Last Modified: October 30, 2013
Contents: Basics; Mounting Considerations; Why Use An Active One?; Why Use A Passive One; Plain Old Speakers; Powered Speakers; Using Vehicle Stereo Systems;
If you cannot understand the other station, it might be you, not him.
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. The only issue was its insertion loss (≈3 dB).
In addition to passive audio filters, there are a number of active filters available for both base and mobile installations. The Hear It® unit shown at right is marketed by GAP It sports a wide range of DSP settings, and sells for about $180. It comes with a headphone jack, a fused power cord.
MFJ makes a similar unit, the MFJ-784B. It is 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 now sells a new model of their popular ClearSpeech®. It is 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.
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.
If you need a bit more audio power, BHI in England makes the DSPKR with 10 watts (RMS) of power which should go a long ways in noisy environments. It is sold by W4RT Electronics here in the US.
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 for the SGC and GAP models, 10 watts RMS for the ClearSpeech® (18 watts with an external speaker), or about four times that of your average transceiver.
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 is 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 is always best to use a RigRunner® or similar device as a power connection point.
All too often, operators mount their speakers too close to their heads. One popular mounting location utilizes the headrest support posts. This isn't a very sound idea (pun intended!), and here's the reasons why. First, modern vehicles have SRS devices (airbags) mounted in every nook and cranny. Besides the dashboard area, they're often mounted in the sides of the seats, the headliner, and even inside the headrests! It should be obvious why these areas are poor mounting locations for speakers, no matter their size.
Secondly, sound pressure levels can be deceiving especially in a noisy environment like the interior of some vehicles (trucks mainly). And, contrary to popular belief, it isn't just the low frequencies which can be damaging to one's hearing. The high frequency hash we all put up with when listening to weak SSB signals is just as annoying, and deaf-producing! Here's a little experiment you can carry out on your own. No matter where your speaker is mounted, after about 10 minutes of listening, just turn off the radio. If you suddenly feel like you're in an insulated sound booth on the set of the $64,000 question quiz show, chances are your speaker volume setting is too high!
Here (and hear) is a suggestion to minimize the hash. Place the speaker under the front driver's seat, pointing upward. This tends to increase the bass response while muffling the high-frequency hash we all have to contend with. Up under the dashboard pointing into the foot well works almost as good. Either way, you'll end up using less audio gain, which is a good thing in more ways than one!
Why Use An Active One?
I think this is a personal choice. I once to used an SGC ADSP2 speaker (out of production), and if I were still using an Icom IC-706MkIIg, I'd probably still be relying on 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, although some do have narrow settings for use with CW. They're of little use for AM, and worthless for FM.
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. Note the reduction in high frequencies, which rapidly cause listening fatigue.
It should be noted that passive filters have insertion loss (3 to 5 dB depending on the design), and if your DSP-equipped mobile transceiver is already lacking in the audio out department, you might consider the SCAF-1 mentioned above.
Plain Old Speakers
There is nothing wrong with plain old speakers. However, 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 be accented which reduces readability.
I wouldn't recommend a brand on a bet, because each person's hearing is slightly different. Just don't buy one smaller than 3 inches by 5 inches. Those tiny 2 inch jobs don't sound very good unless you mount them very close to your ear.
The audio amplifiers in modern transceivers seldom top 3 watts, which is barely adequate especially in a noisy mobile environment. Fortunately, there are several companies making amplified speakers these days, including the MFJ-382 shown at right. It sells for under $40. Midland®, Cobra®, and Shakespeare® also offer amplified speakers for about the same price.
Occasionally, you'll find old Motorola HSN1000B amplified speakers at hamfests for as little as $10. New ones cost about $75. It's amazing how good they are as long as the speaker cone hasn't deteriorated—caveat emptor.
Most of the current commercial offerings use amplifier inputs which can be summed, allowing multiple radios to use one speaker. If you're interested in learning how summing is accomplished, here's a web site with more information. Be advised, however, the treatise is not a how-to.
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. In the end, 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.
Albeit hard to find, there are a few hardware-based, (analog) audio to MP3 converters available, and at least one which operates on 12 volts. They allow older model (analog) tape and CD players to feed audio to vehicle sound systems which have an MP3 input port. You're not going to like them either, as high frequency SSB hash causes distortion in their outputs. They're also prone to RFI.