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NEScaf: A Switched Capacitive Audio Filter

Contributed by Dave Siegrist NT1U

The NEScaf is a switched capacitive audio filter. It is designed to be simple to build and use.

NEScaf PC boardIt is more flexible than previous filters for QRP rigs. It has two panel-mounted controls — a single center-detent potentiometer that sets the center frequency of the filter's bandpass (default value settable between ~415Hz and ~1kHz), and a dual-ganged potentiometer that provides a continuously variable bandwidth control (from ~60Hz to ~1500Hz). This ability to smoothly vary the bandwidth is very useful.

The NEScaf is designed to be driven with a phones or speaker level signal. No hacking into your rig — just connect it to the headphone or speaker output of the radio. This makes it usable with several radios. The board is small enough, at about 2 inches square, to build inside many radios however, if you so choose.

This kit is suitable for all experience levels. It uses all thru-hole components, and the construction and adjustment is very straight-forward. It is reasonably priced and will be a nice addition to the QRPer's arsenal of tools.

The NEScaf is only $31 postpaid in the U.S. For your convenience, we offer a connector set for just $4. Visit the NEQRP Club Store to purchase the NEScaf.


The integrated circuit at the heart of the NESCaf is made up of two CMOS active filters. These filters are extremely configurable (low pass, band pass, notch etc). We have chosen to set up both filters as Butterworth band pass filters and to cascade the filters. Butterworth filters have the characteristic of constant amplitude in the band pass region, while the cutoff knee is not be as sharp as if the filter were configured as a Chebychev. We considered this an acceptable tradeoff, wanting constant volume out regardless of the bandwidth or center frequency setting of the filter. There are two on-board trimmer pots. One is used to "calibrate" the center frequency pot. This allows you to adjust the frequency at which the center detent occurs. If you are using a rig with a transmit offset and sidetone of say, 700 Hz, you can use this trimmer to make that the center-detent frequency. The other on-board trimmer adjusts the audio level into the filters output amp. Using this pot, you can set the overall gain of the filter. This can be used to set the filter for unity gain, if desired. This way, the filter could be switched in and out, and still maintain a comparable volume level in the headphones. The NEScaf has no panel mounted volume control of its own. It is designed so that you use the volume or RF gain control (if present) on the rig it is being used with.

Fine Print

Please Note: A case and audio/power connectors are NOT supplied. Since each builder will have his or her own personal favorite styles for these, we are not including these parts. All board-mounted parts, plus the two panel-mounted potentiometers, a power/bypass switch, and a power-indicator LED are included in the kit. You supply your own case. You can supply your own connectors for audio in and out and power, or you can order a connector kit from us with your NEScaf filter.

The NEScaf was originally kitted with the MF10. That was replaced by the LMF100, a drop-in replacement for the MF10. These were superseded by the LTC1060 in spring 2010.


Hi Carl.

The NEScaf has been used by some for SSB, though it was designed primarily for CW use. You would set the center frequency somewhat higher than you would for CW, and use the filter near the upper end of it's bandwidth range. One could also recalculate resistor values in the filter stage to provide a wider response.

That said, the K2 has excellent IF-stage filtering already. The K2 XFIL values can be adjusted to your preferences, and tailored to your specific likings using a program like Spectrogram. The four available settings would be in discrete steps, however.

I have tried my NEScaf on my K2, with the K2 XFIL set for it's widest settings, and it works pretty well.

Bruce N1RX

A very nice filter!

I was particularly impressed with the quality of the pots and power connector. Where did you obtain them from ? It is hard to tell what you are getting when ordering things online.


Phil Sanders

Hi Phil. Thanks for the nice comments.

We did a bit of research on the potentiometers during the prototype phase. The ones used in the NEScaf kit are purchased from Mouser. The manufacturer is Alpha.

The power connectors are from Jameco.

Bruce N1RX

'rx'd the kit a few days ago and fired it up smoke & it even faith in kit building is restored! all that remainn is to complete the custom enclosure.
thanks, Randy

Argo 515 & IC706MKII
truck&rv mobile too

I would also be interested in using the NEScaf on SSB and even AM if possible. While I do a lot of QRP, I would like to have a filter to get rid of all the hum and noise in vintage tube recievers, especially when listening on modern headphones which seem to be almost too sensitive for these old sets. I have a LogiKit SCAF-1 dedicated to one rig now, and would like a smaller unit to use with phones on other rigs. Can you provide any guidance on recalculating the componet values to get 2500-3000 Hz bandwidths?


My NESCAF is cutting out after a few minutes, works fine again upon recycling power; then audio gradually dissappears. I've reflowed all the solder joints, checked the voltages. Nothing obvious.

When I take IC1 out of the socket, the audio is fine and doesn't cut out.

Since I didn't observe strict anti-static practices; I've ordered a new MF100.

BTW, tThe filter works great - just what was needed for my Rock-Mites. I just hope I can get it to work longer at a stretch :>)


Jim, K4AXF
Strasburg, VA

I reported gradual distortion coupled with loss of audio through the filter. Although the filter seemed to be working fine, it would go dead after a few minutes. I replaced C17, the input cap with an exact replacement from Radio Shack and the problem ceased. My NEScaf now works perfectly. Thanks to Bruce, N1RX for some helpful hints.


Jim, K4AXF
Strasburg, VA

I am trying to finish building a NeSCAF filter, and it is not working properly. I think it was working properly, but I didn't have long enough leads on the various offboard components, so I removed and resoldered all of them. Now, the audio is fine until I switch the filter on, then it makes a loud buzzing noise. I thought that I may have torn up the traces when I removed the old wires, but I checked all of the leads with an ohmmeter, and they all appear to be correctly soldered. The tone of the buzzing noise changes when I adjust R14, but adjusting any of the other variable resistors has no effect. Any ideas would be greatly appreciated!

Mike, KL7MJ

Dear folks,

did somebody tried to improve the noise floor of the LM386 by adding the 10k resistor and 10n capacitor in series between pin 8 and the output pin as it was published in the Oklahoma QRP News Letter and then after by KA4UOS in Sprat nr.79 please?
I am interested in the results...

Thank You for responses in advance.

73 - Petr, OK1RP

Well, we shovled out the bathrooms....then it was time to attack the Argo 509 with soldering iron and duct tape!!

Got the stock 4-pol filter replaced with the INRAD #751. It is a wire-in mod NOT a drop in replacment as the older T-T filter was. The results (viewed on Spectragram) are very interesting. The RX bandwidth is definately narrower, still not narrow enough for serious CW but definately much better than the stock fiter.

Got the SCAF filter out of the bag, put it on the ops table with some clipleads and tied it all together on the AF output of the 509. MAN!!! What a filter!!! I LOVE the continuously variable BW. It really makes a difference. I only wish I had it up and running in a box to demo at the local club meeting last night. It would have really raised some eyebrows!!

At any rate, it will be going in a box (sans clipleads) and I will use it until I order another one (with the newer chip) to include in the "Argo-Mate". As I understand it, from reading the comments, the newer chip seems to be quiteter than the older version I have. Also, there was some discussion about fiddling with the LM-386 to reduce noise. Will have to look into that.

All in all, gang, you guys done good!!! I am writing Dave Ingram's "How it Works" column while Dave is in hospital recovering from a wicked heart attack. Just got the word from Rich Mossesson, W2VU at CQ, who asked me to pinch hit for Dave for the forseeable future until he gets back on his feet. Realize this: I will be sure to plug the SCAF filoter in that column. NO reason for anyone to go to AF DSP with the additional costs when this $30 wonder works so damned well!!

Vy 73

Rich Arland, K7SZ

It turned out that I had one (or more) cold solder joints. A few minutes reworking them last night and all is well

This is a great filter!

Mike, KL7MJ

I just ordered two of the new NESCaf kits. Is there any change in the schematics/layout or can the old Manual Rev. 2008 still be used?

73 de Hermann W4/DL8MCP

Hello Hermann.
There are only a few minor changes to the parts included in the kit. The layout on the board has not changed. One capacitor, C3 has changed to 100 uF from 10 uF. We are using a new SCAF chip, the LTC1060. Also, the trimmer potentiometers may come in one or two style of pin layout, due to parts availability. Either will work fine with the board. I am completing a revision to the manual that will include these changes. I hope to have it on the web page by 30 April.
-Bruce N1RX

Ordered 4/22 and still have not received it. Why? George/W2BPI

all kits on order will be shipped by this weekend.

The fine shipping department (me) has been overwhelmed by work from my employer. Happily I remain gainfully employed as a full time employee cranking out microprocessors by the dozen, and I have a personal goal of remaining that way. In the week following our announcement of NEScaf kits available we received over 100 has taken me and my immediate-non-ham-family that long to process them all, and get some number to the post office. As I said, the rest will go out by this Saturday.

So if you have not received your kit by 14 May, drop me a note. As amazing as the shipping dept is, I have been known to drop stuff on the floor now and again.

thanks for your patience.
nt1u for the neqrp.

Thanks Dave, for your volunteer efforts with the NEScaf.

As Dave said, we have been overwhelmed with orders from the pent-up demand for this kit. In fact, we have ordered parts for another round. Due to some back-orders of parts, we may have to suspend orders temporarily while we wait on parts delivery. Notices will be posted here and on the order page if this becomes necessary.

Thanks all, for your continued interest in the NEScaf kit.
Bruce N1RX

Got my kit today. Thank you guys for all the hard work. I've been wanting one of these filters for years. Now it's here. Too bad I have to head to Dayton so soon as I don't have time to build it until I get back home...snif snif...


Received mine on Saturday, now I need to find time to build it! Thanks for kitting this.

Randy, W0GK

Hi folks,

did You compare the results of the NESCAF filter using the old MF100 and new LTC1060 replacement please? I am interesting the differences and results...

Thank You for Your time,

73 - Petr, OK1RP

Hi Petr.
I have compared the published spec sheets for each. There are some minor differences, but neither stands out as a clear winner. I have done some "A/B" audio testing as well. I have not noted any significant difference between the LMF100 and the LTC1060. In our application, they appear to be operationally equivalent.
Bruce N1RX


thank You a lot for comment. More information I am going to post on my blog over here on NESCAF club web.

73 - Petr, OK1RP

I am surprised that no one has mentioned this. C17 should be reversed, with the + end going to the filter IC, and the - going to the audio input. The input pin on the IC is sitting at ~ +4.5 volts, and the audio input should be at 0 volts DC level. With the capacitor reversed bias as shown in the diagram, it will draw significant amounts of leakage current. I found that the leakage from having the capacitor reversed would pull the bias at the IC down and cause distortion, when I switched the input from one rig or another. With C17 put in the right way, and a 150 ohm resistor (or even smaller) at the input to assure that the external source is at zero bias, this is no longer happening.

Hi Al.

Thanks for your post on this. What you say makes sense. It -is- interesting that you seem to be the first to have a cap leaky enough to cause a loading issue. I agree that it would be a good idea to install C17 with the positive lead toward R2, and the negative lead to the audio input jack.


I will look into changing the documentation and possibly the board silk screen on any later runs.


72, Bruce N1RX

Great filter, worked very well... I do get a bit of RF in the headphones on transmit, but I am working on it, and can live with it if I had to.  The audio level pot was touchy for headphones, but it is pretty close to what is switched in and out, level wise. 

I had a broken dpdt switch from the kit, but I don't need to turn off power, so I had a spdt switch floating around and that worked FB.  Great kit, does exactly what it should, pots are smooth, and I can dial out the QRM great now!


EDIT/ADDITION:  Actually the DPDT is important, the audio level isn't right when the audio is bypassing the scaf - it is good when power is pulled, so the dpdt helps in that regard.  The RF in the headphones is only when power is applied to the scaf, so it isn't the internal wiring, it is the SCAF board picking up RF... will keep working at it to try to eliminate


Other than that, the unit works great!



Can the NESCAF be used on digital modes like PSK31, RTTY, Olivia, etc?




Spend some time listening to true binaural CW and you will discover a new listening experience. If you have a few moments... read on.

A bit of background:

I believe it was a lat 70s article in Ham Radio Magazine (no longer published) that first introduced me to "binaural" CW reception. Over the years I have build several variations of "binaural" CW audio filters. The intention was to purposely split the incoming audio into two bandwidths - at and lower than the desired CW beat and at and above that beat.  As I suspected, when accomplished correctly the resultant "panoramic" sound image adds a new dimension to CW listening. The first thing you notice is that received background noise is divided into a lower and a higher audible frequecy spectrum with each ear hearing only one. This splitting action significantly reduces the apparent overall "heard" noise levels. But more importantly, incoming CW signals are heard literally through your head... from one ear through to the other. Tune through a single signal and listen as it is first heard in one ear and then slowly moves through your mind's sound stage to the center. Keep tuning at it moves from that center off to the other side. Have trouble zero beating a signal? Binaural reception makes it almost automatic.

It is amazing how much filtering our brain can accomplish automatically if we apply differing audio signals to each ear. You don't need complicated stereo simulators either. Simply use two filters to split the audio for the left and right ears. A low pass design with its high cutoff right at the desired CW beat, and a highpass filter with its lower frequency cuttoff at that same beat. Now rather than add increasingly sharp filters (with distortion and ringing) to get a single CW signal, you open up the receiver bandwidth (say 900 hz of so) and listen to the clean audio images as they image through your head. The signal you want is heard in the center.... signals lower in frequency are heard progressively toward one ear, while signls higher in frequency are heard progressively toward the other ear. Add to that natural separation, the broadbanded background noise is also split into two frequency halves and that additional separation creates an amazing overall reduction of apparent "heard" noise.

Enter the NEScaf filter:

Up till now, my binaural filters all had one issue or another that left me unsatisfied, to say the least. About two years ago I first saw info on the NEScaf filter and had a though.... "what if I used two of them?" It took some time for this project to move to the top of the pile, however it did and I am very satisfied.

Naturally the simplest approach was to install to NESaf boards into a single project box... parallel a mono input to both filters and then take the output of filter A and apply it to one half of a stereo headset and the output of filter B to the other half. Simple enough it seemed, but I did encounter two issues with my approach.

1. Hum. This turned out to be easy to fix and really had nothing to do with putting two filters in the same box. My mistake was to use a standard 12 vdc wall wart. The hum completely disappeared once I connected my NEScaf filters to the regulated power supply that I use for the Argonaut V. I could find no mention of this in the manual.

2. Mutual self-oscillation. This one had me going at first. Individually the filters worked great but once they were in the same box in close proximity to one another they output more oscillation spurs than signals by far.... and toucing or adjusting either filter's frequency control only created differing oscillations. Filter one's control effected both filters and visa versa. I quickly deduced that the approximate 70 kHz waveforms from the 555 timers were coupling back and forth to each other. The fix? I stripped off a few inches of RG-8X braid and ran the two signal wires from each board to its center frequency contol. Grounded the braid on the body of the control itself.... Put a little shrinkwrap at each end to protect from shorting other circuits (it is a small project box).... and viola! No feedback.

The NEScaf binaural listening experience:

Paralleling two NESaf filters together really does create a fantastic binaural CW filter. You can adjust both filter's to center the image on theCW offset of choice. And you can talor the upper and lower spectrum halves independently.

Food for thought:

Already have one NEScaf filter? Add a second one. They don't have to share the same box. Just parallel the inputs and apply the outputs to a pair of stereo headphones. Keep note though. If you build your filters into non-shielded enclosures you may experience the self-oscillations problem. If you do just add a piece of braded shield around the two wires to each filter's frequency control.


Jerry, KG6TT

Yes, Pete.

The NEScaf can be used on these digital modes. -Bruce, N1RX

I'd like to order a filter.  Has an date been estabilished for accepting orders?  Also, will the price remain the same as in the past?

Thanks for your work in putting the kits together.

73, Jim


Hi Paul.

Thanks for the inquiry. No date is set. I need to order the outstanding parts and arrange a kitting party. It will probably be sometime next month. Price will probably remain the same, but we'll have to see what may have happened to parts costs since the last round.


Bruce N1RX

Thanks Bruce.

73, Jim

Enjoyed building the NEScafe filter.  One downside was a broken toggle switch that came with the kit.  Got another at 'the shack'.  This little filter does a superb job!




Dick Williams KB3OMJ

NEScaf filter kit is a great project. Performance comparison between Nescaf and Datong FL3 (known to be very effective) are similar.

I hope that the kit is coming soon. I need another one.

73  Ed,  IV3TQE



Just noticed the NEScaf is now on backorder. I ordered my kit (via check and USPS) on 9 February. Any way to confirm I'm on the backorder list or my order was processed and mailed?



Philip, K9PL


I have passed your query on, and we will find out if one was sent yet.

Bruce, N1RX


Thank you for your reply. The kit was in this (28 February) morning's mail. I notified Dave, NT1U, of its arrival.

Many thanks to New England QRP Club for making this useful accessory available.


Philip, K9PL

This kit was a VERY easy build and it performs quite well on the air. I particularly like how it enhances my SW40+ and other less capable Manhattan style receivers and transceivers. I have been meaning to put it in a case for a little while now. The XYL and I were out at a local "dollar" store and I found a bunch of kids puzzles packaged in flashy tins. The filter is mounted in a Winnie the Pooh tin. You can see it in action on my YouTube page:


Many Thanks es 72,

Vaughn  N2BHA

Oops... I believe that the links that I provided were of the same video.

Here is the other one of the filter in action:


Vaughn  N2BHA

Hey Guys,

I recently ordered and received 2 NEscaf kits.  I am in the process of building them and one of the kits is missing the (3) 1uF tantalum caps.  These are C7, C9, and C17.  Is it possible to get replacements?

I recently re-aquired the NEScaf parts stock from Dave NT1U, who is preparing for an extended sabattical. I will contact you directly, Dave. I will send you replacements for the missing capacitors. -Bruce, N1RX


I just did the same thing.  What were the responses to your query regarding that problem.  I heard a pop, but saw no smoke.  Plan on replacing the voltage regulator but wonder what other things I blew...


Dave K8WPE

Hi, Dave.

Sorry to hear about your mishap. I would suggest looking closely at C15 (0.33uF tantalum). It is directly across the input DC line. It may have failed, and that may have been the "pop" you heard. Also the voltage regulator. Did you use IC sockets? If so, remove the ICs from their sockets after you replace the regulator. Check the DC voltages on the appropriate IC socket pins when you reapply power. In some cases, IC2 (LM386) may be damaged by reverse polarity, as well as C11 (100uF electrolytic).

I suggest adding a series diode with the positive power lead to prevent recurrance. Let me know If I can be of any further help.

GL, Bruce, N1RX

I bought my kit quite awhile back and when it didn't quite fit into the enclosure I had in mind, I shelved it until just recently.  I just repacked it and wow!  My rig has a 500 Hz filter, but this added audio filtering pulls out the ones that I would remark "I know there's a CW signal in there, but I can't copy it, to getting a Q3".  I am very impressed with your kit.


And keep them coming.


Great kit, fun to build. Mine was working at first, until I started putting connectors on and that DPDT SW1 switch. Of course, getting near the end of the build, I think I was rushing and not thinking things through. Power up, no LED light. Hmmm. Switch the switch and in the off position I'm getting the audio straight from the radio...Okay. Switch it on, and still no LED, no audio, and the front panel pots do nothing. Time to take a long pause and get away from the work bench.

Next day...first thing I spot is a solder bridge at C8. Not sure how that happened. Fixed that, then went back and rechecked all solder joints, solder joints to the audio connectors, pots, etc. Powered up and this time it worked (again). It really works nice to filter signals down and will be a nifty companion to the many QRP radios I have. So now I'm working on stuffing it into a small enclosure. Thanks New England QRP club for a fun kit and several hours of building.

73, de KC9CS



I see that the LTC1060 data sheet offers a lower noise/offset configuration on page 18 of the datasheet,

Set Sa/b low, connect S1 to LP out.  We may be talking about miniscule differences here - but I was just curious.  I guess the real question is - with the input disconnected & the volume control on max - is there any audible noise ?? 

Villy  ve6sq


Thanks to all at NEQRP (especially the kitters) for a super kit!  Here's some pics of my completed, packaged NEScaf --




I built up the kit in a couple of evenings (both my eyesight and my manual dexterity are not what they used to be), and was delighted to find it worked first time. I packaged it in a small diecast box, and it's really given my old Kenwood R-1000 receiver a new lease of life on CW! A truly excellent product, well worth the modest cost.

Best wishes from England,


Hi folks,

Found an easy way to get the SCAF properly calibrated using the FT-817:

Set SCAF center to detent and bandwidth to full narrow.

Set 817 to any frequency and mode to CW.

PAH (Press And Hold) the HOME button and screwdriver the pot in the 555 circuit to max volume in the speaker.

Then the filter is set for 770 Hz (or whatever you have the sidetone set to).

Slick, quick and effective. Done deal almost instantly. Only had to flip up the top cover and stick a small screwdriver in and diddle :-).

PAH on the HOME button just emits the sidetone when in CW mode.

Great to zero beat too.

Love the 817 :)!

72 de Vikki (WV9K).

Received my kit here in Japan fine. After building it, it appears trying to work but as I slowly turn the R10 frequency pot from max to min and back there appears to be many spurious oscillations that can even be heard in the receiver without the audio connected. After many inspections of soldering and proper component insertion, I'm at a loss. Has anyone had anything similar happen? Any suggestions would be greatly appreciated.

Rick in Tokyo

I thought I better follow up my previous post with the solution to my problem. Due to my location on the 5th floor of a Tokyo apartment building my receive antenna is a simple active loop located in the same room as the receiver. The original enclosure for the filter was a box that had plastic ABS front and rear panels. Anyway the 555 timer harmonics were being picked up by the antenna!. The solution was to rebuild the filter in a tightly sealed aluminum box, shield all internal audio cables and also to use ferrite clip-ons on the external power and audio cables..
Now the filter REALLY works. Being able to smoothly vary the filter frequency and bandwidth makes using this filter a real joy.
My thanks to all at NEQRP who have made my new toy possible.

I built my NEScaf few years ago and find that it is excellent! I've been thinking of building another one, but modifying it into a notch filter. Has anyone done anything like this? Ideas? Advice?

Thanks and 73!