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

Showing 5 responses by kijanki

kijanki's avatar
kijanki
4,449 posts
 
For your 8 ohm speakers efficiency=sensitivity. Driven by 1W they will deliver 86dB sound level.  30W (30:1 ratio) should give you about 15dB more resulting in 101dB.   It should cover even symphony orchestra (EU directive limits level to 85dB in order to protect musicians).

Efficiency is defined as loudness at 1W power at 1m distance in open space.  Since average room is not an open space (sound is amplified by reflections) it can be assumed to be the same in whole room (unless room is very large).
kijanki's avatar
kijanki
4,449 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?
kijanki's avatar
kijanki
4,449 posts
 
@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's avatar
kijanki
4,449 posts
 
@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's avatar
kijanki
4,449 posts
 
@almarg 
Yes, loss will be 6.54dB, but from rated sensitivity of one speaker it will be only 3.54dB.  For 2m listening distance, that they likely assumed as an average, there will be no error.  Sound level at listening position will be the same as rated sensitivity.

In case on hand each speaker sensitivity is 86dB. Two of them playing at the same time add +3dB, room adds +3dB and distance of 2m causes 6dB drop resulting in 86dB sound level at listening position when 2.83V is applied to both speakers.  Am I missing something?
More to discover