[Elecraft] K3 Audio Low Pass Filter

[Paul Christensen]

> You neglect the effect of the 8.2 Ohm resistor in series with
> the headphone outputs.

Joe, you're right.  I did not take into consideration the 8-ohm build-out 
resistance that Lyle just discussed.  Those build-out resistors do not 
appear on the headphone amp schematics (page 28/52) so until today, I doubt 
anyone else knew they existed <g>.  So, they must appear off-page along with 
the speaker coupling cap that also does not show up on the same schematic.

Older headphones with low Z drivers would still benefit from a value above 
100 uF.  But with most modern headphones in the 32-ohm area, and I'll 
repeat...the majority of users would not benefit from higher C values above 
100 uF at C9/C13.  My choice of going to 330 uF was primarily driven by the 
fact that I didn't want to be concerned with the operating Z of a new set of 
headphones if tried, and I had an older DSP Board wiith 10 uF caps.  Since 
re-work was required anyway, I settled on the 330 uF value and would still 
choose that value if given the chance today.

Although not as critical for communications audio, the use of increasing the 
build-out resistance does in fact lower the damping factor of the amplifier. 
Not as important with newer headphones, but it can be a factor with older 
low-Z headphones with traditional moving coil drivers where more mass is 
moved.

Paul, W9AC






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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