Audio Filtering & Speakers
Last Modified: September 1, 2010
Contents: Basics; Why Use An Active One?; Why Use A Passive One; Plain Old Speakers; Powered Speakers; Using Vehicle Stereo Systems; Odds & Ends;
If you can't 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.
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.
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.
It should be noted that passive filters do 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.
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.
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.
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.
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.