[Elecraft] K2 Headphone Output

[Ron D'Eau Claire]

> One individual said that the headphone output varies widely from K2 to
> K2, even though they all use the LM380N-8 audio amp IC and the same other
> components in that circuit
>
> Most others said to use 32-ohm headphones, which I already do (I also
> tried my 8-ohm headphones which yield slightly less volume with the K2).
> On the bottom of the RF board, I temporarily bridged AF OUT to the tip
> connection at the front panel headphone jack.  This made a BIG, BIG
> difference in the amount of audio to that ear.  I therefore will reduce
> the values of R35 & R36 to around 22 ohms or so.
>
> 73, tnx for the info, de Earl, K6SE

There APPEARS to be a wide variation in the audio channel gain, based on
reports from various ops on this reflector over the past two years. This has
always been a curious issue since all of the K2's use the same audio
components as you noted.

One possible cause for that variation - as someone else mentioned here - is
the impedance of the phones used. Low impedance phones will produce lower
audio levels, since their resistance is placed in series R35 and R36. 8-ohm
headphones, for example, would only get about 10% of the audio voltage from
the amp while the more common 35-ohm "computer" type of phones would see
nearly half of the audio voltage.

A very simple and quick check to see if R35 and R36 are cutting the audio
level too much is to simply plug the phones into the auxiliary speaker jack
(using a suitable adapter because it is a mono jack) to bypass those
resistors.

Your symptoms seem to point to low impedance phones  - although I note that
you are comfortable that they are not - OR possibly a bad connection
somewhere between the r-f board and the front panel.

More than one op has discovered that one pin of a connector or other part
never got soldered so that it left a high-resistance contact in the circuit
somewhere.

Ron AC7AC
K2 # 1289