Recently I was researching computer rotor interfaces and happened across the website of Idiom Press, who makes a product called Rotor Card. While browsing their site I found that they sell an audio filtering kit called the SCAF-1. The filter was designed for older radios without DSP (digital signal processing) or additional filtering to make those heavy QRM filled QSO’s more tolerable. My Icom-746 is one of the first generation radios with DSP and I also have 2 additional filters installed (350hz and 1900hz), but I wanted a little more flexibility in my filtering options.

On my particular rig when I enable noise reduction (NR) I tend to get a fair amount of ringing in the filter, an artifact of the older DSP technology. In addition to that Icom’s PBT (PassBand Tuning) only goes so far. My 1900hz filter in the rig could clean up the IF stage of the receiver, but I needed something more to make QRM filled receive audio more tolerable. I also started thinking about the possibilities with RTTY and PSK31.

After listening to Idiom Press’ audio samples on SSB phone and CW, I was sold. I decided to go with the kit form to save a few bucks and experience the challenge of building a kit. Since my last kit (D-Star hotspot) was a success, I was up for the challenge. $90 and 3 days later the kit arrived.

The included instructions were very clear and very consise and the kit assembled exactly like they had stated. One issue I encountered was wrong resistor (miscount). Where I required a 3rd 10k ohm, I had a 100 ohm. No biggie, nothing that I can’t get at my local Radio Shack. Thankfully mine still sells common electronic components. The other issue was trying to figure out the pin ID on the DPDT switch. Unfortunately I soldered mine backwards and the switch is keyed to install in only one direction. All this really means is the UP position is OFF and DOWN is ON. Again, no big deal.

I had most of the kit assembled in the course of 3 hours while I was carrying participating in my usual 8PM net on 1900khz. Today I was able to pickup the remaining resistors and also some audio cables to interface it into my rig setup. I decided to place it between the AF output of the radio (accessory audio output) and the rig audio input of my Rigblaster. This way I could utilize it for digital modes on the computer, which is a direct feed to my monitor speakers. The filter has 2 audio outputs, one for headphones and external speaker audio. The kit gives you the luxury of choosing which size jack 1/4 or 1/8 inch you wish for either function as well as if you want stereo or mono. Since it’s ham radio and the most audio bandwidth I’ll ever hear (from a properly tuned transmitter) is 6khz (on AM), I decided to go mono for both.

Upon powering up the filter for the first time, I was happy to hear the exact results I had heard on their website as well as nothing blowing up (reverse polarity electrolytic capacitors become missiles). The instructions call for you to adjust the internal potentiometer, but I had found the audio to be consistent with it set to the middle.

Check out the photos below and also take a look at the audio spectrum I had captured with the filter in line as well as out.

So far I’m happy with the results and look forward to using it during contests and busy digital waterfalls.
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