/m, rather than the 88 mentioned above. Based on the assumptions of 195W max amp power into its 4 ohm nominal impedance, a 4-meter (13 foot) listening distance, a 6db reduction in spl for each doubling of distance, and adding 3db to reflect the presence of 2 speakers, I calculate that maximum spl at the listening position will be 100db.

That is certainly good enough for most music at reasonable listening volumes. But ime it is roughly 5db too low to adequately handle music that has very wide dynamic range, such as a lot of classical symphonic music, if the recording is well engineered and minimally compressed.

Best regards,
-- Al

CMalak -- Thanks. The Stereophile site appears to be down right now. Are you sure, though, that those measurements are for the Sophia 2 and not for the Sophia? I had looked for JA measurements of the Sophia 2 before I posted, but I was only able to find measurements for the Sophia.

Weseixas: Wouldn't that be a 6 db gain when you add the presence of the second speaker as well as doubling the power when adding the other channel

Doubling of power, whether acoustical or electrical, corresponds to a 3db increase. The fact that the additional 195W from the second channel of the amp is applied to a second speaker, as opposed to 390W being applied to a hypothetical single speaker, makes no difference for purposes of this calculation.

You might be thinking of doubling of a voltage, which would be a 6db increase.

Best regards,
-- Al

Addendum to my previous post:

The Stereophile site is up again, and it looks like I was correct that JA's measurements were for the Sophia, and not the Sophia 2. Also, his 88.3db measurement was not based on 1 watt and 1 meter, it was based on 2.83 volts and 1 meter (2.83 volts corresponding to 1 watt into 8 ohms).

2.83 volts into the nominally 4 ohm speaker impedance corresponds to 2 watts. So the 88.3db sensitivity JA indicated for the Sophia would be only 85.3db based on 1 watt into its nominally 4 ohm impedance, fairly close to the Sophia 2's specified 86db/1W/1m.

And I note also that the Sophia's spec is indicated in the Stereophile review as being 89db/2.83V/1m, which corresponds to 86db/1W/1m for its 4 ohm impedance, which is the same as the spec for the Sophia 2 that I used in my earlier calculations.

Best regards,
-- Al

Expanding a bit on a point I mentioned earlier, the most important factor that determines how much power is required is the dynamic range of the music being listened to. Meaning the difference in volume between the loudest notes and the softest notes. Or perhaps more relevantly for some music, the difference in volume between the loudest notes and the average note.

It seems clear, based on several comments that have been made and on my calculations, that the amp should have no problem playing highly compressed, narrow dynamic range material, such as most rock recordings, at levels approaching 95db at the listening position. I doubt that any of us would want to listen that loud.

However, a symphonic work having wide dynamic range, if well recorded and minimally compressed, and if played with an average volume of say 70 to 75db, can be expected to at least occasionally have brief musical peaks that exceed the 100db maximum spl at the listening position that the amp/speaker combination is capable of.

Best regards,
-- Al

Pubul57 raises a good question. The XA30.5 is specified to "leave Class A at 60 peak watts." I'm not sure how that should be interpreted, as to whether the 60W is peak, rather than the usual rms definition, and as to whether or not that number applies to a 4 ohm load.

But if we make a very worst case assumption that it leaves Class A at 30 watts into 4 ohms, that is 8db less than 195W. Which means per my previous calculations that the amp would be operating Class A up to an spl of 92db at the listening position, which is quite loud.

-- Al

Tvad is correct.

The point to using a ratio that approaches 100:1, or that is much larger than 10:1, would apply to the situation where the only output impedance specification that is available is a nominal rating, that presumably applies to mid-frequencies. In which case using a high ratio would allow for the rise in output impedance that commonly occurs at very low frequencies, especially in tube preamps, due to the output coupling capacitor.

In this case, worst case output impedance data for the Ref3 across the entire frequency range is available in JA's Stereophile measurements. The worst case, which as can be expected occurs at 20Hz, is 1437 ohms in balanced mode. That is well under one-tenth of the input impedance of the Pass .5 amps, and therefore should not be a problem.

Also, in situations where this issue may be a problem I would expect the symptom to be attenuation of the deepest bass, not muddy bass.

Regards,
-- Al