Class A 30 Watt Amps: Are they enough to drive my book shelfs?

@almarg 
Al, remember that there are two speakers, not one (+3dB).    Room gain adds +3dB (or more)  and I'm sitting at about 3m from the speakers equal to loss of a little more than 6dB (9.5dB?).   Rule of thumb has -3.5dB error here, but works perfectly for 2m listening distance.  Almost constant loudness in my room is likely because of listening angle to the speaker.  When I get close to one of them the other is at an angle and when I get closer to front wall between the speakers both are at angle.

@kijanki

Yes, I of course realize that there are two speakers. But for a listener at a centered position that is equidistant from the two speakers the calculation I provided, involving the difference in SPL at 10 feet vs. 1 meter, applies just as well. Obviously it is not usually possible to be 1 meter in front of both speakers at the same time, but if we hypothetically assume that to be possible (so that we can use the speaker’s specified or measured sensitivity, which of course is based on 1 meter) my calculation of the **difference** in SPL between 1 meter and 10 feet is not affected. All that would be affected is the absolute value of the SPL at the two distances, which the second speaker would cause to increase by the same amount at both distances.

In other words, the second speaker does not compensate for 3 db of the distance-related loss. It just increases the reference point to which that loss applies.

I’m sitting at about 3m from the speakers equal to loss of a little more than 6dB (9.5dB?). Rule of thumb has -3.5dB error here, but works perfectly for 2m listening distance.

Here is the calculation of the 9.68 db difference in SPL that I stated occurs between listening distances of 1 meter and 10 feet, putting aside the effects of room reflections:

If we denote the two distances as D1 and D2 a loss of 6 db per doubling of distance corresponds to a loss of 20 x log(D1/D2).

1 meter is 39.37 inches; 10 feet is 120 inches.

20 x log(120/39.37) = 9.68 db

For your 3 meter listening distance the result would be:

20 x log(3/1) = 9.54 db

Factoring in 3 db or so of room gain brings that loss close to the 6 db figure you cited. Although that amounts to a 6 db error in Benchmark’s rule of thumb guideline, since their guideline asserts that essentially no loss would result.

Best regards,
-- Al

I have the 30.8 and I would be shocked if the 30.5 couldn’t completely drive the heck out of those KEF’s.

Yes but the amp will be pushed close to it’s limit to "drive the heck out of them", and no amp I know sounds it’s best when near it’s limit.
Like I said the 30.5 will be enough, but if you want to go big, then you will hear the strain when compared to say the 60.5

Cheers George

@almarg 
Al, I agree, but shouldn't second speaker make a difference (both at 10' distance)?  Two speakers vs. one should add +3dB making overall error 3.54dB (no error at 2m listening distance)

@kijanki

Hi Kijanki,

Two speakers vs. one will add 3 db, but both speakers will have the same distance-related loss, so as I indicated the second speaker will not compensate for the distance-related loss. It will just make the sound 3 db louder at any given distance.

If 2.83 volts is applied to one speaker rated at 90 db/2.83 volts/1 meter, neglecting room gain the SPL at 3 meters will be 90 db minus 9.54 db.

If 2.83 volts is applied to two of the same speakers, neglecting room gain the SPL at 3 meters will be 93 db minus 9.54 db.

In the second case the SPL at 3 meters will be 3 db louder than in the first case. But the distance-related loss in both cases is 9.54 db. Room gain will compensate for around 3 db of that loss, resulting in a loss of about 6.54 db in both cases. The 6.54 db subtracts from 90 db in the first case, and subtracts from 93 db in the second case.  While the Benchmark statement implies that 0 db should be subtracted from 90 db in the first case, and 0 db should be subtracted from 93 db in the second case.

Best regards,
-- Al