Saturday, March 29, 2014


A few years ago I started experimenting with microphone audio processing as a way to way to improve my HF signal.  I live in a fairly dense suburb and haven't been able to put up a tower, so running >100 watts isn't really an option.  Speech processing such as that described in this article seemed like a good idea.

I started out playing around with a PC app called Voice Shaper by Alex VE3NEA.  (YouTube demo of Voice Shaper.)  This worked fairly well, had all the features I needed such as RF envelope clipping/limiting, compression, and equalization.  The only downside to Alex's app is that there's a digital processing delay, and I like to monitor my transmitted audio with headphones so I can detect if there's any distortion or RFI on the signal.  Voice Shaper's delay was enough to send me looking for other solutions.

Reading around I found that a few hams are using equipment like Mackie tabletop mixers, parametric equalizers, etc.  The one that caught my attention was the dbx 286a, a rack-mount microphone processor for studio work.  I found one used for a decent price and figured I'd be on the air no problems.  As it turns out, I was starting a journey of discovery which would teach me a lot about RFI, filtering, ferrites, and ultimately signal impedance.

I made up an adapter cable from the mic processor to my Heil Pro-Set Plus and another to the mic input on my Kenwood TS-2000 and started testing on the air.  I quickly discovered that while some bands were OK, others were causing some RFI on the transmitted audio.  Depending on which antenna I used, the RFI ranged from barely noticeable to so bad that it blocked my audio completely.  A couple of cases were so bad that even keying the radio caused RFI feedback which continued until I unkeyed.

So I started experimenting with adjusting audio levels, adding ferrites, grounding and tuned grounding, etc.  I found that I could clear up some bands, but others got worse.  After a lot of work I was able to get most bands working, but it bothered me that I couldn't get all of them to work.  The question haunted me, and the dbx processor sat near my station unused, silently accusing me of being an idiot.  Why won't it work?  Other people had clearly made theirs work.  Was I just doomed to wander the earth for the rest of my life in search of a solution?

Every few months I would get an idea and try again.  Modern HF radios use BALANCED inputs, and so I went through and made sure that nothing was pulling the differential pair to ground.  I didn't find anything, but at least I had eliminated that as a possible cause.  Maybe I had faulty bought equipment?  I tested using some other audio gear and found that the problem shifted around; some bands got better, others got worse.  I gave up for about nine months after that.

My most recent attempt proved to be the solution.  I had been listening to Ham Nation while driving and Bob Heil was talking about ground loops.  Something he said made me realize my mistake.  He was talking about balanced microphone inputs (already knew that) but he also mentioned that most radio microphone jacks are expecting a LOW LEVEL audio signal, whereas the auxiliary in port usually wants to see line level signals .  Clearly the mic processor was emitting a line level signal.  What if I connected the mic processor to the AUX IN port on the radio?  This proved to be the solution!  Not a bit of RFI on any band, or on any antenna.

It's a little embarrassing to admit that it took me so long to figure this one out, but I'm really pleased to have finally resolved this one.  I'm looking forward to finally getting some use out of the mic processor I bought!