[PVRCNC] SCAF-1 Audio Filter

[N4CW@aol.com]

A Report on the SCAF-1
by Bert Michaud, N4CW 11/20/03

I. The Kit

First of all, if you want a professional review, refer to "CQ" magazine,
November, 2003, page 34-35. Here's a not-so-professional version.
 
Online, I ordered my SCAF-1 kit along with a Logikey K-3 keyer (assembled
and tested) on a Wednesday from Idiom Press' web site (www.idiompress.com)
and received both the following Friday! Can't beat that with a stick.
The kit was neatly packaged, and the instructions were concise and 
error-free.
They do recommend a magifier, and I'll back that up; the parts are tiny 
enough
that you'll need some visual aid. I used a headband type with the additional 
swing-over magnifying lens. For most things, just the headband with its 
lenses
are enough, but when you want to check to see if you're "bridging" a solder
point with an adjacent one, the additional lens helps.
Although they don't mention it, let me recommend some "solder wick" that 
you can get at Radio Shack -- just in case you have to undo a solder connec-
tion, like I did!
Give yourself two to three eventings (2-3 hours each time) to complete
the job. You will be tired after each session, and you don't want to work on
tiny stuff when you're tired.
After you've finished, you'll have to wire up the power plug. They provide
one...it's a "smidgin" larger than what I use on most other powered devices.
The center/plus pin is fatter, too! Also, you're cautioned not to use a "wall 
wart",
so plan on tapping your 13.8VDC supply. There are two output jacks, one
requires a mini stereo plug, and the other jack is the RCA type. Since each
ham's station is different, you'll have to mate these cables appropriately.

II. Performance and Evaluation

Initially, I tried the SCAF-1 with my IC-706 MkIIG on CW. The first thing
I noticed when I turned the filter on was that it really quieted the audio 
down;
ambient noise on the speaker and in the headphones is definitely reduced.
Then, I turned the "tuning" know counterclockwise, lowering the cut-off
frequency (the SCAF-1 is a low-pass filter with the upper cutoff variable),
and tuned in a CW signal. I played around with the cutoff frequency until
I was happy with the bandpass. Then I turned the SCAF-1 off; what a
difference! Suddenly there were squawks and beeps from nearby stations
that the filter had cut off. As you know, the 706 is notorious for having
a gradual taper to its bandpass filters, often conferring the name "blowby"
to the result. The SCAF-1 effectively eliminates that phenomenon, making
copy much less tiring. In summary: less ambient noise and reduced adja-
cent signal "blowby".
How's it perform on SSB? Well, I used the SCAF-1 with my MP during
SS Phone...it helped a lot on crowded bands. Plus, it improved the 
ambient noise level on the MP also!
So I compared the SCAF-1 with my Timewave DSP-59+. The DSP
has several more features and is more flexible than the SCAF-1. More-
over, using "Spectrascope", a software program that allows you to
check out the bandpass characteristics of your rig's audio thru the
sound card on your computer, I compared the two filters. The DSP-
59+ in bandpass mode is definitely superior to the SCAF-1 in terms
of bandpass skirts. In the Timewave, the skirts are very pronounced,
both on the low and high ends, whereas with the SCAF-1, the bandpass
is more gradual. With Spectrascope, you can see how tuning the SCAF-1
reduces the passband from the high end towards the low end of the
audio spectrum, but not in a real steep cut-off fashion. I can't quantify
my measurements with any precision; but from an observer's perspec-
tive, the differences are apparent.
How does that translate to performance? The SCAF-1 is easier on
the ears, and, especially on phone, the results are more intelligible
than with a narrow bandpass filter from the DSP box.
One more plus for the SCAF-1 is its size...2-1/2" wide, 1-1/2" high,
and 5" deep. The Timewave is 7-1/2" wide, 2" high, and about
8" deep. The SCAF-1 is smaller and cheaper than the Timewave
(which you can't buy new anymore).
All-in-all, the SCAF-1 is a good accessory to the 706 series of
transceivers, as well as to more serious rigs that you thought didn't
need improvement. I recommend the SCAF-1.




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