[Elecraft] K3 Audio Low Pass Filter

[Brett Howard]

Isn't the equation 1/2*pi*r*c?  If I do the math on my headphones at 27 ohms
(a little worse than your 32 ohm example) I get:

1/(2*3.14159*(8.2+27)*100e-6) = 45.2145Hz

The 8.2 ohms is actually in the K3.  If I do it again for your 32 ohm
example I get 39Hz.  With your 220uF cap that moves it down to ~18Hz for a
pair of 32ohm cans.

Personally I think the more robust connection is a good point to keep and
I'm ok with the 45Hz output.  I tried running things through my headphone
amp which has an 18KOhm input impedance (which should give a -3dB point of
0.883Hz even with the 10uF caps) and I wasn't hearing stuff down too low.
I'd have to dump it into a spectrum analyzer to see how low it really goes
and I'm not so sure I have too many sound cards that are all that clean down
there.  Guess I'd have to go looking...

~Brett 

-----Original Message-----
From: Paul Christensen [mailto:w9ac at arrl.net] 
Sent: Monday, July 06, 2009 8:23 AM
To: Brett Howard
Cc: Elecraft at mailman.qth.net
Subject: Re: [Elecraft] K3 Audio Low Pass Filter

> 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