[Elecraft] optimal headphones impedance for K2?

[Jeff Burns]

Before I installed the KDSP2 in my K2 I used my David Clark headphones most
of the time. Their impedance is only 10 ohms so the audio was low but
sufficient.  I found the David Clark's comfortable to wear despite being
heavy.  The isolation form outside noise is also great.

Now with the KDSP2 the David Clark headphones sound bad.  The output is
weaker. To get good output I have tried using the speaker jack on the back.
That gets the volume up but produces more hiss. Some CW sounds are fairly
distorted.  I do not understated what the KDSP2 did. Maybe the signals are
so much cleaner that I am noticing the shortcomings of the headphones?

This has led me to try every pair of headphones in the house. My Pioneer
SE-72 sound very good. Unfortunately, they are the supra-aural type and hurt
my ears in short order.  They also do not block out sound as much as I would
like. Their impedance is around 30 ohms.  I remember someone on this list
stating that this was about the optimal impedance for headphones used with
the K2.

In looking at possible new headphone I have noticed that many of the best
ones are very high impedance.  For instance the Koss Pro 4/aa impedance is
250 ohm, and the sensitivity is 95dB SPE/1mV. Just considering the power
dividing between the resistors in the K2 and the headphones it seems that
the higher the headphone impedance the better. I suppose that at some point
the amplifier will not be able to put any current into circuit and audio
output will decrease.   So what is the optimal impedance for headphones used
with the K2?

I have found one nice set of headphones with an impedance of 55 ohms -  the
AKG K271. They have a sensitivity of 91dB SPE/1mV. These look like great
headphones, but their price is getting a little salty.


Jeff Burns
AD9T