A Low-Pass RC Audio Filter
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After several long portable sessions with noisy bands and a sidetone rich with harmonics, I decided to try a low pass audio filter to reduce ear fatigue. Wanting to do something simple and quick, I turned to a resistor/capacitor design, they are so simple I had to give one a try. These little filters do have a place in ham radio and can live nicely between a rig and earphones. The one above took only a few minutes to stick it together Manhattan style. Solder and glue are great stuff for rapid prototyping. Then it went into the portable box for the next time out. I could have used NMØS Dave Cripe's FB Hi-Per-Mite but I have another use for it sitting on the back burner.

The calculations are simple, but there are online calculators that make it easy to play "what if" games and fine tune the component values vs the corner (cut off) frequency, several links are posted below. It's also easy to whip up a spreadsheet if you like doing that. Loading effects and multiple order designs can be incorporated into the spreadsheet as well.

I chose to use 1000 cycles as the corner frequency in mine. It's just high enough to pass signals at about 600-700 HZ without attenuation, but low enough to provide adequate attenuation of the higher audio frequencies, i.e. noise/hiss/hash. Another thought was to limit attenuation by the series resistors. I dont think this is a real concern, but I intuitively wanted small resistor values in series with the earbuds, so I chose 4.7 ohms (because I had some). When used with a 32 uF capacitor this yields a cutoff frequency of 1058 HZ. I built a 2nd order design in case I needed the effectiveness of two stages. Notice I paralleled (izzat a word?) 10uf and 22uf to get 32uf. They were in the junk box, use whatcha got. The calculated cutoff frequency for both stages calculates the same as for the single stage, but the loading effect of the second stage moves the actual cutoff down in frequency. In this case down to about 700 cycles, so the loading effect should be taken into consideration for a serious design. The first reference discusses this effect, but I made no effort to account for it, just cascading two identical stages to see how that would work.

The proof is in the performance, and this little circuit does fairly well. It "mellowed up" the band noise and sidetone quite a bit. Shorting the sections with a jumper revealed the effectiveness of each stage and the overall filter. Important: Since the BFO on the rig I use was adjusted to place received signals at ~600Hz when they are centered in the passband, on air signals werent attenuated and no distinction in their volume could be heard when jumpering out the whole filter. I've made no attempt to measure this little filter's passband attenuation, feeling that the ear is sensitive enough for this application.

If you want to increase your operating comfort, give one of these very simple little accessories a try. The improvement in tonal quality may be worth the small effort to you. You might consider it an intermediate step on the road to a Hi-Per-Mite.

Links to calculators and tutorials:
VE3WWG calculator  Very nice page with an easy to use calculator and Bode plots.
Electronics-Tutorials filter tutorial.  Excellent tutorial, includes 2nd order calculations, and links to their other tutorials. All good Stuff.
Okawa Electric Design RC Low-pass Filter Design Tool  Good calculator with links to their other tutorials.
Sengpiel audio  Good calculator on an audio site for a slightly differecnt viewpoint.

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