[Elecraft] K3 Audio Low Pass Filter

[Joe Subich, W4TV]

Paul, 

You neglect the effect of the 8.2 Ohm resistor in series with 
the headphone outputs.  With that resistor considered, the 
-3 dB point with 100 uF capacitors as a function of headphone 
impedance is: 

   Z       Frequency 
---------------------- 
   4         130 Hz
   8          98 Hz 
  16          66 Hz 
  32          40 Hz
  64          22 Hz 
 200           8 Hz
 600           3 Hz 

Although 8 Ohm phones might be "marginal" for someone wanting 
maximum low end response, 98 Hz is probably suitable for most 
communications uses.  One of the reasons I have not felt the 
need to change the coupling capacitors in my K3 (although I 
will be installing 100 uF caps when I make some other changes) 
is that the -3dB point is 80 Hz with the 200 Ohm transducers 
in my Heil ProSet.  There is a noticeable difference compared 
to the 980 Hz -3 dB point when I use an old set of Yaesu 
headphones with their 8 Ohm transducers. 

The only reason I plan to change the capacitors is to avoid 
problems when I grab a cheap set of headphones or the rig is 
used by a visitor with their own headphones. 

73, 

   ... Joe, W4TV 
  



> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Paul 
> Christensen
> Sent: Monday, July 06, 2009 11:23 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
>  
> 
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