[Heathkit] Re: Audio filter on HR-10B

[Tom NØJMY - AAR7FV]

Truth be told, there really isn't *any* audio filter that won't create a 
marked improvement in the HR-10B, HW-16, and other receivers of that 
genre. At 2005 Dayton, I picked up a factory-built version of the 
QST-project, "The Crud-O-Ject" which was basically a passive filter made 
from a couple of 88 mH telephone-company toroids with attendant 
capacitors to roll-off the audio frequencies above about 700 Hz.  While 
it didn't make the HW-16 into a contest-rig, even *that* made everyday 
rag-chewing type QSO's much easier.

I like the MFJ because of it's minimalist approach, which is in keeping 
with the spirit of the HW-16, plus it's small enough to install inside 
the rig and operate by putting a push-pull switch-equipped pot in place 
of one of the factory pots (I replace the RF gain pot - no new holes!).

The Heathkit HD-1418 is excellent as a separate unit.  I use one with my 
Atlas 350XL.  IIRC, it's basically a kit version of the Datong FL-2.  
The FL-3 is the next step up and has the ability to automatically change 
it's center frequency to track a wandering CW signal and it also has at 
least one automatic "search and destroy" notch (which was lacking in the 
FL-2) that take out hetrodynes so fast it makes working the 40-meter 
novice band at night a breeze.

The older Datong FL-1 was the ancestor of the -2 and -3 and it also had 
an automatic notch though it took a little longer to find and destroy 
them.  The problem I had with the FL-1 was that the audio output amp 
chip was easily destroyed by sudden high amplitude audio spikes - common 
when the receiver has no AGC, or  it's turned off for CW operation.  I 
had two and blew them both up, whereupon I had to cobble in a 386 chip 
to replace the the factory output amp.

The nice thing about filters that have a variable center frequency like 
the Datongs and the Heathkits is you can set the bandwidth very narrow 
and then "tune" back and forth between very closely separated signals in 
the AF  pass-band.  The drawback *might* be that they have a tendency to 
"ring" at or near maximum selectivity.  I personally never found one 
that was objectionbly so, but YMMV.  

I built an inexpensive  Ramsey Electronics 'AF-1' filter kit and it does 
a fine job of tuning at high-selectivity, as well.  Unfortunately, I 
don't think they make it anymore.  I wonder if anybody else picked it 
up?  You could buy it without an enclosure and slip it into a short 6"x 
6" slope-top project box with the controls and headphone jack on the 
front and a speaker out the top -- paint it two-tone green and have a 
whale of a factory-looking speaker-accessory box for a HW-8 or a -16, 
etc. (Non-conformists like myself would also add a switch to select 
manual T/R switching for the HW-16, using Steve WD8DAS's mod).
 
I don't have enough experience with glowbug-type filters, such as the 
Autek's but the seem to have a devoted following as well.

Hombrewing of audio filters is very rewarding, too, if you enjoy simple, 
inexpensive, weekender projects.  Just Google "C.W. Audio Filter" or 
similar and you'll get lots of ideas.  And a Distinguished Hamming Cross 
is awaiting those who continue to develop  "binaural" filters with which 
the various cw signals "move" back and forth across your brain as you 
swish your receiver tuning around and their audio frequencies change.

Good Luck to all and if anyone invents something, please let us know!  
We will praise your inventive genius to high Heaven as we unashamedly 
glom onto the circuit for our own use hi HI!

Vy 73,
Tom n0jmy
Heathkit HW-16 panel overlays: $14.95 at www.hayseedhamfest.com
 

Jim Brannigan wrote:
> If you can find one, the Super SCAF by Aftronics is a great audio filter.
>
> Thumbwheel switches allow you to adjust the HP and LP frequencies. 
> Bandwidth is from AM to very narrow CW.
> A 2W audio amplifier is built in.
>
> Jim
>
>
>> I've followed with interest the discussion of audio filters. Of the 
>> filters mentioned so far, I
>> wonder how they compare to the Heath HD-1418 Audio Filter. Anyone 
>> with experiences?
>> Am I talking apples & oranges?
>>
>> 73 Ken kb7rgg
>>
>> ps - I was just looking at my old 1964 copy of the ARRL Hadnbook 
>> where they describe the
>> Selectojet. Anyone ever build one?
>>
>
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