[Elecraft] K2

[Jim Brown]

On Thu, 17 Jun 2004 00:33:23 -0400, Don Wilhelm wrote:

>I have always thought the K2 to be a bit 'wimpy' on the audio
>output - but I have found it is really adequate when driving the internal
>speaker, just not a lot of reserve to overcome a high ambient acoustical
>noise level.

Wayne has done a really good job of engineering the audio chain on the
receiver to provide enough loudness (and good quality) without wasting
battery current. Part of that good engineering was finding a really efficient
loudspeaker to go inside the K2. 

When you want to add an external loudspeaker to the K2, you must do the same
level of good engineering. That means NOT grabbing whatever cheapo
loudspeaker you have in the junk box, but rather looking for a loudspeaker
with relatively high efficiency (and, as you correctly observed, a 4 ohm
loudspeaker has an inherent 6 dB advantage).  You have a very big advantage
over Wayne though -- in general, the larger the loudspeaker, the easier it is
to find one with high efficiency!  

The spec you are looking for is dBSPL for 1W at 1 m.  (that is, dB sound
pressure level at 1 meter away from the loudspeaker with 1 watt electrical
power). The higher the number, the louder it will be. Hi-fi loudspeakers are
generally LOW efficiency. PA loudspeakers are generally HIGH efficiency.  A
good number for an 8-inch loudspeaker is in the high 80's or low 90's.  

To translate this, the K2 puts out 1 watt sine wave at clip. Speech typically
has a 10-14 dB peak to average ratio, so if your loudspeaker is putting out
90 dBSPL at the peak of that sine wave, the average level will be 10 dB below
that (80 dBSPL). Few of us listen 1 m (39") away from our radio -- half that
distance is far more typical, (19"), and gives us 6 dB more level, so we're
back up to 86 dB. And that assumes no sound is reflected from nearby walls,
shelves, etc., which can also add a dB or two to the loudness. The bottom
line is that 86 dB is plenty loud unless you've got severe hearing loss or
are in a very noisy environment.  And, of course, if you listen closer, it
will be even louder. 

On the other hand, let's say you hang an 8 ohm hi-fi loudspeaker (you bought
it at Radio Shack or your local hi-fi shop) and it has a sensitivity of only
80 dB for 1w at 1m. The same math puts you 10 dB lower in level for the lower
sensitivity, and anothe 6 dB lower because it's an 8 ohm loudspeaker. That's
only 70 dBSPL, which is only marginally loud enough in a noisy room (or if
you have a lot of hearing loss). 

Jim Brown  K9YC
http://audiosystemsgroup.com