[Elecraft] New CW audio recording: K3 vs. K4D

[Bob Wilson, N6TV]

This afternoon I made a 3-1/2 minute audio recording of two stations in QSO
with each other on 40 CW, using both a K3 and K4D, and switching back and
forth.  Download here (2.3 MB):

https://www.kkn.net/~n6tv/K3_vs_K4D_CW_Audio_by_N6TV_20200625.mp3

Recorded off rear headphone jack using an MK2R+ USB Sound Card.

In sum, I think the K4D audio sounds quite a bit better, but the "500 Hz"
passband of the K4 seems wider than the K3 (wider skirts?).  The AGC attack
time seems slower in the K4 (similar to Icom IC-7851 fast AGC, IMHO).  So
the "leading edge" of each dot or dash is louder, which may improve copy a
bit.

The loud station (about 599 + 10 dB) is AC2K in AZ, and the weaker one
(about S-6 to S-7) is NZ0T in MO.

Timestamp      What you're hearing
------------   -------------------
00:00 - 00:16  K3
00:19 - 01:00  K4
01:01 - 01:21  K4 with NR engaged and adjusted for best copy ("Level 3")
                  (Tuned range 0 to 10, the complete range)
01:22 - 01:25  K3
01:26 - 01:58  K3 with NR F1-2 (didn't adjust)
02:00 - 02:35  K4 NR off I think
02:37 - 03:10  K3 NR off
03:13 - 03:20  K4

K3 is S/N 1495 with KSYN3A upgrade but original DSP board and Inrad 500 Hz
8-pole CW crystal filter, with W9AC Audio Mod
<http://web.archive.org/web/20091011115528/http://n1eu.com/K3/k3audiomod.htm>.
(capacitors changed in headphone circuit to increase low end audio response)
K4D Field Test Unit S/N 0008

K3 and K4 were set with identical parameters where possible:

AGC-F, ATT OFF, PRE OFF
RF Gain about 3 O'Clock on K3.
RF Gain 0 (max) on K4
CW mode, 500 Hz bandwidth
Pitch 480 Hz / center of passband IF shift
AGC Attack:  210 (K4 only)
AGC DCY Soft (K3 only)
AGC HLD 0.05 (K3 only)
AGC PLS NOR/ON
AGC SLP 008 (K3)
AGC Slope 15 (K4 -- units are different)
AGC THR 010 (K3)
AGC Threshold 4 (K4 -- units are different)
AGC-F / AGC Decay, Fast:120
AGC-S / AGC Decay, Slow:  20

73,
Bob, N6TV