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Reviews Categories | Filters, Audio: (DSP and others) | W3NQN Passive Audio Filter Help

Reviews Summary for W3NQN Passive Audio Filter
W3NQN Passive Audio Filter Reviews: 37 Average rating: 4.9/5 MSRP: $70 USD $75 Cdn $90. overseas
Description: Passive audio bandpass filter for CW
Product is not in production.
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N0XC Rating: 5/5 Jan 28, 2018 17:57 Send this review to a friend
An excellent new design  Time owned: more than 12 months
About 10 years ago, I was introduced to Ed Wetherhold's passive CW filter by a fellow club member at a field day event. I was instantly impressed with the "ring" free tone and QRM fighting qualities it provided. During Field Day the filter really helped to minimize the noise fatigue that I was experiencing. Some months later I saw Ed's schematic in the ARRL Handbook and set out to build one for myself. The description in the handbook was straight forward and I had little trouble building the filter to success. It should be said though, that ten years ago sourcing the 88 mH telephone inductors was a fairly simple task especially as I had a relative who worked for a rural phone company.

Fast forward to today. A few months back, I became interested in building another filter for a friend and I set out searching the net for the coveted inductors. It appears they are now only made of pure "unobtanium" and not even found on Epay. During my search I came across Ed's website (W3NQN.COM) where he mentioned a newer design that he was involved with. A quick email to Ed and I was back in business. Ed mentioned that he is sourcing the inductors for a newer design by Jim Tonne, W4ENE. Ed was extremely helpful in assisting me while building the filter. The instructions, part lists (with Mouser part numbers) and schematics were easy to follow, and the filter worked wonderfully after I corrected a simple soldering mistake.

Bottom line: If you are a CW operator and want an exceptional passive audio filter, you can't go wrong with the W3NQN\W4ENE filter. Even operators using newer rigs with DSP filters may find that this filter will remove some of the hiss and artifacts that some find objectionable. What are you waiting for? Send Ed an email for details. You can find his contact info on his web page W3NQN.COM

73, Steve N0XC
W1PJE Rating: 5/5 Aug 9, 2017 08:49 Send this review to a friend
Essential for all serious CW operators!  Time owned: 0 to 3 months
This is a remarkable filter with a solid theory behind it and really excellent results in practice. Let's see why.

In situations with many CW signals crowding the band (e.g. contest time), it helps greatly to have a very selective receiver with a narrow IF filter to both enhance a particular signal and reject others. Various attempts at doing this go all the way back to tube/valve days, with mixed success. Much of this marginal success is due to group delay problems through the filter, where frequency components at the edge of the filter pass through at different rates compared to frequency components in the middle of the passband. Such filters are inherently dispersive - i.e. nonlinear - and suffer from unpleasant 'ringing' and hollow sounding background noise. Any filter with a flat top in general can cause these problems, and using one for long periods of time is fatiguing.

Enter Jim Tonne W4ENE and Ed Wetherhold W3NQN, both with stellar reputations hard-earned in the amateur community. Jim has written an excellent recent QEX article (Mar/Apr 2017) describing the theory and practice mentioned above for CW filters, showing why this unpleasant ringing occurs for most narrow audio filters in CW reception including DSP based filters with a flat top. He then lays out a clever fix: use an add-on filter that has a low Q and rounded Butterworth shape, which attenuates the frequencies at the band edge that have significantly different group delays. The result is a large reduction in unpleasant artifacts. The arithmetically symmetric nature of the filter, compared to most filters which are geometrically symmetric, also is key to suppressing the ringing at the edges of the pulse as it suppresses the higher frequencies better.

W4ENE further shows that a multistage passive LC mesh network filter implementing the low Q / Butterworth shape of the add-on filter is simpler and requires no power compared to the active equivalent involving opamps and the like. The filter design is tuned for a lower tone near 500 Hz, but this is actually easier in my experience to comprehend compared to the higher 700 Hz BFO normally used. (As ears get older, the lower frequency helps too.)

After reading this description, I was quite interested in constructing this filter and seeing how theory met the real world, so I contacted W3NQN by email. Ed told me that he was not selling fully assembled filters any more, but that he would supply the four large value (78 to 80 mH; that's MILLIhenries!) potted inductors and help instruct me on the filter assembly.

This turned out to be a delightful experience. W3NQN gave one of the clearest detailed explanations I have ever experienced for a circuit assembly, helped me select the right capacitors from Mouser electronics, and supplied not only the unusual inductors but other items such as the filter housing box, switch, the input and output audio transformers, and lots of practical advice on how to manage the assembly along with clear documentation. All of it worked extremely well. (By the way, Ed reports that the best capacitors to use are Panasonic 5% metallized film versions, also available from Digikey, Mouser, etc.)

As others have mentioned here, it is very simple to use the filter - simply insert it inline with your speaker or headphones. There is a level-matched lever switch that makes it easy to do In/Out comparisons A/B style. NOTE: The filter is set up for a larger mono audio plug (1/4" in and out). This is not as common in the current age of smaller, stereo audio jacks. I recommend obtaining from eBay a set of 3.5mm to 6.5mm adapter jacks in both male and female versions (search for "3.5mm 1/8" Stereo Female to 6.3mm 1/4" Mono Male Headphone Audio Adapter" and "1/4" Mono Female to 3.5mm 1/8" Stereo Male Headphone Audio Adapter"). They are not expensive. With these adapter jacks that translate stereo to mono, the listener gets the filtered signal in both ears at once as intended.

The results in practice are nothing short of astounding and truly remarkable. Switching the filter in and out, I found that the lack of ringing gave an almost magical boost in intelligibility to fast CW signals, just as advertised. To explore further, I used the audio add-on filter with a software defined receiver to test the effect of the IF signal's passband shape on the qualities of the audio add-on filter. I was indeed able to verify W4ENE's statements that a flat IF filter was just the thing to use with the audio add-on filter. In fact, the best way to listen to CW turned out to be to use a simple 2.7 kHz flat SSB filter, at least to my ears!

Tuning the SSB passband around in this configuration, the desired CW signal would literally leap out of the background mix when it hit the ~500 Hz center band, with clear separation of mark and space compared to the much more diffuse results with the filter in 'bypass' mode. Background noise was amazingly well suppressed as well and lacked the hollow 'tube'-like sound of a narrow digital CW filter - I vastly preferred the W4ENE filter!

In short, it really is amazing how well this filter performs. You will enjoy it immensely! Hats off to Jim W4ENE for a superb design, and to Ed W3NQN for his true Elmering spirit and his long contributions to the art, which were very much in evidence here.
S57OX Rating: 5/5 Sep 24, 2016 09:40 Send this review to a friend
S57OX very good  Time owned: 0 to 3 months
Three monts ago I bought used ft-817 transceiver. There was no filters installed. When I was looking for improvement especialy for cw work, I found Ed,s web page with description of his arithmeticaly-symetric filter. I contacted Ed and recieved filter about 14 days after first contact.

Filter is equipped with mono audio ¼ inch input and output. My filter,s center frequency is 499 Hz. It is on lower side but after you listen cw a couple of hours it is more pleasant for me to listen lower pitch.

What amazed me most is qrm reduction. When you put filter to work, qrm almost completely gone.
Second plus is no need for separate power so it is convenient also for portable work when transceiver is powered with ACCU or batteries.

For now i will judge only filter in conjunction with FT 817 and combination is good. Audio output from transceiver is sufficient. Just make sure that all connections are mono.

If you have older rig or you looking for cw filter consider this option. You wil not regreted.

73 from Janez
OZ1DTF Rating: 5/5 Sep 10, 2016 12:03 Send this review to a friend
A very good passive CW audio BP-filter  Time owned: 0 to 3 months
Even if you have a modern transceiver you may feel the need for an external audio filter. I did, and my rig is the Kenwood TS-590S. Inspired by the fine reviews here I got curious about the filter with 'Arithmetically – Symmetrical ' design by Jim Tonne W4ENE. This is assembled and marketed by Ed Wetherhold W3NQN.

The filter is only available in ready-assembled and tested 490 Hz and 600 Hz (Center Frequency) versions i.e. no more kits.

As the CF is not adjustable, take your time to find out if 490 Hz or 600 Hz is best for you. On the TS-590S the pitch and sidetone is by factory default set to 800 Hz but is adjustable in 50 Hz steps. Fact is that our hearing get worse by age and the deterioration is worse at higher frequencies. In my case both frequencies work equally well.

Installing ("plug-and-play") and using the W3NQN filter is very simple:

1) INPUT: Integrated cable terminated with a 1/4 inch mono-plug for connection to the speaker or headphone socket of the rig.
2) OUTPUT: 1/4 mono-jack for connection of a speaker or headphones
3) IN/OUT Switch: A sturdy lever- switch for by-passing the filter.

The claimed advantage, in addition to the simplicity, is "no ringing effect". This is also what I can confirm. The filter is a great help in digging the wanted signal out of noise and QRM.

The mechanical design is simple and robust. Should you e.g. wish some other type of connector you can easily drill a hole and mount it.

By coincidence I acquired the W3NQF filter and the old active filter Datong FL1 at the same time. I have done a lot of A/B comparison. I get similar good results with the FL1 but that is another story. I will definitely keep both!

I highly recommend the W3NQN filter as a simple yet very effective CW audio filter.

Notice that Ed's homepage is currently not up-to-date and his email address is
W2UDT Rating: 5/5 Aug 10, 2016 19:32 Send this review to a friend
Good filter  Time owned: 0 to 3 months
This filter is passive and requires no power. If you are over 50( like me) you may want the 490hz one. E-mail W3NQN about what you want. Also, please note this only works with mono headphones.
SV2BBK Rating: 5/5 Aug 4, 2016 01:37 Send this review to a friend
Excellent passive CW audio filter  Time owned: 0 to 3 months
I was looking for a CW audio filter and after an internet search I found Ed Wetherhold's W3NQN website and read about his 'Arithmetically – Symmetrical ' passive CW filter.
I e-mailed Ed and we arranged to send me the filter with a center frequency of 600 Hz ( it also comes with a center frequency of 490 Hz ) .
Within a week the filter arrived to Greece in excellent condition.

The filter's container is a heavy duty plastic case with silicone type feet and a sturdy in/out switch.The filter simply plugs into the transceiver with the supplied ¼ inch mono-phone plug. It also has a ¼ -inch phone jack into which a headset or external speaker will plug. The 600 Hz filter is designed by Jim Tonne, W4ENE and Ed, W3NQN does the assembly work.

I use the filter with my Kenwood TS-430S transceiver and the HS-6 12,5 ohm impedance headphones.I live in an urban area and have quite high level noise. First thing I noticed when using the filter was the incredible amount of noise that had disappeared.It does an excellent work in eliminating background noise leaving only clear CW signal.

Concerning QRM from adjacent CW signals the filter is doing a great job in eliminating the interfering stations.
No more undesired signals on the sides, no audio hiss, no ringing, only clean, crisp CW signal !
The filter has an Arithmetically-Symmetrical response that is optimum for the minimum distortion reception of pulse-type CW audio Morse-code transmissions.
The output tone is clear, a pleasure to listen to.

As I mentioned above the filter is being used with my old TS-430S transceiver, it is possible that the addition of the filter may not be as effective as hoped when added to the more advanced receivers.

The use of Ed' s passive filter reduces the fatigue factor and makes listening to CW more enjoyable.I am very pleased with it's use and I highly recommend it to hams with older rigs. They will not be disappointed.

You can e-mail Ed , W3NQN for any further information concerning Tonne's new CW filter design. Ed is a real gentleman, his service is excellent and is a pleasure to deal with him.

Theophanis, SV2BBK
AG8B Rating: 5/5 Jun 26, 2016 08:49 Send this review to a friend
Helps on modern as well as boat anchors  Time owned: 0 to 3 months
I have used this on my Elecraft K-3 to reduce audio hiss fatigue and it does help on dsp artifacts and residual dsp filter bleed through. Alot of K-3 users use the APF function set at 200 hz with the dsp filter bw at 200 hz to stop any ringing. I have found I can run the k-3 dsp at 100 to 150 hz with tolerable ringing and this passive filter cleans up the hiss, artifacts, and bleed through better than the APF. This is just my subjective opinion and others may feel otherwise.

Where this filter really shines is on old BA equipment like the HRO-60, AR-88, and SX-28 that do not have narrow 8 pole IF filters. It allows you to use these receivers for more crowded CW conditions. It may not be effective as a 8 pole 300 hz IF filter, but it surely helps and does it without the ringing that you sometimes get with narrow bandwidth IF CW filters.

When I get done with a long sit during a DX contest, I'm not fatigued as much. I think this passive filter is more effective than an active audio filter and I highly recommend it.

Dave AG8B DXCC 356/338 Mixed
W4II Rating: 5/5 May 26, 2016 09:35 Send this review to a friend
Excellent  Time owned: 0 to 3 months
My station is modest. The radio is a Kenwood TS-590S, which has an excellent receiver on the down-conversion bands. It ranks well on Rob Sherwoods chart for 2KHz DR.

While I've been very pleased with the receiver, I couldn't resist the temptation to play with one of the Arithmetically- Symmetrical passive CW filters made by Ed W3NQN, and designed by Jim W4ENE. The filter has added additional capabilities to my radio. While there is some insertion loss, it is minimal. I typically back off on my RF gain and ride the AF Gain. This works great with the passive filter.

It removes a lot of the background noise, and adds to the roofing/DSP filtering handled by the radio. Even with the 500 Hz roofing filter engaged, and the DSP bandwidth cranked down between 50-400 Hz, the passive filter still adds to the reduction of nearby signals. It is especially good for weak signal work on pulse-type signals such as CW.

The filter is simple. It goes between your headphones or external speaker and the radio. A single, sturdy, in/out switch is the only control. Since it is passive, no power is required. I opted for the 490 Hz center freq, since it works best with my hearing.

I can only imagine how this would improve CW copy on older receivers such as the Drake 2B, etc.

I have what Ed calls the prototype version. You will not see a high grade PCB, with robotically placed SMT parts. It looks hand-built. But, personally, I view this as part of the fun. I added an additional phone jack to facilitate the various speaker/headphone combos I use. The container is a very nice, heavy duty, plastic case, with silicone-type feet.

Try one..........improve your CW copy, and have fun.

John W4II
VK6VZ Rating: 5/5 May 2, 2016 06:03 Send this review to a friend
Indispensable CW tool for modern and boat-anchor radio users  Time owned: 0 to 3 months
I am a 99-per cent CW operator with an unfortunate liking for DXing on the lower and noisy 160m-to-30m bands. Over 45 years of low-band operation, I‘ve tried most kinds of audio filtering - from DSP filters to active-audio filters. However, recently, through my friend and fellow low-band enthusiast, VK6GX, I was introduced to passive, resonant can speakers. Impressed by their improvement in CW-signal readability, I then realised I’d never tried a passive LC-audio filter.

After an internet search, I found Ed Wetherhold W3NQN’s web site and read about Jim Tonne W4ENE’s “Arithmetically-Symmetrical” bandpass filter design with a 3-dB bandwidth of 190 Hz that Ed is currently assembling and retailing. Traditional audio filters, such ones with Butterworth, Chebyshev and Cauer responses, use Geometric Symmetry. However, according to Ed, filters with the newer Arithmetically-Symmetrical design are optimum for filtering pulsed audio signals such as CW and consequently sound better.

I emailed Ed to see if a 500-Hz centre frequency /pitch version could be made for me - the standard version is 600 Hz. Ed rapidly reworked the W4ENE four-inductor design, found he could build one for 490 Hz and sent me a graph of the response. Two weeks later I had the customised filter.

The reason for me asking for the relatively low centre-frequency is that it is generally easier for most operators to separate closely-spaced CW signals from each other if they are being heard at a relatively low pitch. For example, a 550-Hz tone is 10 % higher than a 500-Hz tone, while an 850-Hz tone is just over 6 % higher than an 800-Hz tone. This might not sound a lot, but in a CW pile-up my experience is the percentage improvement is ‘night and day’ in terms of actually separating competing signals. Also, my old ears hear a 500-Hz pitch best.

The W4ENE/W3NQN audio filter is used with either a pair of 8-ohm mono headphones (such as the famous Kenwood HS-5) or a single speaker and thus has audio transformers on its input and output so it can simply plug into the extension speaker socket of a transceiver or receiver, with the supplied quarter-inch/6 mm jack-plug flying lead. Your speaker (using an identical size jack plug) or mono headphones are then plugged into the filter.

The audio filter has a sturdy IN/OUT toggle switch, so it is easy to assess its performance, and there is an equalisation resistor inside the filter so the audio output is the same for IN and OUT positions.

Initially the audio filter was used with my Orion II crystal/DSP filtering set to 1 kHz (3-dB bandwith) to ensure the W4ENE/W3NQN filter was the final determining element in the audio chain) and a Mission centre channel speaker (which cuts off below 70 Hz but passes only up to 3,500 Hz). The Orion was put on the CW end of 40m, in which wanted signals competed with S9+ SSB Indonesian pirate stations and the usual heterodynes.

The effect of switching in the audio filter was stunning - a weak UA0 working a Chinese station in the middle of the din went from barely readable to Q5 copy. The Orion-filter bandwidth was then reduced to 500-Hz (3-db bandwidth)and the audio filter switched back in – again increasing the readability of the UA0.

Perhaps the most remarkable and interesting effect was when the filter bandwidth of the Orion was reduced to the same 3-dB bandwidth (190 Hz) as the audio filter. Switching in the audio filter still markedly improved the ease of readability of the weak UA0, particularly when the signal faded down. Changing the speaker over to the 500-Hz pitch can speaker further improved the signal’s readability, but nowhere near as much as the addition of the W4ENE/W3NQN filter had done.

I then tried out the filter out on my old favourite Ten Tec Corsair, which has some seriously hissy operational amplifiers in its audio chain, with the transceiver crystal filtering set to 500 Hz (3-dB bandwidth). The hiss totally disappeared, leaving only the desired CW signals – and the Corsair sounded absolutely gorgeous, beautifully quiet and even nicer than the Orion II.

For me, what is great is the way that the filter makes weak CW signals pop out of the noise using a loudspeaker as effectively as using headphones (which more and more I want to avoid using for hearing-related issues). I wish I’d tried one years’ ago.

Not having used an older-type Geometric Symmetry audio filter, I can’t make a performance comparison with the Arithmetically-Symmetrical design of the W4ENE/W3NQN filter. However, what I can say is Jim and Ed’s filter doesn’t ring in the slightest and the CW signal at its output sounds purer than any active audio filter I’ve used.

As far as I know, this is the only passive LC-audio filter currently being sold in the world – and it is simply brilliant and now used on all the VK6VZ radios. Ed and Jim aren’t getting any younger, so if you are a serious CW op, get one while you still can.

Interested readers can contact Ed via email at: for purchasing details. You can buy either a fully-assembled CW filter for a 490-Hz or 600-Hz centre frequency. If you are a home brewer, Ed can sell you the four potted inductors so you can build your own filter.

Steve Ireland, VK6VZ, Glen Forrest, Western Australia
VA3GUY Rating: 5/5 Apr 16, 2015 20:07 Send this review to a friend
EXCELLENT!!!  Time owned: 6 to 12 months
I was searching for a passive CW audio filter when I came across Ed's site. As it turned out, he ran out of the older '5-inductor stack, three-resonator' components so he recently made some filters of a new type. These new filters use four potted inductors and are of a design by Jim Tonne (W4ENE) and feature arithmetic symmetry and improved group delay. This new design is intended to improve the quality of the received signal.

I was extremely fortunate to become a beta-tester for Ed so this report is based on quite a few tests in different situations. My testing comprised of using a mixture of 5 different headsets/headphones with various impedances. I did many comparisons of the DSP in my radio versus Ed's filter. I also tried using both at the same time. It should be noted that this filter has a fixed center frequency of 600 Hz (450 Hz available) with a 3-dB passband of about 200 Hz and is not adjustable.

The first thing I noticed about the filter was that there was no 'ringing' in the audio. When I set my DSP bandwidth on my radio down to 600 Hz and activate it, I get some ringing on the audio. Not so when using Ed's filter alone.

The second thing I noticed was the incredible amount of noise that had disappeared...including electrical noise, atmospheric noise and even some ignition type pulses. And once I 'zeroed' in on a CW station and activated the filter, I was amazed at the amount of rejection it produced. I was able to tune in the single station I desired and got rid of the majority, if not all, of the undesired signals. It seemed that with some weaker stations, when I flicked the filter on, there was almost an audio amplification effect. There isn' just sounds like it because there is no more QRM. There is a little of the typical filter hiss but very minimal. I tried using the DSP on my rig and the filter together and in most cases, there wasn't enough of a change to warrant using both. I found that I am extremely satisfied using Ed's filter on its own.

On many filters, when it is activated and/or deactivated, the volume of the audio can change and if the user is wearing headphones, it can hurt the ears. Ed has taken care of that problem by soldering a 33 ohm resistor across the switch to equalize the audio output so there is very little variance in volume level when the filter is activated or not. A nice touch!

The new filter is available (assembled) for $70 USD plus $15 USD Priority USPS. As mentioned above, the standard center frequencies are 450 Hz and 600 Hz and a custom center frequency is available for an extra $10 USD. Just email Ed and let him know. I can pretty well guarantee that you will NOT regret spending one cent...and you WILL be using it a lot!

Every CW operator should have one of these little wonders. Actually, it should be standard equipment in the shack!

Thank you very much Ed & Jim.

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