[Elecraft] K3 Audio Low Pass Filter

[Paul Christensen]

> I've been informed Elecraft is already replacing those caps with 100uF
> caps in the current production units.

100 uF is the maximum size compatible with the existing PCB pads for C9/C13 
on the main DSP Board.  Moving up beyond 100 uF presents a problem.  Even at 
100 uF, I believe the SMT caps are now rated at close to 10 WVDC.  There's 
just no more scaling back on the SMT cap's voltage rating in order to 
accommodate a fit with a higher C value above 100 uF.

Why would anyone want a higher value than 100 uF at C9/C13?  Consider a set 
of modern, medium-Z headphones with a Z of 32 ohms at 100 Hz.  The -3dB 
turnover point calculates to roughly 150 Hz.  Anyone wanting to take full 
advantage of the lowest vocal fundamental (e.g., AM or ESSB modes) wants 
that point down to ~ 70 Hz.   So, a cap value of 220 uF would be preferred. 
I went to 330 uF in a radial style to allow extra margin for a wide range of 
headphone vintages and types.  To make most use of the 100 uF coupling caps, 
I would ensure that my headphones are of the newer, medium Z type and not 
the older style with 4-8 ohm drivers.

My guess is that scaling the caps beyond 100 uF is only of interest to a 
small minority of ops.  The majority will probably be happy with the current 
choice of 100 uF.  Going from 10 uF in the prior design to 100 uF is already 
a significant step in the right direction.

Paul, W9AC