[Elecraft] [K3] K3 Audio Response - Version 3.33 Firmware

[Paul Christensen]

> Is this measured with high impedance probes? If I remember correctly, 
> small
> caps limit low-frequency response of K3 especially with low impedance
> phones/speakers.
> Ignacy

Correct, measured with a 2K-ohm load resistance at the sound card input. 
The purpose of the test was to measure only the response plots as a result 
of changes in DSP versions.  To get a ruler-flat response down to 20 Hz with 
headphones may require a change in C9/C13 on the Main DSP Board, depending 
on the Z of your headphones at that frequency.  I changed C9/C13 with 330uF 
units and this work well with any commercial headphones.

If you refer to the K3's schematic, note that 8-ohm series build-out 
resistors are used and appear "off-page."  The addition of the build-out 
resistors has the effect of improving low-end response as the headphone Z 
decreases.

As a practical matter, and assuming one is using newer headphones with a Z 
of at least 30 ohms at roughly 50 Hz, then Elecraft's revised Main DSP Board 
with 100uF caps at C9/C13 are completely sufficient.

More info here:  http://www.n1eu.com/K3/k3audiomod.htm

W9AC