Audio filters for CW

Wolfwave. Peak in graph indicates that signal is tuned to sidetone frequency. Box with jacks, upper right, is the transmit bypass circuit.

I admit I've had a bit of an obsession with audio filters. This is partly because I’ve worked with a variety of vintage rigs and with QRP rigs, both of which often have fairly limited filter options. But the other reason is my aging hearing: now in my mid-70s, I seem to have become less tolerant of background noise levels. As with my choice of vehicles, I’m in search of comfort.

I’ve tried many analog and digital filters, some external, and some I have built into rigs. A long-time favorite is the Hi-Per-Mite designed by David Cripe, NM0S, and sold by the Four State QRP Group for around $25 plus postage.[i] While it doesn’t necessarily remove background noise, it often makes the signal “pop” out of the noise. I’ve used them with many of my rigs, including my TS-590SG. I even have one built into my KX1 (photo below). Although the center frequency is fixed and the “default” is 700 Hz, a provided chart gives the substitute components needed to change that. It is compact, an easy build and the price is right.

More recently I discovered the CALF audio filter and limiter by KC9ON.[ii] This can be purchased as a kit or assembled ($55-$80 plus postage) and comes with a nice case. It too has a center frequency of 700 Hz, though a chart is supplied to allow that to be altered by changing some resistance and capacitance values. It provides six stages of bandpass filtering and even the narrowest allow comfortable listening and reduce the noise level significantly. I used it for some time with my TS-590SG and later with a low power SDR rig to add filtering options and make the audio less harsh. It is a very nice unit overall and again, the price is very reasonable.

A recently-released product by SOTAbeams[iii] but also sold by DXEngineering[iv] is the most sophisticated. The Wolfwave[v] Advanced Audio Processor is not only effective but very cool to watch and use. Center frequency and bandpass width can be continuously selected with a push and turn of the encoder and are graphically shown on the display. Even the narrowest width (50Hz) is usable without much ringing. Also included is adjustable DSP noise reduction and several other features. The display provides a graph of the audio within the selected bandpass range, and the peak representing the signal makes an excellent zero beat indicator. The menu includes a variety of options and is easily mastered. Firmware updates are relatively simple and the on-line instructions are good. The latest firmware even includes a decoder that works well (just push the Mode button to switch it on or off). The unit is built into a sturdy, compact plastic case and it also works on SSB and digital modes. It is small enough to be used portable or to fit easily into the workflow of your shack. (Photo at top of page.)

The only downsides are the price ($275) at DXEngineering) and the inevitable lag caused by digital processing. The lag time is small – “typically less than 60ms,” the specs say, and the effect in the sidetone is pretty minimal. I found it a bit bothersome at speeds of around 20 wpm or so, however, and although I probably could have learned to ignore it, I decided to eliminate the filter during transmit. I built a 12v DC relay, triggered during transmit from a control circuit in my TS-590sg, into a small box to bypass the filter.

It is not quite the Holy Grail of cw audio filters for me, but it is definitely close. The Wolfwave made listening and copying cw much more enjoyable to my aging ears.

[i] http://www.4sqrp.com/hipermite.php [ii] https://kc9on.com/product/calf-cw-audio-limiter-filter/ [iii] https://www.sotabeams.co.uk/audio-filters-and-more/ [iv] https://www.dxengineering.com/search/product-line/sotabeams-wolf-100-wolfwave-advanced-audio-processor?autoview=SKU&keyword=Wolfwave&sortby=BestKeywordMatch&sortorder=Ascending [v] http://www.wolfwave.co.uk