Audio Filter

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. 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 served me well for many years.

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 jack, comes with a power cord, but without a 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, but doesn't have a CW setting. It sells for about $180. It comes with a headphone jack, a fused power cord.

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.

West Mountain Radio sells the ClearSpeech^®. If you click on the photo, it'll open to actual size. As you can see, it's larger than its competitors mainly because of its larger speaker. As a result, it does have more bass response than the others. Whether this is an attribute remains with the listener. I've never owned one, but those who have say they're top notch.

WMR also sells a unit without a built in speaker, which may be handier for some folks. Both units sell for $220, and are available from any WMR distributor.

All of 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. The ClearSpeech and MFJ units are rated at 2 watts which is about average for most mobile transceivers, unfortunately.

One caveat. Using the accessory jack to power ancillary devices isn't always a good idea. The limiting factor isn't necessarily the current rating (typically one amp). Rather, it's the voltage drop through the radio; as much as 2 volts in some cases. This can cause some ancillary devices (powered speakers especially) to operate erratically. It's always best to use a RigRunner^® or similar device as a power connection point.

I think this is a personal choice. I used to use an SGC ADSP² speaker, and if I were still using an Icom IC-706MkIIg, I'd probably still be using it. Although the audio DSP built in to the 706 is fairly good, the colorization it adds to the receive audio is more severe than the ADSP² 's. There is the added level of operating inconvenience which for some is mediated by the extra audio power. If your radio doesn't have any DSP, all of these devices are a worth-while investment. It should be mentioned that DSP speakers are designed for SSB operation. They're of little use for AM, and worthless for FM.

There's another kind of filter which can be of benefit, especially for removing the high frequency hash. The SCAF-1 shown at left if from Idiom Press. It comes in both kit and preassembled versions. The kit is about $100 including shipping, and takes about 4 to 8 hours to assemble depending on your expertise. It has 96 dB per octave rolloff, and the cutoff is adjustable from 3.5 kHz, down to 450 Hz. Its audio power output is only rated at 500 milliwatts, but has proved to be adequate in my personal installation.

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.

Plain Old Speakers

There is nothing wrong with plain old speakers, except some of them are really plain looking (pun intended). That's fine too, but there are a few things to consider when selecting one.

First, price means nothing. Some really cheap speakers will out perform ones costing 10 times as much. One of the reasons for this is, expensive speakers tend to have a much wider bandwidth (high frequencies especially) than cheaper ones, and they're typically larger in size. While they might sound a little fuller at home, when you're mobile you need all of the definition you can get. Conversely, you don't want one too small or you won't have enough bass response, and the highs will accented which reduces readability.

I wouldn't recommend a brand on a bet, because each person's hearing is slightly different. However, here's a few things to consider. About as small as you want one is 3 inches by 5 inches. Those little tiny 2 inch jobs don't sound very good unless you mount them very close to your ear. Some of the 2 inch by 3 inch ones are fairly good, but might not have enough bass to suit some folks. If that's the case, try mounting it under the seat pointing upward. This tends to increase the bass response while muffling the highs.

Speaking of mounting. I see a lot of folks mount their speakers on headrests. I don't think that's a safe place. Neither is mounting one near the headliner especially if your vehicle has side air bags.

Using Vehicle Stereo Systems

In most cases, vehicle stereo systems are designed specifically for each model in an effort to maximize linear frequency response. The higher end systems even have built in equalizers which allow owners to further tailor the frequency response to suit their desires. All and good, but most EQs do not have the range to effectively cut off frequencies above 2.7 kHz. Even if your radio had a decent IF or AF DSP, listening to the high frequency band noise gets real old in a big hurry!

It doesn't make much difference if your stereo has all manner of input capabilities, using one in place of a decent mobile speaker, is a stopgap solution you'll soon change.