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AR-2A's running on tubes


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This B.A.S. article on speaker efficiency may or may not be useful:


It does not list the 2a but it does include the 2ax and shows them to be pretty inefficient, requiring 60w for 100db.

My guess is there are a lot of other factors to consider. I used to drive my AR4x's perfectly well with a dyna 17wpc tube amp. YMMV. A lot depends on the source, room size and volume. For heavy metal head-bangin' music you'll want an amp that goes up to 11 :D *

What size amp do you have/are you considering?


*Spinal Tap reference

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The 2a and 2ax have essentially the same sensitivity.

I have AR-3a’s in a small 11 x 13 room. First, consider that the sensitivity specification, as measured one meter on axis for a 1-watt input, will yield a figure of about 86dB for an AR-3a. That’s considered “inefficient.” The 2's are similar (but their higher impedance will draw less current from a solid-state amp, so they may appear to be of "lower efficiency" at the same physical setting of the volume control compared to the 3/3a).

But 86dB is really a pretty healthy level, much more than background listening. It may not be loud enough in a dealer showroom, but in your quiet listening room, you’d have to turn it down to answer the phone. I regularly use a Radio Shack SPL meter to check the playback levels in my room, and it's amazing how loud low-mid-90's dB SPL is.

Now, you’ve got two speakers, so there’s some addition to the 86 SPL figure because of that. You are also about 8-10 feet away from the two speakers (and somewhat off axis, to boot), so there’s some reduction because of that (the inverse square law, which says that SPL reduces by -6 dB as the distance doubles). Throw in your room’s absorptive characteristics, any open walls that lead to the next room, etc, and what you’re left with is that the raw sensitivity figure for one speaker is a pretty decent number to work with as to how loud two speakers will sound from your listening position with a one-watt input. There are a LOT of variables, obviously, but 86dB for 1 watt for a pair of 3a’s from your listening chair is not a bad estimate. The 2a will be very similar, within a few dB.

Every doubling of power is another 3dB of loudness. So:

2 w = 89 dB

4 w = 92 dB

8 w = 95 dB

16 w = 98 dB

32 w = 101 dB

64 w = 104 dB

128 w = 107 dB

256 w = 110 dB

I’m not even sure how cleanly the 2a can play at or above 110dB, but any competent amp has enough juice to push the 2a to 95-100dB in a normal listening room. And that is very, very loud. As mentioned in previous posts, make sure you’re using a good amp that’s not apt to be driven into distortion. The 2a doesn't have a ferro-fluid cooled tweeter, so be careful not to abuse the speaker by pushing the amp into gross audible distortion.

Caution and common sense are always the guidelines when using 30-50 year-old classic speakers that have irreplaceable parts.

Steve F.

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Wow, lots of science and math involved! I don't have a particular amp I am looking at quite yet. Most of them have been the the 15w rms per channel range. I mostly listen to classical and am radio with a bit of country and classic rock tossed in between. I don't listen at loud levels, mostly slightly higher than background volume. I am not sure of the room size but it isn't very big.

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It is worth noting that:

85-95 dB is the range in which sustained exposure begins to cause permanent hearing loss. Real-world examples include a garbage disposal or blender or a passing big rig truck.

60-70 dB is the range of normal conversation.

45-55 dB is the level of background noise in an average residence.

30 dB is the level of your public library's reading room.

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Agreed--hence my observation that mid-90's on my SPL meter is extremely loud and that in the mid-80's, you'll have to turn down the system to answer the phone. As I said, 15-20 watts will drive the 2a to very nice levels, and as long as you don't push the amp into gross audible distortion, both your speakers and your ears will be fine.

Steve F.

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