Here's the complete B.O.M. to improve the K3's receive and transmit audio frequency response.  This mod re-scales several coupling caps
in the audio paths to extend transmitted and receive low-frequency audio response by more than one full octave.  If the user is not 
comfortable with SMT re-work, it is suggested that s/he seek qualified assistance before proceeding.  All changes affect only the K3's 
Main DSP Board:

DIGIKEY
QTY.    DIGIKEY- P/N     DESCRIPTION                K3 SCHEMATIC DESIGNATOR

(4)     PCC2396CT-ND    (4.7 uF SMT Size "603")     C33, C34, C36, C37
(2)     PCC2275CT-ND    (0.47 uF SMT Size "603")    C44, C45
(1)     PCC1915CT-ND    (1.0 uF uF SMT Size "603")  C28


MOUSER ELECTRONICS
Choose one of the following cap types for C9, C13.  The radial caps offer lower frequency response, while the SMT caps will be easier to 
change.  In my opinion, the 100uF caps are perfectly adaquate since the K3's headphone output stage utilizes 8-ohm series build-out resistors.  
In essence, if an 8-ohm load is present at the K3, the amplifier actually sees 16-ohms, owing to the build-out resistors.

QTY. MOUSER P/N          DESCRIPTION                 K3 SCHEMATIC DESIGNATOR

(2)   647-UKW1A331MED   (330uF @ 10 WVDC Radial) C9, C13

Or,

(2)   647-UCD0J101MCL1GS   (100uF @ 10 WVDC - SMT)    C9, C13

The replacement 330uF headphone coupling caps are audiophile-grade, radial-lead Nichicon Gold Audio Electrolytics, rated at
2000 Hrs @ 85°C:  http://products.nichicon.co.jp/en/pdf/XJA043/e-kw.pdf

Note that the original headphone coupling caps as used in the K3 are only 10uF with 8-ohms of fixed, series build-out resistance.  Depending on 
the type of headphones used, the low-frequency turnover point can typically vary between  400 Hz (32-ohm) to 1000 Hz (8-ohm).   That's a lot of 
low frequency attenuation.  After the mod, the - 3dB turnover point for 32-ohm phones calculates to 40 Hz (for 100uF coupling caps) and 
approximately 12 Hz for 330 uF caps.

Even for older 8-ohm headphones, a change of C9, C13 to 100 uF results in a -3dB point of  100 Hz -- still respectable -- and as low as 30 Hz for 
330 uF caps.  It's important to note that the true headphone Z varies as a function of frequency. Thus, a set of "8-ohm" headphones may have a 
substantially different Z at some frequencies with respect to others -- and with most 8-ohm phones, the effect of low-end attenuation is actually 
quite a bit better than the computed amount.

If you decide on upgrading to the 330uF caps, the replacement C9, C13 caps can be carefully soldered to the existing SMT solder pads and placed 
against the insulating material on the headphone jack.  After soldering the new caps, use a hot glue gun to bond the caps against the headphone 
jack. However, if SMT pads become stressed as a result of leveraged pressure against the cap (e.g., trying to straighten the cap after soldering),  
it is a certainty that the SMT pad will lift from the PC board.  If that occurs, the main DSP board may become permanently damaged.  Because the 
330 uF radial caps introduce a greater propensity for damage and offer little additional audio benefit, the 100uF SMT caps will be the best choice 
for the majority of users.

For those of you not wanting to embark on these changes, Elecraft may offer a Main DSP Board-exchange upgrade program in the future.  Recently 
produced K3 transceivers include all necessary frequency response modifications.

In addition to the aforementioned mod, it is recommended that the user make the remaining audio/grounding mods to the K3 as directed by Elecraft:

http://www.elecraft.com/K3/mods/K3_Application_Note_Front_Panel_Microphone_Grounding.pdf

http://www.elecraft.com/K3/mods/ELECRAFT_Application_Note_Improving_the_Immunity_of_the_Rear-Panel_KIO3_RS232_and_Audio_Connectors_to_RF.pdf

http://www.elecraft.com/K3/mods/K3_AF_Stage_Upgrade_Instructions_Rev_B.pdf

Disclaimer of Warranty:  This mod is offered to the Elecraft user community for informational purposes only.  No warranty of merchantability 
or fitness for any use is implied with the dissemination of this information.  With the exception of the authorized application notes stated 
above, please DO NOT contact Elecraft concerning this equipment modification.

Paul, W9AC