[Elecraft] My K3's hissy and noisy audio

[Paul Christensen]

Steve,

Someone else responded to me in private with the same question.  Here's my 
answer:

"The K3 speaker output has something like 470 uF of C in series with the 
speaker output amp.  That's reasonably adequate.  The problem with the K3 
speaker's bass frequency response is not the electrical circuit.  Rather, 
the K3's speaker has low-end response limitations due to the fact that the 
speaker is in not encased by an adequate baffle.  The back wave of any 
speaker driven by a coil is 180-degrees out of phase with the forward wave. 
Although the K3's speaker is mounted to a metal surface, too many openings 
near it allow for the short circuit of the acoustical front and back waves 
at low frequencies.

The degree to which acoustical wave cancellation occurs is a function of 
distance.  At low frequencies, quite a bit of isolation distance is 
required.  Typically, commercial speakers are encased (although some are 
ported to a resonance) to produce a virtual infinite distance through the 
use of the so-called infinite baffle.

We've all done this experiment before at some point in our electronics 
education: Simply place an open speaker driver on a desk just sitting there 
and listen to low frequency tones.  Then, take that same speaker and place 
it in an enclosure, or behind a large surface.  The speaker placed by itself 
on the desktop sounds vastly inferior in bass
response when compared to the same speaker enclosed in a box or placed 
behind a surface like wall or ceiling.

For the K3, the fix is relatively easy.  Simply enclose the back of the 
speaker driver with a solid substance like plastic.  I used the top of a 
spray paint can and modified it slightly to fit around the K3's support 
truss."

Incidentally, there's another interesting acoustical mechanism that assists 
our brains with bass content.  If it were not for this effect, our 
perception of low-frequency content would be worse than it is when we listen 
to small speakers whose low-end response is restricted.  In effect, our 
brains create a missing fundamental when it detects the presence of multiple 
harmonics of a single fundamental that should be there in the first place. 
Among auditory professionals, this effect is referred to as "Phenomenon of 
the Missing Fundamental."

http://www.phys.uconn.edu/~gibson/Notes/Section7_4/Sec7_4.htm

Paul, W9AC

----- Original Message ----- 
From: "Steve Ellington" <n4lq at carolina.rr.com>
To: "Paul Christensen" <w9ac at arrl.net>; <elecraft at mailman.qth.net>
Sent: Monday, April 06, 2009 9:31 AM
Subject: Re: [Elecraft] My K3's hissy and noisy audio


> Paul
> I can see how this mod would improve headphone audio but what about low 
> frequency audio for the speaker(s)?
> Steve Ellington
> N4LQ at carolina.rr.com
> ----- Original Message ----- 
> From: "Paul Christensen" <w9ac at arrl.net>
> To: <elecraft at mailman.qth.net>
> Sent: Monday, April 06, 2009 6:54 AM
> Subject: Re: [Elecraft] My K3's hissy and noisy audio
>
>
>>> Perhaps the band noise is more obvious in the K3 because the low 
>>> frequency
>>> audio is restricted causing the noise to be distributed over a more 
>>> narrow
>>> range. This lack of bass tends to induce for ear fatigue.
>>> Steve Ellington
>>
>> I agree with Steve on this one.  Once more low-end was rolled into my 
>> audio
>> hardware changes, I noticed a significant reduction in my own listener
>> fatigue.
>>
>> Others have noted the K3's audio frequency response variance across
>> different headphone models, most notably the Heil product.  The K3's
>> headphone circuit uses 10 uF of series capacitance from the headphone amp 
>> to
>> each of the headphone coils.
>>
>> For the moment, assume that your headphones are 32-ohms (ref 100Hz). 
>> What
>> is the best possible low-end response from the K3?  The answer is 500 Hz
>> @ -3dB.  That part of the circuit needs some help.  With a headphone Z 
>> even
>> lower toward 8-ohms, the attenuation of bass response is worse.
>>
>> Now upgrade the series caps to 330 uF and re-run the calculation.  With
>> 32-ohm phones (ref 100 Hz), the frequency response improves to 15 Hz. 
>> Now
>> that's t real Hi-Fi on par with what Icom delivers and the difference is
>> immediately noticeable to anyone -- not just the folks with "golden 
>> ears."
>>
>> Finally, compare the result of the modified K3 with the Icom '7700 or 
>> '7800.
>>
>> Paul, W9AC
>>
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