[Elecraft] Increasing K2 Audio Output

[Wayne Burdick]

Increasing K2 Audio Output

I have two K2s in my shack: s/n 1, the first rev A K2, and s/n 3000, the first
rev. B unit. Both have identical and quite loud audio output--in my opinion. 

About once a year I discover that either (1) not all K2s are built and aligned
the same as mine, or (1) my definition of "loud" is not the same as everyone
else's! I'll take the blame either way. I wrote the manual, which may not be
100% clear in every detail. I also convinced Eric that we needed to optimize the
design for both low current drain and ease of construction by using single
conversion. 

But those who want more audio from their K2 can get it, without much difficulty.
I recommend the following steps, one of which *will* do the trick. Please do
"before" and "after" measurements to make sure you know how much things improved.

1. THE BASICS: Make sure that that the rig is assembled and aligned correctly,
according to the manual. 

2. TROUBLESHOOTING: If you suspect something may not be working right, start by
using the extensive troubleshooting section (K2 manual, appendix E). We included
full signal-tracing instructions, as well as all of the parts needed to build an
RF probe. There's even a schematic for a very simple crystal oscillator which
can be used to inject a signal. Comparing your K2 with a known-working unit is
another effective technique when signal tracing.

3. SUPPORT: If you find this signal-tracing technique is too cumbersome, or it
doesn't work for you, you might e-mail Gary for assistance. He can also fix the
rig for you if you prefer.

4. FINE-TUNING: Let's say you've been through the alignment and signal tracing
and can't find anything wrong. Here are several details to check:

  A. Turn the AGC off and see if the background noise goes up significantly. If
it does, you probably have the threshold set too high. The ideal set point will
vary from rig to rig, because there are a number of components in the signal
patch, each of which can have lower or higher gain than average.

  B. Your BFOs might not be adjust optimally with respect to the center of the
crystal filter. This is especially noticeable with narrow filter bandwidths. You
can try aligning the BFOs using Spectrogram, but first just move them around a
bit to see if they might not be at their ideal settings. I align my BFOs by ear,
but we also provide tables in the manual showing what BFO frequencies are the
best starting points. The frequency counter can display the BFO frequencies
while you tune them.

  C. Turn the preamp on if you've been leaving it off. The K2 has gobs of
dynamic range, so it's safe--in the vast majority of cases--to turn it on, even
on lower bands. You can always turn it off it you think that there's an intermod
issue, but in my experience, this is unlikely.

  D. Make sure you have OPT=PERF set in the menu. The OPT=BATT setting causes a
1 or 2 dB loss in signal strength in some cases.

  E. Make sure L34 is set correctly (and be sure to add the 2nd XFIL mod if you
haven't already--see below).

5. MINOR MODIFICATIONS: The K2 is not a one-size-fits-all-hams transceiver. Even
if it's working right, some operators may want more audio than it is designed to
put out. Here are a few things you can do to increase it:

  A. Change R76--the RF preamp gain-setting resistor--to a smaller value. This
will increase preamp gain. 90% of operators could increase the gain of this
stage and still have more than enough dynamic range. Try decreasing it from 10
ohms to something between 3 and 5 ohms. The smaller the value, the more the
gain. Re-peak the band-pass filters (as described in the manual) after you
choose a new value, just to ensure that you've optimized the filters vis-a-vis
the preamp's slightly modified input impedance.

  B. Install a KAF2 or KDSP2 option. The KAF2 increases audio output by about 2
dB, nominally, and you can increase the gain further if desired by selecting a
couple of different resistors. (Anyone who has done this might comment on what
value they used.) The KDSP2 can be programmed to increase gain even more than
this--download the manual for details.

  C. If you have an older K2 that does not have the 2nd Crystal filter mod
installed, you should do so. This modification optimizes the second filter for
both CW and SSB reception so that L34 can be set a its ideal position. Without
this mod installed, you can optimize for CW or SSB but not both.

  D. Increase post-mixer-amp gain. I haven't tried this myself, but it should be
possible to eke out an extra dB or two by reducing the size of R83 from 4.7 ohms
to something in the 2-3 ohm range. If you try this (or have tried it)
successfully, please post your results. NOTE: Without careful lab testing, I
can't vouch for this modification. While it will increase gain, it might also
worsen IMD on some bands due to non-optimal termination of the receive mixer, Z1.

  E. Some older K2s had narrower or slightly lossier crystal filters. For the
past two years (or more) we've been matching the crystals more closely, and some
builders have seen an improvement in performance with a new set.

6. EXTERNAL ENHANCEMENT: The K2's built-in speaker is extremely efficient--95 dB
SPL. But it does aim *up*, which may not be the best direction for your shack.
Generally it works well if you have a wall behind the K2, but if the wall is too
far away or covered with dense tapestries, the sound could be deadened. To
enhance the sound level, you can:
  
  A. Place a small, stiff card on the top of the rig, behind the speaker.

  B. Use an external speaker that's front-facing.

  C. Use an external amplified speaker.


I'm sure that others may have additional things they've tried and could add to
this list.

73,
Wayne
N6KR