Topband: CW Speaker Mystery - My Experience

[David Raymond]

Greetings Topbanders. . .

There has been some discussion in the Topband community over the past 
year about "CW speakers."  The CW speaker has had a small but 
enthusiastic following relating first hand experiences with the device 
remarkably and easily pulling weak signals out of the noise that were at 
or below the receiver's real time effective noise floor.  I wish to 
relate my own experience with the CW speaker.

A little over a year ago as part of my effort to downsize I 
decommissioned my station built in 1996 on a very "quiet" QTH on 35 
acres in rural Iowa.  When I say the QTH was quiet I mean in terms of 
very low noise level on 160m.  Back then the noise floor on my FT1000MP 
was in the -130  dBm range listening on a quarter wave vertical 
antenna.   At high noon in the winter I could disconnect the vertical 
from the receiver and noise would only drop about three or four db, if 
that.  My four square TX antenna was remarkably quiet to the extent that 
beverage antennas rarely offered any improvement in copying weak 
signals.  It was a perfect place to chase 160m DX.  Over the years the 
noise floor increased with with the construction of  new houses within a 
mile or so along with the the flood of new electronic/digital devices. 
  By the time I decommissioned the station in 2024 the noise floor had 
increased a good 10 db or more.  However, even at the end my overall 
receive capability still remained fairly good.   The remarkable Hi-Z 
eight circle array (200' optimized for 160) was my salvation.

In June 2025 I moved to a suburban QTH with the usual HOA restrictions. 
. . uggh.  Fortunately, I had the option of remoting into another nearby 
well equipped rural station providing me a good place to continue 
operating and chasing DX (CW) on Topband. I began 160m operations with 
my usual routine of being on in the evenings for Europe and again on 
mornings for VK/ZL/Asia.  I continued my long standing tradition of 
having QSOs with Topband stalwart and beacon, Adrian/VK2WF.  While the 
remote station had (and has) noise problems in some directions, 
fortunately, the noise floor looking west toward VK was pretty good. . 
.about -129 dBm on the west Beverage.  During my dozens of QSOs with 
VK2WF it became quickly apparent that he was hearing me significantly 
better than I was hear him.  Some of that was attributable to the 
difference in transmit antennas.  Adrian had a shortened (but very 
efficient) single element vertical while I was running a full size four 
square of Rohn 45 sitting on miles of copper. However, accounting for 
the differences, Adrian nevertheless clearly heard better. . .many times 
a lot better.  He was hearing me consistently Q5 at times when I could 
hardly tell he was even there.  I was a bit frustrated as I had always 
"heard" very well at my old QTH.  At that point  Adrian began mentioning 
on KST chat something about a "CW speaker" which made possible his 
excellent receive capability.  Along about the same time N4IS/JC was 
mentioning something about a similar speaker, singing its praises.  
Clearly, something was afoot.  Adrian and JC both explained their unique 
but effective approaches to the CW speaker.  Being an acoustical and not 
an electronic device JC's approach used PVC tubing for the acoustically 
resonant element while Adrian's approach used small square metal 
tubing.  Both devices use a small speaker driving an acoustical chamber 
which is coupled with acoustically resonant tubing.  Obviously both 
construction approaches, while somewhat different, were very effective 
and worked well.  At that point I approached Adrian about building me a 
unit.  He did and a couple of months later the unit arrived in Iowa from 
Australia in good condition being very carefully and substantially 
packed. Upon inspection Adrian's CW speaker was impressive. . .compact 
and beautifully constructed of very high quality materials.  I quickly 
unpacked it and got it installed.  I used the "Line Out" audio on the 
Flex Maestro console for an audio source and a small power supply for 
the internal amplifier.  The unit has a switch for traditional "straight 
through" broad band audio and a position for "narrow" audio.

After hooking up the unit I was excited to see how it performed. 
However, I was quickly disappointed with the results.  I could see no 
improvement whatsoever in the ability to copy weak CW signals. I was 
stumped and a little disappointed.  Adrian had supplied a carefully 
measured audio frequency response graph which showed a significant 
improvement (about 20 db!) in S/N at my chosen frequency of 610 Hz.  I 
have tried various combinations of RX and TX antennas, different 
bandwidth settings on the Maestro but to no avail.  The CW speaker's 
performance simply wasn't matching up with neither Adrian's nor JC's 
observations of truly significant improvement in the ability to 
literally pull weak CW signals out of the noise.  My unit simply didn't 
do that.  I disconnected the unit and put it on my bench (with modest 
test equipment) and did a simple audio frequency scan with an audio 
oscillator to see the response.  The unit had a very large and sharp 
(about 10 or 15 Hz bandwidth) audio response in the "narrow" position.  
I reinstalled the unit for further testing and evaluation thinking I 
must be missing something.  After several weeks I finally approached 
Adrian to tell him I saw no improvement.  I was hesitant and cautious as 
I know Adrian and JC are both quite technically competent, credible, and 
would not be making unsubstantiated claims.  After much thought and 
further off line discussion with Adrian I believe we have resolved the 
mystery.  Adrian is running an analog receiver while I am running an SDR 
receiver (Flex).  It appears that the SDR receiver will not provide 
recoverable audio below the real time receiver noise floor threshold 
while the analog receiver can.  The real explanation is probably more 
complex than that but that's the essence of our observations. That said, 
I think it's spot on.

It's entirely possible I may have missed something here along the way.  
That said, from my perspective, don't expect a miracle from a "CW 
speaker" if you're using an SDR receiver. . .YMMV.

73 from Iowa. . . Dave, W0FLS
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 2025-11-04 16.51.11.jpg
Type: image/jpeg
Size: 2606744 bytes
Desc: not available
URL: <http://lists.contesting.com/pipermail/topband/attachments/20260207/42aedf4c/attachment-0001.jpg>