2. amps-preamps
  3. 152150-19072

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

Currently looking at buying a Pass Labs XA 30.5 to drive my Kef 201/2 speakers which are rated at 86 DB sensitivity.  Is this a bad move?
puffbojie
kijanki's avatar
kijanki
4,428 posts
 
@almarg 

For a non-planar non-line source speaker, especially a relatively small bookshelf speaker, putting aside the effects of room reflections SPL will fall off at 6 db per doubling of distance.

Al, yes but doesn't it apply to open space or anechoic chamber only?

I found this published by Benchmark Media (Rules of Thumb)

Rule 13: In a home environment, the SPL at the listening position is about the same as the 1m response of the speaker in an anechoic chamber.

If a speaker has a measured output of 90 dB SPL at 1 meter when fed with one watt, you can expect about 90 dB SPL at your listening position when driving your speaker with one watt. Room reflections supplement the output of the speakers and compensate for the fact that you are more than 1 meter from your speakers. If you are outdoors, this rule does not apply. But, in a typical home listening room, the rule works reasonably well for a quick approximation.

I believe this to be true, since I hear pretty much constant loudness walking in my room. What do you think?
almarg's avatar
Ag insider logo xs@2x
almarg
9,670 posts
 
Hi Kijanki,

Al, yes but doesn't it apply to open space or anechoic chamber only? 

Yes, as I indicated the 6 db of falloff per doubling of distance applies if the effects of room reflections are put aside, i.e., if they are not included.

I hear pretty much constant loudness walking in my room. What do you think?

I've seen a number of references over the years indicating that "room gain" typically adds something like 3 db or so to the SPL that is heard at typical distances in typical rooms, for non-planar non-line source speakers.  Obviously that number will vary somewhat depending on the room, its acoustic characteristics, the dispersion characteristics of the speaker at various frequencies, and other variables.

Best regards,
-- Al
 
bigkidz
3,378 posts
 
I drive a pair of Vandersteen 5As with a Class A 30wpc amp.  They are 87db but have powered subs.  Should not be an issue as Class A is high current.  Happy Listening.
almarg's avatar
Ag insider logo xs@2x
almarg
9,670 posts
 
P.S. to my previous post, regarding the Benchmark statement that was quoted by Kijanki:

Looking at it quantitatively, and choosing a listening distance of 10 feet as an example, and putting aside room effects for the moment, an SPL reduction of 6 db per doubling of distance can be calculated to result in a reduction of 9.68 db at a listening distance of 10 feet, compared to a distance of 1 meter.

A difference of 9.68 db corresponds to a difference of 9.28 times in terms of power. So at that 10 foot listening distance only 1/9.28th as much acoustic power would reach the listener’s ears via the direct path from the speaker compared to what would reach the listener’s ears at 1 meter, in the absence of room reflections.

So for room reflections to compensate for the distance-related loss of 9.68 db it would mean that 8.28 times as much acoustic power would have to be reaching the listener’s ears as a result of room reflections than would be reaching the listener’s ears via the direct path from the speakers. And that would seem to be much too large an effect to be expectable under usual conditions. Especially under conditions that are conducive to good sonics.

Best regards,
-- Al

ratboysr's avatar
ratboysr
60 posts
 
I just bought that same amp here. I have Sonus faber Chameleon B's, 4 ohm 87 db, and was worried about driving them. But I can tell you it drives them well beyond my listening needs and never once has left Class A. Fantastic amp.
More to discover