NEScaf: A Switched Capacitive Audio Filter

I saw two new NEScafs at our local Saturday morning ham breakfast today. They were built by Gene (AB1FN) and his son Vince (KB1JHG) of Charlestown, NH. Both got the kits at Boxboro last weekend. Gene won his kit at the QRP forum.

Both looked great. Gene built his into a nice black enclosure with beautiful panel labeling he did. He added a "VU" signal level meter to the front panel. It looked really slick.

Vince had built a 40M RockMite into an Altoids tin and wanted to add the filter. He had previously removed all the paint from the RockMite's tin. He did the same to a second tin, and built the NEScaf into that. he then soldered the two tins together to make a nice compact package.

Both gents said that they really liked the performance of the filter. I hope to have pictures to post of these sometime soon. Those who have built a NEScaf and would like to share a photo can post it to the NEScaf Gallery page.

Steve KA4NRF, Came back from Boxboro with the kit and before I could build it received the 2 LCD freq display kits so I was swamped. So between the Kids and work finally got around to building all 3 kits and the NEScaf is awesome,share it with my FT817 and SW40+. Worth its weight in gold.

hi guys,

I really liked the New England QRP club's switched capacitor audio filter kit,

but i thought it could be better, a little quieter. at first i thought the lm386 was adding some hiss. instead of using the lm386 i used a tda2003. used the circuit from page 27 of the sprat passed out at lobstercon, titled "High Performance Headphone Amplifier". i think most of the noise thats added is coming from the scaf itself and not just the 386. if the band is noisey its not objectionable but if the band is very quiet and especially if the signal is not very strong then there is room for improvement. regardless I still find it very useful for figuring out where zero beat is, and i have been amazed at how well i can sometimes isolate a signal from nearby interference.

if you used a 20 pin socket (radio shack 276-1991) then replacing the scaf chip is very easy. National made the MF10 supplied with the kit. if you go to their site they recomend an LMF100 as an upgrade. this is a 'pin compatible' drop in replacement. couldn't find it at mouser but they did have them at digikey for $6.09 for one (in addition to tax and shipping, digikey adds a $5 charge if your order total is less than $25). the other alternative drop in replacement is a MF10 made by maxim.
http://www.maxim-ic.com/parts.cfm?p=MF10
couldn't find them for sale anywhere but if you sign up with maxim you can request a sample, the MF10CN and MF10BN are 20 pin dips, not sure about the others or what the cn & bn mean.

i have tried both the LMF100 and the maxim chips and they are quieter than the National MF10. not sure which is the best as both the replacements seem about the same to me but enough different from the stock National MF10 that i think you will notice it and be pleased with the improvement.

72
jerry terres aa1of
franconia nh

(with copy to QRP-L)

OK one more up and running on the Left Coast this time. Set it up for my NorCal 40A and centered the pass band on the side tone. Besides the noise reduction, it really does indicate "zero beat" with the filter set with a very narrow pass band.

Neat project, thanks for doing this guys!!

73

Doug W7RDP

2/2/07 OK I installed the LMF100 and the CMOS 555, not sure what the CMOS 555 did but the LMF100 is much more quiet than the LM10, very big decrease in noise level. I will have to see if MAXIM will send me some samples of their version to compare with the LMF100.

Still a very cool project, you guys just need to stock up on some XL t-shirts ;-)

73, Doug W7RDP

I have built the NEScaf into 3 rigs, and find it terrific. I am ordering one more to set up separately. VERY useful kit!

John-W2AGN

The minute I put power to my newly built SCAF, it came to life and worked! I guess you could say it was CLEAN!

Dennis--K1LGQ
Brookline, NH

Hi Jerry,

as I checked with Maxim (even directly with their manager) they are not able to produce the MF10 -> they are not sending the samples MF10BN/CN as You mentioned.
Can You specify where You got them please?

73, Petr OK1RP

(continued)
under "annual usage" i estimated "<1000"
click "continue"
click "Register if you are a new user"
register for membership by filling out the contact information and click "submit" then check your email.
To my ears the Maxim brand MF10 sounded as good as the MF100. If the kind folks at Maxim decide to send you a sample then you can decide for yourself.
72
jerry terres aa1of
franconia nh

Jerry,
thank You for reply. I know how to order Maxim samples and I did exactly as You mentioned but...instead of the shipping I got message from manager of Maxim that they are not able to produce that product and that is not possible to ship any samples. Even for my second request I got reply that there is no way.

73, Petr OK1RP

I recently received my NEScaf kit, and the .33uF capacitor is missing. I was unable to locate a like-value replacement at Radio Shack. Is there a substitution value that is suitable for this component; or can I get a replacement from NEQRP?

Thanks,
Roger Lange, KC3DI

Hi Roger.

Sorry for the delay in replying. I have forwarded your message on to Dave, NT1U. He should be able to get you a replacement cap shortly.
73,
Bruce N1RX

Nice Kit! I have it hooked to my Rock Mite 40m. The only snag I have run into is setting it up, I find the instructions here Very confusing.

Page 49(!) says to hook the right tab, front view, to the "correct" circuit board location. The schematic shows the wiper hooked to one of the other tabs, which one, who knows! So it is anyones guess which way to hook it up.

Page 59 says to adjust the Bandwidth (R7?) to it's widest position, not sure which way, I adjusted it for loudest sidetone.

Then it says to narrow the bandwidth and adjust the center freq. to keep it audible. It then says to set the BW to its narrowest setting and peak the audio with the Center Freq control. What is the point? Why not set the BW to minimum and peak with the CF. What are we doing here?

What about R9, the pot on the PCB going to the 555? No mention of it's setting that I can find.....

Thanks for the help

Rich

Hi Rich.

I don't see any other replies yet, so here goes.

First off, the manual needs some updating/correction. When reading the manual, "page49" should be read as "4 of 9". Editing oversight. On to your specific questions...

R7 is the front-panel mounted, dual section potentiometer. It will control the bandwidth of the filter. Each section (front and rear) are wired to a separate set of holes on the PC board. For each section, the "wiper" or middle lead, goes to either R7a_W or R7b_W. doesn't matter which, as long as you wire the corresponding "end" connection for each section to it's corresponding pad.

So, if you call the front section "a", then the wiper of the front-most pot section will go to point R7a_W. One of the end connections will go to R7a. Which "end" of each pot section you use depends upon your personal preference for how you want this control to behave. The filter is at it's narrowest (minimum bandwidth) when the pot is turned towards it's minimum resistance end.

If you want the filter to get narrower as you turn the pot clockwise, then use the pot connection that the wiper moves toward when you turn the pot clockwise. If you want the filter to get wider as you turn the knob clockwise, then use the other end. The important thing is to wire both pot sections the same. You can always swap the leads over to the other side of the pot if your preference changes.

Re page 59 (5/9)bandwidth pot initial setting: The intent here is to initially set the bandwidth pot (R7) to it's widest setting. Which way to turn it will be determined by how you wired the pot in the step above.

Thoughout the procedure that follows, the front panel frequency pot (R10) should be left in it's center-detent position. The frequency pot pot being adjusted in this process should be R9, the board mounted trimmer for the 555 clock generator. The goal through this procedure is to end up with the center detent spot on the front panel pot to be set at your sidetone (transmit offset) of choice.

I hope this helps to clarify things.

73,
Bruce N1RX

Thank you Bruce, that helps a lot. I think the key missing bit was any mention of R9.

I will try it again. Other than the setup instructions the filter kit is very nicely done and a real value.

73

Rich

They may or may not be willing to send you a sample, its up to them. the "+" designation at the end of the part number, for Maxim, means lead free. the BN version is a DIP (dual inline pin vs surface mount type).to request a sample try using this link: http://www.maxim-ic.com/parts.cfm?p=MF10
next to "MF10BN+", click on "sample"
on the resulting page click on "sample now"
up near the top of the same page click on "Sample Cart"
put what you want under "customer part or reference number#"
under "sample quantity" change to "1" if you want (2 is ok)
under "annual usage" i estimated "
(contiued)

Jerry,
thank You for help -> but again...there is copy paste of the reply from Maxim:

Petr,

We would like to sample you this product, but we are no longer manufacturing this part. We do have many filters, perhaps you could browse our webpage. Maxim-ic.com and find an alternative. From what I know, the MF10 series has been killed for more than 6 months.

Thank you,

Kyoung Kim
Maxim Integrated Products
Inside Sales Associate
Phone: (408)331-4222
eMail: Dongjoo_Kim@maximhq.com

Just finished building the NESCAF and wow, it works great. Received it in 2004 I think and finally got around to it. It works really well. I hooked it to my MicroR2 receiver that was just completed a couple of days ago. Makes a great combination.

Just ordered the M100 chip. Thanks for a great kit!!

John Arnold WA6YSY
QTH: Ocean Shores, WA abt 75 miles West of Olympia

John wb1hbe

Think I will give this kit a try Dennis.

I just completed my NESCAF and I've tested it out. I built it to use with my backup rig, an Alinco DX-77T. I have to say that I am completely impressed. Not only has it worked well for me on CW, but I cranked open the bandwidth, moved up the center frequency, and listened to some SSB using it.

I put my unit in a Bud cast aluminum enclosure painted gray to match a passive SSB LP filter I built previously. With both of these in line the band noise is cut down considerably.

Excellent job to all who put this one together.

Andy N1KSN
NEQRP #176

I made a soldering error while building the kit. Has anyone had luck with desoldering from the board?

Hi Bill.

There is nothing special about the PC board for the NEScaf that makes it particulary difficult to remove a component. I'm not sure what you mean by "has anyone had luck..."

Use enough heat, and your favorite solder removal method (i.e. solder-wick desoldering braid, or solder sucker etc.) I have found that in general, it can be helpful to add a little more solder to a joint when attempting to remove a component. This allows the newly-flowed solder to help conduct heat into the joint, and introduces a little more flux for better flow.

If there is a specific question about parts removal from a PC board, then please repost back with some specifics. I am sure you will be able to get your questions answered.

GL,
Bruce N1RX