Passive audio filters have been around for many years. The Skytec CW-1, made during the 70s, is a good example. It utilized a ported 2 inch speaker mounted in a tuned cavity made from thin wall PVC pipe. At the mouth was a little sleeve you could adjust to set the center of the bandpass. If your receiver didn't have a CW filter, this little jewel worked fairly well. It sold for about $20 at the time.

Passive filters for SSB have also been produced over the years by just about every manufacturer of amateur equipment. When these units were first introduced, few radios had built in bandpass or IF shift features, to say nothing of the DSP systems we have nowadays.

Unlike the aforementioned CW filter, all of these designs used a combination of inductors and capacitors typically built into a station speaker. The schematic at left is that of the Icom SP-20 (click on it for a larger view). Note the switches to select the desired mode; lowpass, highpass, or both. Also note the passive elements are in series with the speaker for good reason; you have to be careful using directly shunted elements with single-ended audio amplifiers (Icom, Yaesu, et. al mobile radios). Doing so can cause them to fail.

In the early 80s, I built a similar passive filter into an old Heathkit mobile speaker. The parts (coils and capacitors) I used were scavenged from a couple of HiFi speaker crossover networks. Besides the bandpass filter, there is a separate lowpass tailored to remove most frequencies over 2,400 Hz. The latter is a direct shunt element, but it cannot be used unless the bandpass is also being used. Thus, it avoids the aforementioned failure mode. This speaker has served me well for many years, and I still use it!

In addition to passive audio filters, there are a number of active filters available for both base and mobile installations. The unit at the left is SGC's ADSP². One advantage it has over its competition is its three distinct filter positions; 100 Hz, 500 Hz, and 1,800 Hz. It has a headphone, comes with a power cord without an fuse. Street price is about $159. SGC also offers stand-alone versions (with and without an audio amplifier) for building into radios not so equipped.

GAP also markets one (right photo). It sports a wider range of DSP settings than does the SGC. It comes with a headphone jack, a fused power cord, but doesn't have a CW setting. It sells for about $180.

MFJ makes a similar unit, the MFJ-784B. It's a bit cumbersome for mobile operation, as it has a bunch of controls to contend with. With only 2 watts of audio power, and no built in speaker, its $300 street price is a bit steep.

These devices use an audio-interfaced DSP (Digital Signal Processor) to remove band noise, static peaks, and other bothersome background hash. In some cases, they'll out perform built in audio-based DSP units, however, none of them are as good as a properly designed IF DSP. The main reason is, most built in IF units are placed before the AGC loop, so there is less problems with nearby large signals.

There is no argument about their usefulness, as long as you don't mind the additional level of complexity. They do require DC power which must be switched on, and they must be positioned to allow operation of their controls. One very good advantage is their audio output: Five watts RMS (SGC and GAP models only), or about double that of your average transceiver.

Why Use A Passive One?

You might be asking why add a passive audio filter to a radio with a decent IF based DSP? If it were a radio primarily designed for base station use, that question might be difficult to answer. But for a mobile installation, no matter how good the DSP, some high frequency rolloff increases readability. Here a 3 second sound bite of the rolloff starting at the 2 second mark.

I first wrote (parts of) this article for publication on eham.net. It received the usual number of rebuffs particularly from the HiFi SSB, wideband AM, and other enhanced mode aficionados. Apparently, they've never operated HF mobile where readability is much more important than bandwidth and fidelity.

Designing & Building A Passive Filter

Designing an audio-based DSP speaker is beyond the capabilities of most amateurs. Even if you have the wherewithal, making a worthwhile improvement over and above the available commercial units isn't going to be easy. Therefore, we're sticking to designing a passive audio filter; a much less daunting task.

Audio lowpass, bandpass, and highpass filters aren't any different than their RF counterparts, except for the frequency in question. That, and they're less susceptible to stray capacitance. All of the formulas are available in the ARRL Handbook, and even on line.

You could duplicate the Icom unit rather than design one, but some of the necessary parts, especially a suitable circuit board are hard to find. While a circuit board isn't strictly needed, it does make assembly and installation a lot easier. So, for the casual home brewer, having most of the hard-to-find parts all in one envelope would be a bonus.

In a recent QST, a New Product announcement described a passive bandpass CW filter from the Xtal Set Society (don't be fooled by the name). What caught my eye was the fact the filter could be configured for SSB (as shown in the left photo). A quick look on their web site confirmed this, and that it was designed by Phil Anderson, WØXI, of Kantronics fame. It cost all of $19.95 plus shipping. It took two days to reach me via priority mail.

It comes with everything you see in the photo except for 10 feet of #24 hookup wire. About all you need otherwise, is a box to mount it in if there isn't room inside your mobile speaker, perhaps a jack and matching plug, and some basic hardware.

The instruction manual (to be honest) is a little lame. It's geared for the CW version, but an addendum is included covering the SSB version. It also states the filter is in bypass when the switch is in, instead of out. Perhaps this is nit-picking, but to a neophyte it's important fodder. It took me about 30 minutes to put the filter together including the winding of the three toroids. With the help of a little super glue (a hot glue gun would suffice), the toroids were secured in place.

The circuit board is double sided, but it is NOT plated through, so you'll need to apply solder to both sides of the board to assure a good connection. I found this out the hard way! It also helps to look at the pictorial which comes with the filter.

There are a couple of caveats to keep in mind. First, as I mentioned previously, the audio amplifiers in amateur radio equipment are almost always single-ended. This usually means that both leads to the speaker are above ground potential. In other words, you can't chassis ground either of them, and if you do you run the risk of destroying the audio IC. Therefore, care must be used when mounting the filter either inside your speaker or in its own box.

One drawback is a reduced audio output level. The amount of reduction depends on the design and cutoff frequencies of the filter in question. It may be anywhere from 2 dB to as much as 12 dB. In this case, it's about 4 dB, so you'll have to turn up the audio to compensate. However, too much audio (way too much actually) can cause the filter to ring, which will cause clearly heard distortion.

The filter works as advertised, although I'd like a little more high frequency cutoff (you might not). The above chart depicting the SSB response curve exemplifies my bench results. However, I should point out that there can be slight variations depending on the feed and terminating impedances if other than 8 ohms.

Odds & Ends

Building a passive bandpass filter is a good, hands-on learning tool. Speaking of which, the Hand-On Radio column, starting with the March 2007 issue of QST, starts a series of articles describing the basics of filter design.

One of the shortcomings almost all mobile transceivers share (and a lot of base ones too), is a lack of audio output power. Two to 2.5 watts RMS seems to be the standard. Yaesu's new FTM-10 dual band radio has an 8 watt amplifier which is a step in the right direction. After all, even a cheap CB radio has 5 watts of audio. As a result, some amateurs use adaptors or auxiliary audio inputs to their vehicle's stereo system. From experience, this is not a very good solution, no matter how you set the tone controls.