Last Modified: April 5, 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.
Antonym: If it is him, tell him to read this article.
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 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 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, 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.
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. If you don't operate CW, there is a version just for SSB which makes adjustment a bit easier.
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, except some of them are really plain looking (pun intended). 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. 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 you use an amplified speaker, like the Radio Shack® model 21-541 (shown left), and you need more inputs, here's a way you can add up to three more. If you need more than this, perhaps you need to rethink your installation. So we know where we're starting from, shown at right is the schematic of the Radio Shack speaker (double click to enlarge).
The circuitry starting with C1, and continuing to Q1, and beyond, comprise the auto-on feature, and are left intact. All we're going to do, is add two (or three) new transformers (Radio Shack part number 273-1380), and new input jacks (Radio Shack part number 274-251). Incidentally, the 273-1380 transformer's impedance is 8 ohms to 1,000 ohms. The original is slightly higher (≈2,000 ohms), so there will be a slight difference is sensitivity between the inputs. Also note that the speaker jacks float above ground, and this is intentional as some amateur transceivers use double ended audio outputs, just like the KIA6227H audio chip does.
In the case of the Radio Shack speaker, the original audio input jack is mounted on the circuit board. The other inputs can be mounted in the side or rear of the speaker's plastic case. Be careful, however, as the magnet assembly fills the center portion, so the additional jacks should be placed towards the outside edges. The transformers have rather long leads which eases installation. They can be super glued to the inside of the case. At left is the schematic for the additional inputs.
The circuit trace for the ground side of the existing input transformer is cut (see schematic). The new transformers bridge this cut. This places the secondary windings of the input transformers in series.
The original 100 ohm R3 should be replaced with a 220 ohm resistor. If you need a little more gain, use a 330 or a 470 ohm resistor. Don't make it any larger, or the auto-on circuit won't work properly. In practice, the audio gain of the attached transceivers is adjusted to give the desired audio level for each one.
The inputs of most medium to high Z, all-in-one chip amplifiers can be easily summed as shown here, as long as you don't need more than 3 or 4 inputs. If you only need two inputs, you can use a dual voice coil speaker. Small (5.25 inch) mobile subwoofers work well in this application, as they roll off the highs. Models (speaker only, no enclosure) can be purchased for under $25 if you Google around.
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. Schematics are available on-line. Midland®, Cobra®, and Shakespeare® also have amplified speakers available.
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.