[Elecraft] K3 Interior Speaker Low End Roll Off Below 400 Htz

[Paul Christensen]

> "Yes, I know that it is a small speaker but I believe that this is more 
> than is caused by a small speaker. This restriction does not seem to be in 
> the exterior speaker jack, or the head phone jacks...I would like to 
> modify this to allow the response down to 100Htz or so. Any suggestions on 
> where the restriction is, or how to modify it?"

Dean,

The K3's low frequency response limitation is not related to the speaker 
amplifier circuitry.  It's capable of excellent low frequency response.  The 
roll-off you describe is caused by an acoustical short circuit created by 
the wave cancellation of the forward and back acoustical waves.  All 
speakers that allow for a short conduction path between the front and back 
of the speaker exhibit this behavior.

You can prove this effect by removing the K3's speaker, laying it out on a 
table, then feed it from a high-quality source of your choice.  Even from an 
audiophile-grade amplifier, you will hear substantial diminished low 
frequency content.  Next, place the speaker in a small sealed box, and note 
the results.  The low end response will dramatically improve.

As a further experiment, you can take a full sheet of plywood and cut a hole 
in it exactly the same size as the K3 speaker.  Place the speaker in the 
hole.  The increase in low frequency content will also dramatically increase 
and you've not used an enclosure.  Why?  You've just increased the 
acoustical circuit path of the low frequency content.  The longer the path 
between the forward and reverse acoustical wave, the larger the downward 
shift of the low frequency short circuit point.

Back to the K3.  To increase the low frequency content of the K3's internal 
speaker requires sealing and isolating  its back side.  The short circuit is 
presently being allowed through the many ventilation openings on the top 
cover, near the speaker.  Of course, we cannot close those vent openings so 
to compensate, a small plastic cap can be added around the back side of the 
speaker.  The more mass of the cap, the better.  I used the top of  plastic 
spray can and modified it to cover the entire back side of the speaker. 
Low end response through the speaker is excellent.

Quite honestly, the best approach is to simply use a small external speaker 
with an infinite baffle (sealed box).  I also use several Realistic Minimus 
7 speakers for station audio.  This is one of the few R/S products I will 
actually admit to owning.  Back in the late '70s, they did a marvelous job 
with the Minimus series of speakers.  They can be purchased on the used 
market for reasonable prices, although demand for them remains high today.

Paul, W9AC