Review of Various Commercial Audio Filters With the widespread introduction and adoption of DSP filters by Hams, older outboard active filters are widely available for cheap prices at most Ham Fest Swapmeets. With many commercial rigs including built in DSP, many outboard DSP filters are also becoming available at reasonable prices. I started buying these "obsolete filters" a couple of years ago and have a collection of several. I thought you might be interested in my comments on them. DIgital Filters W9GR-2 I recently picked up a W9GR-2 DSP. This is the revised unit from the original September 92 QST article which contains all of the different filters in a single ROM. The original aritcle had the same filters, but only one filter per ROM. The - 2 has 10 filters in a single ROM. It is not the - 3 model that has been advertised in Ham magazines the last few years. The -3 has improved ergonomics and CW filter choices. I paid $30 for the filter. It was built from a kit and in an enclosure that was provided by the builder. The buyer said it worked and when I got it home it did. It has an automatic denoiser, an automatic notch filter, 200 Hz CW filters centered at 400, 600, 750, and 1000 Hz, a 30 Hz filter at 750 Hz, and an HF packet/RTTY filter. There are no narrow SSB filters. The ergonomics are not particualrly convenient. Switching modes requires two switches to be thrown three times and you must wait for the proper mode to appear in the LED display. It is not possible to switch modes on the fly, which makes comparing different modes difficult. Having said that, the denoiser is very nice, nearly transparent in that it does not significantly color the signal being received. It is much better than the Timewave 9 which I have in this respect. The denoiser is very nice for weak to moderate signal SSB work. Noisy signals appear to have broadcast quality. Unfortunately, the denoiser is not available in conjunction with the narrow CW filters. However, CW signals sound surprisingly good with the W9GR denoiser used and selectivity provided by a filter in the rig or another audio flter. The CW filters are good, and compare well with the Timewave. However, I would prefer a filter centered at 500 Hz or 550 Hz rather than the 400, 600, 750, or 1000 Hz offered, but the Timewave has this problwm as well. Many QRP transceivers have CW filters that are centered at 700 Hz, and many ops like this center frequency, so this may not be a major concern. There is more "noise" character to the W9GR CW filters than the Timewave has. The Timewave 100 Hz and 200 Hz filters sound almost like listening to a code practice oscillator. Not everybody likes to listen to CW this way, but for casual operating like ragchewing I like it. For contest work, I like to hear a bit of noise. The 30 Hz wide 750 Hz filter is amazing. Signals that are barely discernable at normal bandwidths are clearly readable at the narrow bandwidth. There is no ringing. However I do not like to listen to weak signals at this high pitch, so I will probably not use it much. There is still noise audible at this narrow bandwidth, and I am not sure that I like it better than the Timewave 100 Hz position. I have not tried to use the RTTY and Packet filters, but will do so later. I like the filter. I use it for SSB work, and in conjunction with the Timewave or Datong FL-3 to get the CW selectivity at the frequency I like and the nice denoiser of the W9GR filter. At high volume levels some clock feedthrough is evident in the speaker. During normal operation it is not too noticable. A bandpass filter following the W9GR will eliminate this effect. The filter has uses an 8 bit ADC. There is no AGC function in the DSP. There is an LED bargraph that displays the signal level. Manual adjustments to the input signal are required to avoid saturating the A/D. In operation I did not find these limitations a problem. The filter draws 500 mA at 12 V, so it is not quite ready for portable use. It is substantially less than the 1 A that the Radio Shack DSP and the Timewave 9 draw though!! At $30 this was a good buy. It is a particularly nice filter if you like to operate SSB or RTTY. I would reccommend it if you can get one at $50 or less. I do not see them often at Hamfests, but if you want to play with DSP this is a good inexpensive way to do it. Timewave 9 This is my favorite audio filter. I bought mine for $50 and have seen them up to $75 at Ham swaps. It is an excellent value at these prices. The filter features frequency bandwidths of 100, 200 and 500 Hz for CW at center frequencies of 600 Hz and 700 Hz or (selectable with internal jumpers) 400 Hz and 500 Hz. Adaptive noise reduction is available and can be switched on or off. On voice, bandwidths of 1.8, 2.4, and 3.1 kHz or (selectable with internal jumpers) 1.6, 2.1, and 2.4kHz are available. The adaptive noise reduction is available as well in voice mode, as is a automatic notch filter. The filter skirts are very sharp. Signals 50 Hz off the edge are inaudible. There is no ringing. The noise reduction works well, but is a little aggresive for my taste. It is not noticible on CW, but voice signals sound like someone is talking at the end of a hall when the noise reduciton is in use. None the less, it can make signals at the noise level quite audible. CW signals heard with the 100 Hz filter and noise reduction turned on sound like a keyed sine wave oscillator. It sounds more like a code practice oscillator or computer generated code than off the air code. There is little if any evidence of the digital nature of the processor in modes other than the noise reduction. There is no detectable noise or digital artifacts. It is "clean". The filter has built in AGC. The filtering seems most effective when the AGC in the rig is turned off. The internal amplifier is good quality. There is no provision for bypassing the filter when it is turned off. The filter is a joy to use on CW. It is very effective at dealing with pileups and the 100 Hz position is great in fox hunts. I do not like the fixed center frequencies as I like to copy CW at 550 Hz and I would like the amount of adaptive noise reduction to be variable, but these are not major problems in my opinion. The Timewave has displaced the DaTong as my favorite filter, but it is not the best choice for a filter for the digital modes as it does not have variable center frequencies. The filter requires a 12 VDC supply. At a 1 Ampere current requirement it is not a filter for the field. Radio Shack DSP Radio Shack introduced these about 5 years ago at $85. While they had a CW position to appeal to Hams, they were aimed at CBers. CBers didn't buy them and Radio Shack closed them out at steeper and steeper discounts, $45, $35, and the final few went out the door at $25. These are usually available at $20 to $30 at swap meets. I think they represent a good value at that price. I paid $20 for mine. The Radio Shack is a bare bones digital signal processor. It provides frequency selectivity and an automatic notch filter (in the SSB positions) only. There is no noise reduction function despite the switch label. In the NR position the SSB filters have a higher bass cut and lower treble cut which makes most voices sound funny, but does little to reduce the noise. The filter skirts are steep and effective in eliminating off channel interference. The CW filters are centered at 700 Hz. Since I prefer a lower note, I have problems using it at the narrowest setting. The filter imparts a slightly raspy undertone to both CW and SSB. It is more objectionable to me on CW. I must admit I used the filter for many months and thought it was good. Then I got the DaTong (see below) and found out how good sudio qualtiy could be in a steep filter. If you are into voice the automatic notch filter is very good. It makes 40 M at night an entirely different band. The amplifier and speaker sound very good when the filter is switched out. The speaker is better than that built into most rigs. At $20 this is probably a good buy just for the amplifier and speaker! It is not a Timewave or NIR, but it is a good introduction to DSP. It is the ideal companion for an older, inexpensive rig such as the HTX-100 that has minimal filtering built in. I use it with my HTX-100 and 2 meter multimode rig. I think that these filters have an undeservedly poor reputation in many quarters. I know an excellent op who loves his and leaves it connected in the mid CW position all the time. They are not state of the art DSP, but they are very useful in many applications. I must admit I think less of this filter after using the Timewave 9. The DSP requires an external 12 VDC supply to run which means it can be used portable. It draws an amp or so though so it is not very "field friendly". Analog FIlters DaTong FL-3 This has to be one of the high points of analog design in amateur radio. It has an automatic notch filter implemented in analog circuitry, seperately adjustable high and low pass filters, two different peak functions, a manual notch function, and the ability to put the notch in the middle of the peak function for RTTY use. Built in England, it originally sold for between $200 and $300. They appear at swapmeets for between $40 and $60. They are an excellent value at this price. I paid $35 for mine. The FL-2 is similar, but does not have the automatic notch function. Since I originally wrote this, the Timewave has displaced the DaTong as my favorite filter. However, there are still circumstances when I prefer the DaTong. In particular, data modes are much better on the DaTong as it has much more adjustability than does the Timewave. It is nearly infinitely adjustable and has excellent audio quality. The audio amplifier is only capable of driving headphones though, so if you like speaker audio an external amplifier and speaker are necessary. The Radio Shack DSP is good for this purpose. :^) The filters are 5 pole elliptic and bandwidths down to 100 Hz are easily obtained with little objectionable ringing. The notch is steep and effective. The automatic notch feature is as quick as that on the Radio Shack and Timewave DSP, but it only handles one heterodyne at a time. The filter requires external 12 V and is useful for portable use. It is hard to say anything bad about this filter. I love mine and am tempted to get another one. Autek QF-1 This is a simple barebones audio filter. Built in the 70s it sold for $75 or so. They were soon replaced by the QF-1A (see below); a much different filter than the adding of an "A" would indicate. They are not too common at Hamfests. I paid $8 for mine. The seller was asking $10. I consider it a good value at that price. It has peak, notch, and lowpass functions. The frequency and selectivity are separately adjustable. This is a simple filter to operate and use. It is capable of bandwidths as narrow as 75 Hz before ringing becomes a significant problem. The filters are 4 pole I believe. The ringing at narrow bandwidths, while not bad, is not as good as the DaTong. This filter has a headphone output, so an external amplifier is necessary to use it with a speaker. I leave this filter connected to my OHR Classic when using it in the shack. Although it could be used on voice I have not done so since the DaTong is much better for voice. This is a good, basic, easy to operate filter. The filter is AC powered and there is no provision for external DC power. The QF-1 off switch does not bypass the filter, so when it is turned off you don't get any audio at the output. Autek QF-1A This is a different filter than the QF-1. It sold for $100 from the late 70s through the early 90s, it is fairly common at swapmeets. They sell for $20 to $40. At $20 they are a good buy, but at the higher end of the range I would avoid them. I paid $20 for mine. To the QF-1, Autek added a high pass function and an auxillary manual notch filter. They also labeled the selectivity and frequency knobs with frequencies, but this does not make the filter easier to operate. The filter has similar performance to the QF-1. I find it a bit more confusing to operate though. I have operated it in tandem with the QF-1 using the QF-1A as a high pass filter and the QF-1 as a low pass filter to get a bandpass function with variable low and high cuts. While cumbersome, the two together provide effective filtering. The filter is AC powered. No provision is made for driving a speaker. Autek provided a hole in the back panel for an external DC connector and provided instructions for hooking up external power in the instrucitons. Thus it could be made to work in a portable situation. Of all my filters I use this one the least. It is not a bad filter. I think that the extra functions that I do not use put me off somehow. KISS. :^) The off switch bypasses the filter so unfiltered audio can be heard when the filter is off. Both of my Auteks have some AC hum. I don't know if this is inherent in the design or if I have a ground loop that I need to pay attention to. The manual suggests using a resistor in series with the headphones to reduce the hum, but I have not tried this "fix". Others have also told me about hum problems. EG&G 114 Signal Conditioning Amplifier This is not an amateur piece of gear, but a commercial piece of test equipment designed for precise measurements. It has both low and high pass filters seperately adjustable from 10 Hz to 100 kHz in 1,3,10 steps. The input modules are changeable. I only have a 50 Ohm differential input module. The gain is variable from 1 to 10,000. It has 50 Ohm and 600 Ohm outputs. I paid $5 for it in a test gear auction. I am sure it sold for $500 to $1000 when new. This filter is not designed for amateur use, but with the high pass filter set for 300 Hz and the low pass set for 1000 Hz it makes a nice CW filter and with the low pass set for 3000 Hz it makes a nice voice filter. The filter is AC operated and quite big; twice the size of my OHR CLassic. Definitely not for portable use. I put this in to let you know that equipment not designed for Ham radio use can be quite useful. Similar amplifier/filters are also made by Khrone-Hite, Beckman, and Ithaco. If you work for a high tech company you might survey the surplus list for similar amp/filter combinations. They are very useful as CW filters. I spent more on adapters for the BNC connectors than I did on the filter. MFJ CWF-2 In the 70s MFJ made several inexpenisve CW active filters with selectable bandwidths of 80 Hz, 120 Hz and 180 Hz (?). At one time you could buy a bare board version. MFJ sold scads of these. Before I became interested in active filters I used to see them at nearly every hamfest I went to. Now that I am looking for one I have not seen one in several years! These should be inexpensive. I used one when they were first brought out and recall it as being effective. I would like to have one to compare to my other filters. I think it operated with a 9 V battery so it would be nice for portable use. I have since acquired a partial kit version of this filter. I have not completed the kit, but will post the results as I get them. OHR SCAF OHR made a switched capacitor filter, the SCAF-20 (?), until National Semiconductor stopped making the MF-8 chip it used. I don't have one, but I used one that Roger Hightower had at the 4 corners event. It is a bandpass filter only, but has selectable bandwidths. I recall it being easy to use with no perceptable digital noise or artifacts. These should be available for near the price OHR sold them for, $50, and represent a good value at that price. I hope that these thoughts are useful. Use them at the next swapmeet you go to.