the sproutie mk ii hf regen receiver _ dave richards aa7ee

7/25/2019 The Sproutie MK II HF Regen Receiver _ Dave Richards AA7EE

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The Sproutie MK II HF Regen Receiver | Dave Richards AA7EE

https://aa7ee wordpress com/2015/09/14/the-sproutie-mk-ii-hf-regen-receiver/[07 11 2015 23:26:39]

September 14, 2015

The Sproutie MK II HF Regen Receiver

Filed under: Amateur Radio,homebrew radio,Shortwave Radio — AA7EE @ 6:28 pm

Tags: Front Panel Express, HF Regen Receiver, Jim K4XAF, John Farnsworth KW2N, NR5Q, regen receiver, Regenerative receiver, Seaside

Chassis, The Sproutie MK II, The Sproutie Regen

Note – If you have read this article before and are checking back in, it would be a good idea to clear your cache, to ensure that you are

viewing the very latest version of this post. I do add material and make corrections from time to time.

It’s been about a year since I finished building The Sproutie, and it’s been a good year. Of all my scratch-built projects, it has been the

most satisfying to own. It works well, looks pretty good and also, there is always the lure of possible of tweaks and improvements. This is

partially because it’s a home-brew project, and also because it’s a regen It was really enjoyable to build a receiver with the basic circuit

architecture taken from the 1930’s, but with a combination of solid state devices and lovely old vintage parts.

I have continued to occasionally purchase vintage reduction drives and variable capacitors. After using a National N Dial for the main tuningcontrol in The Sproutie, I became quite pre-occupied with what, to me, is close to the ultimate dial and drive for an analog receiver – the

classic National HRO micrometer-type dial and gear drive. I wanted to find a good example of one of these, and use it in a regen. I also

spent quite a bit of time performing Google searches using phrases such as “best regen receiver ever”, and “the ultimate regen”. These are

the kinds of things I search for when at a loose end, in the vague hope that I’ll magically find the most amazing regenerative receiver ever

designed and built! One very inspirational regen I did discover while searching for the “ultimate regen”, was Jim K4XAF’s build of Bruce

NR5Q’s “Ultimate Regen”. What a beautiful receiver! It’s a tube set, built on 2 separate chassis. One chassis contains the main receiver,

while the other houses the power supply, the speaker, and the “Selecto-O-Jet” audio filtering. It makes use of a National HRO dial and gear

drive for the main tuning, along with National “Velvet Vernier” drives to control the regeneration and variable antenna coupling. Now this

was the type of regen that inspires true longing, and convinced me that as enjoyable as The Sproutie was to build and own, I needed to

build just one more regen

Initially, I was hoping to use a different type of circuit for this receiver from the tried-and-tested front end used in The Sproutie. I did buildVE7BPO’s regen #4 and had some trouble with it picking up a local FM broadcast station. In retrospect, I should have realized that I have

had this issue with other simple receivers at this location, until they were cased up and grounded properly. The problem seemed to be a

little worse than normal, but this could well have been due to the amplification factor of Professor Vasily Ivanenko’s hycas detector. I gave

up far too soon and headed for the security of the front end I used in both the first version of my Sproutie, and the WBR. It is, of course,

the circuit used in Nicky’s TRF, as featured in issue 70 of SPRAT (with a few corrections and suggested mods in issue 72). Incidentally,

“Bear” NH7SR built a very functional version of Prof V’s Regen #4 which he described in this thread over on The Radio Board.

However, I didn’t just want to exactly duplicate the circuit of The Sproutie, even if the new receiver was going to have a different physical

form and different hardware. This new receiver would have to have some alternate type of circuitry that would make it worth building. I

was interested in trying a different type of filtering in the audio chain, and a tip from Prof V in his Solid State Regenerative Receivers group

Dave Richards AA7EE

https://aa7ee.wordpress.com/2014/08/21/the-sproutie-a-general-coverage-regen-receiver-with-plug-in-coils/

http://radioheaven.homestead.com/NRQ-62_The_Ultimate_Regen.html


http://qrp-popcorn.blogspot.com/2015/02/regenerative-receiver-4.html

http://theradioboard.com/rb/viewtopic.php?f=4&t=6223&hilit=bear

https://plus.google.com/communities/111700710411953988036?hl=en

https://aa7ee.wordpress.com/

https://aa7ee.wordpress.com/

https://plus.google.com/communities/111700710411953988036?hl=en

http://theradioboard.com/rb/viewtopic.php?f=4&t=6223&hilit=bear

http://qrp-popcorn.blogspot.com/2015/02/regenerative-receiver-4.html



https://aa7ee.wordpress.com/2014/08/21/the-sproutie-a-general-coverage-regen-receiver-with-plug-in-coils/



https://aa7ee wordpress com/2015/09/14/the-sproutie-mk-ii-hf-regen-receiver/[07 11 2015 23:26:39]

on Google+ clued me in to a great tool for designing active audio filters (more on that later). The pieces were beginning to come together.

I had a bunch of NE5532’s in my parts stash that had sat unused for a couple of years and it struck me that a regen which utilized a series

of active audio filters for different bandwidths, switched from the front panel, might be an interesting idea for a receiver. The LM380 output

stage I had used in The Sproutie works well, so I saw no reason to change it. It is fairly low noise, a welcome factor that makes it possible

to listen to a receiver comfortably for long periods of time.

Here’s the block diagram of The Sproutie MK II. As it contains 6 separate AF filters, I decided to also switch the +ve supply to the filter. A

5532 active filter draws about 7mA (14mA if using 2 x op-amp stages). Although it’s not a lot of current, it’s a fairly significant amount

relative to the total consumption of this receiver if all 6 filters are continuously powered. One of the reasons I prefer solid state over tubes

is the power efficiency, so no reason to keep all 6 filters powered if only one is being used at a time –

Fig 1 – Block diagram of The Sproutie MKII. Note that S1a, S1b and S1c are all part of the same rotary switch.



The front end, as I mentioned, is exactly the same one I used in the original Sproutie. It is the one used in Nicky’s TRF featured in issue 70

of SPRAT. I thoroughly recommend joining G-QRP. Your initial membership includes an archival DVD of past issues of the club magazine

SPRAT, which is a very valuable resource for homebrewers. If you have access to this archive, you should also take a look at issue 105, in

which a slightly different version of the same receiver is featured. It employs a simple passive LC audio filter, if you’re not keen on the

extra complexity that my version here entails.

Here’s the schematic of the front end. The oscillator tank circuit has been simplified to just one variable capacitor, and all details of the

plug-in coils removed, purely for the purposes of making the circuit a bit easier to understand. If I drew the octal coil socket without the

coil (as I did with the schematic for The Sproutie) it would make the process of understanding the circuit diagram a bit less intuitive –

http://www.gqrp.com/

http://www.gqrp.com/



Fig 2 – The Sproutie MKII front end, with details of plug-in coils and fine tuning capacitor removed for simplicity.

Here are details of the coil base, using an octal tube socket. You can use any pin configuration you like – this is the one that worked for

me. It is the same configuration as used in The Sproutie –




Fig 3 – Plug-in coil base wiring

The final AF amp is a simple LM380 circuit. It’s easy to build, is fairly low-noise, and it works. As well as a phone jack, I included a jack for

an external speaker on the rear panel. It took me a while to figure out how to wire the internal speaker and the 2 jacks properly. I

wanted the internal speaker to cut out if either headphones or an external speaker were plugged in. I also wanted the the external speaker

to cut out if the headphones were plugged in. It’s a simple problem really, but simple things often elude me. I got there in the end –



Fig 4 – The AF output stage. The “bass” switch only gives a very gentle lift to the lower frequencies. The effect is so subtle that you won’t be

missing much if you leave it out.

The thing that makes this receiver different from the original Sproutie, electrically speaking, is the bank of switched active audio filters. If

you don’t want to be bothered with building multiple filters, and switching them all with a switch, you could permanently wire just one filter

into the circuit. Another idea would be to replace this bank of switched filters with an adjustable filter made from op-amps, with the center

frequency and bandwidth controlled by potentiometers on the front panel. Once you bring op-amps into the mix, all sorts of things are

possible. Another idea suggested by Bear NH7SR, is a 5KHz audio notch filter, which could be quite useful for AM SWBC listening. The

design tool that made all this happen for me was by Texas Instruments (thanks Prof V). There is an online version called Webench Filter

Designer. It has a user-friendly interface that actually made the process of filter design harder for me than the offline software they also

offer, called Filter Pro. Use which one works best for you – they are both accessible from this page (opens in a new browser window). Of

the two, I recommend Filter Pro. You can use this software to design low-pass, high-pass, bandwidth, allpass (time delay) and notch filters.

I stuck with low-pass filters. I was tempted to try a bandpass design for the CW filter, and may still do at some point. The CW filter I

constructed was the very last filter out of 6. By that time, I didn’t have the patience for the slightly more complex design of the bandpass

filter. I also rationalized that I might need to tune through a CW signal to hear the other side of it, if trying to escape QRM, so a lowpass

would make this easier, as I’d be able to hear the signal all the way through to zero-beat and out the other side. This might simply have

been my excuse for not wanting to build a bandpass filter

I wanted a “straight-through” position to give me something to compare the other filters to. All the filters, with the exception of the narrow

CW filter, were designed with a 6dB gain, so I designed my “straight-thru” filter with a 6dB gain also, so I could step through the

http://www.ti.com/lsds/ti/analog/webench/webench-filters.page?DCMP=sva-web-filter-en&HQS=sva-web-filter-pr-en

http://www.ti.com/lsds/ti/analog/webench/webench-filters.page?DCMP=sva-web-filter-en&HQS=sva-web-filter-pr-en



bandwidths seamlessly. If doing this again, I would have given the filters a bit more gain. I’ll explain why later. Dan N7VE gave a talk to

the Arizona QRP Scorpions a few years ago on (among other things) designing active audio filters. It’s definitely worth taking a look at his

presentation, which is available here. In fact, I wish I’d paid attention to it before embarking on designing the filters for this receiver, as I

would have tweaked some of the resistor and capacitor values a bit. Dan explains how it’s desirable to keep the resistors in the main signal

chain fairly low in value, to avoid noise. He recommends trying to stay under 1K. I only read the presentation before designing the very

final filter – the CW one with a cut-off of 700Hz – so while my resistor values in that filter are nice and low, they are not quite so low in

the others (though in my defense, they are not atrociously high either).

Here’s the first, and widest filter. As far the ear is concerned, it’s not really a filter, as it has a cut-off set at 20KHz, with a gain of 6dB –

Fig 5 – The “straight-through” filter (an LPF with a cut-off of ~20KHz)

I wasn’t interested in the shape of the response as, for this stage, all I wanted was effectively an unfiltered stage with a gain of 6dB. For

this reason, I used just one half of a dual op-amp 5532 package as a real-pole filter. Filter Pro doesn’t show the power supply and biasing

arrangements, so I added the 2 x 47K resistors to keep the input biased at about half of the supply voltage. I also added the 10uF

http://www.azscqrpions.org/Tuthill_filter_presentation_08-09.pdf

http://www.azscqrpions.org/Tuthill_filter_presentation_08-09.pdf



capacitor, which keeps the bottom end of the 1K resistor at ground potential for audio signals, while blocking the DC bias. I also added the

lowpass filter formed by the 10 ohm resistor and the 100uF electrolytic on the supply line, as well as the 0.1uF ceramic RF bypass cap on

pin 8 of the IC (mounted close to the pin). I don’t know how essential these 0.1uF caps are, but the datasheet suggests them, and they

can’t do any harm.

The other filters were all 4th order low-pass filters (2 stages = 1 x 5532 dual op-amp package), with the exception of the 2.4KHz filter,

which was an 8th order low-pass filter (4 stages = 2 x 5532 dual op-amp packages). The 8th order filter has a sharper cut-off, of course.

Feel free to design your own filters, with the help of Filter Pro, for whatever cut-off frequency and rate of roll-off you wish. I’ll show you

the R and C values I used for my filters but you might want to fiddle around with the software and come up with your own values that

keep the R values in the main signal chain at or below 1K, if possible. The resistors in the first stage of the filter are particularly important,

as the noise they produce is amplified more than noise produced in later stages. Just click on a component in Filter Pro, enter a differentvalue, and hit return to see what new values of the other components the software has calculated. A bit of trial and error should get you

close. Also note that you can specify the series of resistor and capacitor values you want to use (E96, E48, E12 etc), and watch how the

filter response curve changes as you change the tolerances and values.

First of all, here’s the gentler roll-off 4th order filter that uses just one 5532 8-pin dual op-amp IC – or use the op-amp of your choice. I

chose the 5532 because I had a bunch of them in my parts stash and because they are the 2N2222 of the op-amp world – plentiful,

reasonably priced, and all over the place –

http://www.ti.com/lsds/ti/analog/webench/webench-filters.page?DCMP=sva-web-filter-en&HQS=sva-web-filter-pr-en#active-filter

http://www.ti.com/lsds/ti/analog/webench/webench-filters.page?DCMP=sva-web-filter-en&HQS=sva-web-filter-pr-en#active-filter



Fig 6 – Schematic for the gentler roll-off 2-stage LPF

Here are the component values I used for my 4th order filters –



Fig 7 – Component values for the 2-stage filters in my Sproutie MK II

For a sharper roll-off, an 8th order filter, which uses 2 x 5532 dual op-amp packages (or equivalent) –



Fig 7 – Schematic for the 4-stage sharper roll-off LPF

The 2.4KHz 8th order filter I used, although a bit on the narrow side for SSB, is good for listening when there are nearby stations higher in

pitch that need cutting out. If you think about it, this 2.4KHz LPF is going to sound roughly like the 2.1KHz filter in a regular SSB rig. The

reason for this is that your regular SSB filter is a bandpass filter, with the bottom edge being set to cut off at about 300Hz. This means

that a 2.1KHz SSB bandpass filter will pass frequencies up to about 2.4KHz (2.1KHz + 300Hz). Here are the values I used in mine –



Fig 8 – Component values for the 4-stage filter

After I had built the receiver and all these filters, and done some listening, I concluded that for SSB and CW, a bit more filter gain would

be helpful. The set has plenty of gain when listening to AM but on CW/SSB, the RF gain has to be wound right down to prevent theoscillator pulling. This creates a need for more AF gain in the CW/SSB modes. At the time of writing this, I have only just finished building

this set and have no enthusiasm for building more filters. I actually had to build 8 filter boards to get the 6 that I used, and 3 front end

boards to arrive at the final one. Together with the physical side of the construction, I am tapped out right now and have no desire to

construct anything else at all for a while!

If you want to use this receiver mainly for SSB and/or CW, you may want to experiment with the value of the NPO capacitor in the front

end that connects the hot end of the main tuning coil to the base of the 2N3904 oscillator transistor. It is listed on the schematic as being

39pF, and that is the value I used. However, it is possible that a lower value will cause the oscillator to pull less on strong signals. Of

course, the lower value might also reduce the signal strength into the detector which will put you back to square one. It’s worth trying

though. I’d be tempted to try a value as low as just a few pF. Remember that changing this capacitor will affect the frequency coverage –



particularly at the top end of each range.

When building the filters, I originally built the 700Hz CW filter with a gain of 6dB, like the other filters. The idea is that if they all have the

same gain within their passband, the operator can step through the different bandwidths without a change in the volume of the wanted

signal in the speaker. This was the way it worked except with the 2 narrowest filters. The 2.4KHz 4 stage filter had a slight, but noticeable

drop-off in volume. The effect was very pronounced with the 700Hz filter – so much so that I redesigned it with a gain of 20dB and still

found that there was a slight drop-off in volume within the passband as compared to the other filters. I don’t know the reason for this.

EDIT – Thomas LA3PNA Tweeted the following explanation – “The perceived loss when changing filters is because the power delivered to

your ear is 10log(BW of filter) and less with less BW. So basically, the reduction in noise makes it sound like the volume goes down” He

also gave a very useful tip for adjusting the gain of the filters so as to preserve the perception of constant volume – “I like to add gain in a

filter circuit after the formula 20log(bw/orginal bw) for AF filters” That is very useful information Thomas. I’m a little tapped out afterbuilding The Sproutie, but if and when I decide to revamp the filter bank, I’ll be paying attention to this formula.

I may, at some point, rebuild the 3 narrowest filters with higher gains. If that ever happens, I’ll report the results here in this post.

Incidentally, at this point, allow me to say one more thing about the filters. If building and wiring up all these filters sounds like it is

making the construction of a regen overly-complicated, I can definitely sympathize. If you want to use this set for CW, SSB and AM and

you want to permanently wire in just one filter, I’d go for a 4th order (2-stage) LPF with a cut-off of 3KHz. The one I have is perhaps a

touch wide for SSB (it’s roughly equivalent to a 3.3KHz passband filter, as explained earlier) and a bit narrow for AM broadcast, but it’s a

good compromise for both. If it were the only filter I had, I know I would get used to the sound of it. As for the gain, mine has a gain of

just 6dB, but I’d like to up it in order to have a good volume when turning the RF right down, as is necessary to prevent oscillator pulling

on SSB/CW. I can’t know until I’ve tried it, but I’m thinking something along the lines of 26dB gain. Just make sure to be careful when on

AM, as you may find that you have way more gain than you need – so keep an eye (and a hand) on that RF gain control.

A big part of the inspiration for building this receiver, as I mentioned earlier, was the physical form of K4XAF’s version of NR5Q’s Ultimate

Regen. In the search for a National HRO dial and gear drive in really nice condition, I bought several, and finally came up with a dial and

drive combination that just cried out to be included in this receiver. This gear drive has a shaft rotation limiter, which was perfect, as the

tuning capacitor I wanted to use didn’t have any kind of rotation limiting built in – it was the capacitor in the first photo in this post – a

Hammarlund MCD-50-M. The final M stands for midline, referring to the fact that the off-center shaft and shape of the vanes help to make

the tuning a lot more linear than with regular variable capacitors. With a standard capacitor, you’d find that the frequencies would become

very compressed at the top of the tuning range i.e. the tuning would get a lot more fiddly. Try to get a midline unit. I believe they also go

by other names, depending on the manufacturer.

Of course, a big dial and gear drive need a big chassis, and Terry from Seaside Chassis, who made the chassis for The Sproutie, came to

the rescue again. I decided to use a chassis and front panel that would be compatible with 19″ rack cabinets, for a variety of good

enclosure options. A chassis that big needs to be fairly thick in order to still be stout and solid. Terry does offer the use of 12 gauge

aluminum for bigger enclosures, and I wanted this receiver to be big and solid (although compared to your average boat anchor, it’s still

relatively light). As well as a large, stout chassis, I decided that I wanted to try designing a custom front panel with the services of Front

Panel Express in Seattle using their free design software. Right at the beginning of this whole project, in the first month or two of 2015, I

downloaded their software and casually laid out a very rough front panel, mainly for the practice, and the fun of learning something new.

As the project progressed, I’d spend a few weeks working on circuit boards, then go back to the front panel, then do a bit of work on the

plans for the chassis, to send to Terry. I had an idea that, with a bit of luck, I’d complete the whole thing in or around the fall, and that’s

how it worked out. At no point did I rush though. Why rush? Besides, the longer a project takes, the less it costs per month. I could see

that building this regen in the way I had chosen to build it was not going to be a cheap affair, so I took my sweet time.

Here is the chassis as it arrived from Seaside Chassis, along with 2 side braces for supporting the front panel, 2 mounting brackets for the



https://aa7ee.wordpress.com/2015/04/08/holy-grail-attained-a-national-hro-npw-gear-drive-and-dial-in-fb-condition/


https://aa7ee.wordpress.com/2015/02/26/a-popcorn-qrp-regen-receiver-and-lots-more-air-spaced-variable-capacitors/


http://www.seasidechassisdesign.com/

http://www.frontpanelexpress.com/














main tuning capacitor, and 2 mounting brackets for the regeneration pot. I only needed 1 of each of these brackets, but like to have extras

on hand. As it turned out, an extra bracket was needed to help secure the main tuning capacitor which I forgot to ask Terry for, so I put in

an extra order. The shipping from Canada dwarfed the cost of the bracket but at this point, it was easier to ask him for it than to find

someone local and besides – I just wanted him to fabricate all the chassis components. Terry’s work is first-rate. It’s good to give him as

much relevant information as you can. Simple drawings with penciled-in dimensions work well. If it’s important to you, remember to take

into account the thickness of the aluminum if there are any dimensions that are particularly critical. Also remember that he is bending and

fabricating these components by hand, so allow for a certain amount of tolerance in the final dimensions. Having said that, the chassis he

supplied was remarkably close to the exact dimensions I requested, and within the tolerances I had allowed for. If you have any

dimensions that are particularly critical or non-critical, I think this is all good information to pass onto him when making your request –



Figuring out exactly where to drill holes for controls in front panels and enclosures usually takes quite a lot of time. It’s a bit like a game of

chess in that every decision you make affects everything else down the road. To make matters tougher, I have trouble thinking about more

than one thing at a time, so juggling all the variables in my head takes a lot of thinking, measuring, and drawing. For front panels, I always

draw the shape of the panel on a full-size sheet of paper, and place all the knobs and controls on it to see how they look in various



configurations. Just when I think I have it right, I leave it and walk away, often overnight. On returning, I inevitably come up with an

improvement or two. Building something like this is all baby steps for me. I am impressed and amazed by builders who claim to be able to

throw something like this together in a few afternoons – this one took me over 6 months. Heaven knows how long a more complex

receiver, such as a multiband superhet, would take me.

I took a great deal of time and care in designing the front panel. They are worth every penny, but they are not cheap. I didn’t want to

make a mistake that would result in having to re-order the whole thing. So after checking, rechecking, going to sleep, then waking up and

rechecking again, I went through this whole process several more times before finally clicking “order”. A week or so later, this beautiful

4mm thick aluminum panel arrived via UPS, packed with a little bag of gummie bears –



Gummy bears!



The front panel as it arrived from Front Panel Express, vacuum packed to a stiff baseboard. The metal ruler is 18″ long (the panel is 19″ wide).

Look at this beautiful, black anodized front panel!



I just couldn’t get enough of this thing when I first saw it –



There were some scratches in the black finish on the rear, but this is normal. I later found out that it is possible to add a note when

ordering, to ask the people working with the milling machinery to take extra care with the back side of the panel. The front surface is

guaranteed, but not the back. I decided I was OK with the rear of my panel as, well, it was the rear, and the bottom half of it would be in

direct contact with the front of the chassis anyway –



You’ll notice a number of “blind” holes milled on both the front and rear. The panel is so thick (4mm) that controls sticking through both

the chassis and this panel wouldn’t protrude far enough for the nuts to thread onto the bushings. For the RF gain, AF gain and filter rotary

switch, the blind hole was milled on the front side, as the knob would cover it. For the phone jack and bass switch, the blind holes were

milled on the rear. Here’s a close-up of the blind hole on the rear side for the bass toggle switch. You’ve probably figured (if you didn’t



already know) that a “blind” hole is one that doesn’t go all the way through the panel –



works quite well. I mark the edges of the cutout with a pen or pencil, then with a hand-drill, drill lots of small holes around the perimeter.

Then, with an old screwdriver, I knock out the piece of metal in the center, and clean up the edges with files, usually using a bastard file

first, and finishing off with something finer. These photos should help illustrate the process. The speaker cut-out was inspired by a WW2-

era British military R107 receiver that I owned as a teenager. It is simple – just 4 large holes arranged in a square. This is the “during”

photo, showing the series of small holes drilled around the edges of the holes. The rectangular cut-outs to the right were made using the

same technique, incidentally –

After the initial euphoria of receiving this fantastic front panel had subsided a little, it was time to put some time and labor into making all

the remaining cut-outs in the chassis. I had asked Terry to make the holes for the octal tube socket and the main controls, but there were

others that still needed to be done. My usual method of making non-standard cut-outs and holes is very time and labor-intensive, but it



– and after –



Here’s another photo, taken a bit later during the assembly, showing the placement of some of the main components. This particular

National HRO NPW gear drive, unlike most that I have seen, has a shaft rotation limiter. The tuning shaft is a little on the short side. I

needed to mount the gear box as close to the front panel as possible in order to be able to mount the dial properly. If you scroll back and

look at the photos of the front panel, you’ll see there are 3 smaller holes located around the main hole for the gear drive. These holes

h l d i l i h b l h f l ibl ( h 3 h d fi i h 3 ll h l h f l)



helped in locating the gear box as close to the front panel as possible (the 3 screw heads fit into the 3 smaller holes on the front panel).

Most of these gear boxes don’t have this rotation limiter, so the extra holes won’t be necessary. Also, do you see the aluminum shaft

couplers on the regen pot and fine tuning capacitor? Those are quality parts personally machined by John Farnsworth KW2N. The one on

the right is a standard 1/4″ to 1/4″ coupler, while the one on the left was made to order. It couples the 1/4″ shaft of the 10-turn

regeneration pot to a short 3/16″ shaft that the National knob fits onto. I wanted to use the same type of National Velvet Vernier knob and

escutcheon plate for the regeneration that I used for the fine tuning, but I didn’t want to use the 5:1 reduction drive. I wanted to use a

10-turn wirewound pot instead, as I like the feel of those pots. From the front (as you will see in later photos) the 2 National knobs and

escutcheon plates look the same. However, the knob on the left is connected directly to the 10-turn pot and not to a National Velvet

Vernier reduction drive. The black escutcheon plate for the regen control is spaced away from the front panel by one washer thickness, and

bolted to the front panel with 4-40 hardware. It is not used for anything, other than looks.



Now, let’s look at some of the boards. They were built, as always, with W1REX’s very useful MeSQUARES and MePADS. This is the AF

output stage and the 4KHz filter mounted on one board, and installed in the chassis. The idea was that this board, together with the main

RF board, would form a working receiver, after which I could build and install the other filters, one by one –

http://www.qrpme.com/?p=product&id=MES

http://www.qrpme.com/?p=product&id=MEP





Mounted above the AF output stage, on the stand-offs, is this next filter board, carrying 2 LPF’s. The first filter to be built was the 6KHz

one –



Next came the 3KHz filter (in the foreground of the next shot). The grey rectangular poly capacitors were from Tayda Electronics. Thier

prices are low, and the caps seem good. The resistors are 2 types – either 5% carbon film from the parts stash I had as a kid in England in

the early 80’s. They lasted a long time, but I am beginning to run out of them. The others are newly-acquired Xicon 1% metal film parts,

purchased in lots of 200 from Mouser –



The same board, taken from above (3KHz filter on the left, 6KHz filter on the right) –



The same board, with the 3KHz and 6KHz filters, mounted in the chassis above the 4KHz filter and AF output stage –



Here are 2 shots of the 4-stage (8th order) 2.4KHz LPF, with temporary leads in place for testing. It’s quite the QRM-buster –



Here’s a wider view of the underside of the chassis at this stage of the construction, showing the main RF board wired to the octal tube

socket, as well as the AF output stage with 4KHz filter, the 3KHz and 6KHz filter board on top of it, and the 2.4KHz filter board sitting on

it’s own for the time being –



A closer view from a slightly different angle. At this point, it was beginning to dawn on me that keeping all this wiring tidy would take a bit

more work than I had anticipated. I never did get the wiring as tidy as I wanted, but it’ll do –



and the 700Hz CW low-pass filter –



Coils were constructed in the same fashion as the coils for The Sproutie. In fact, the pinouts used on the octal socket are the same, so my

Sproutie coils work in the Sproutie MK II, though they cover a wider range, due to the greater maximum capacitance of the tuning

capacitor I used – a 2 x 50pF instead of the 2 x 35pF used in the original Sproutie. I decided to wind a complete set of new coils for this

receiver. As of writing this post, I have 6 coils wound with a few more to go, as needs and desires dictate.



I like these ceramic bases, because they are just a little higher than the phenolic ones, offering a bit more protection to the toroid.

Wherever you get yours from, if they’re ceramic, they may well be the same ones as these, as most of these bases and sockets seem to be

made in China these days –



“Take us to your leader”

You’ll need to figure out the exact details of your coils with the help of online calculators (I like the ones on W8DIZ’ site) and good old trial

and error, but here are my details – they should give you a start. Remember to take into account the values of main and fine tuning

capacitors, if they are different from mine – and the value of that 39pF capacitor between the tank and the base of the oscillator

transistor, if you try a different value. I’ll update this table as I wind more coils. The plan is to wind general coverage coils up to about

21MHz or so, and a few more coils for specific bands. It is much easier, with the aid of the dial calibration graphs, to pinpoint exactly which

http://www.kitsandparts.com/

http://www.kitsandparts.com/



, p , g p , p p y

5KHz channel you are on, when the band coverage is limited to 1MHz or less. With the 20:1 reduction ratio of this National HRO drive, and

the large, relatively massive dial, I found it quite easy to tune in stations even on the 13500-18300KHz coil, which spans almost 5MHz. For

pinpointing which 5KHz “channel” I am on though, a general coverage receiver to listen to the oscillator of The Sproutie is more reliable

(and faster) than reading dial calibration graphs.



that would still look good, regardless of how my personal aesthetic might change. Minimalism is the key, though the one extravagance I did

allow myself was the larger “Sproutie MK II” declaration, and my callsign. I did try my callsign in red but decided that it looked gaudy. Best



to play it safe, I think. I was also concerned that the finish might be a bit too shiny or glossy, but it turned out to be matt with a slight

sheen. I’m very happy with how this looks –



Here’s The Sproutie MK II with her little sister, for size comparison –



A quick word about that regen control. On first installing the main RF board, one filter, and the AF amp into the chassis, I noticed that

occasionally, when receiving a strong carrier, I’d hear a ringing in the speaker. My first thought was that it was microphonics caused by

physical feedback between the internal speaker, which was bolted to the chassis, and some part of the circuit. Plugging in headphones



https://aa7ee.wordpress.com/2015/09/14/the-sproutie-mk-ii-hf-regen-receiver/[07.11.2015 23:26:39]

I like this receiver as it is, with the partially open chassis. From using The Sproutie, I have become used to seeing the vanes of the variable

capacitor rotate as I tune the band, and I like that. I like seeing these vintage radio parts in action. However, I did learn from The Sproutie

that whatever isn’t covered picks up dust – and when you’re living with 3 cats, 2 of whom are long-haired, a lot of cat hair too. I designed

this receiver so that it would fit any standard 19″ enclosure that is also 6RU (rack units) or more high. The first plan was to make use of a



And in case anyone ever wonders who made this fantastic cabinet for my regen, David left his mark. What a quality job! –



The National HRO gear box does have a small amount of backlash – even when I apply as much tension to the anti-backlash gear as my

poor little fingers can manage. At first, the backlash was something like 1 – 1 1/2 dial divisions. After increasing the tension on the anti-

backlash gear as much as I could reasonably easily manage without the use of tools, the backlash, though still there, decreased to about

1/2 a dial division. It is a small amount, and also predictable, so not really a problem. For the purposes of dial calibration, I always turn



The second thing is that, because the cabinet is wood, the receiver is not completely shielded. This would be useful were I to wind a coil



The second thing is that, because the cabinet is wood, the receiver is not completely shielded. This would be useful were I to wind a coil

for the 2-3MHz region and use The Sproutie MK II in conjunction with crystal-controlled converters to cover specific bands. This is a

Regenorodyne approach, like Gary WD4NKA’s inspiring Regenerodyne receiver here. It would also be nice to reduce the possibility of picking

up very local QRM in the shack. I could achieve better shielding with my Sproutie MK II by either simply housing it in an all-metal rack

cabinet, or by cladding the interior of the existing wooden rack cabinet with thin metal plate or mesh. There is absolutely no hand-

capacitance effect when using the set, due to the metal front panel, but when my cat Sprout jumps up on top of it (as she often does) the

frequency shifts by about 20Hz. This is also due to the lack of shielding on top of the set. Again, it is not much, but it is there.


http://www.qsl.net/wd4nka/TEXTS/REGENf~1.HTM

http://www.qsl.net/wd4nka/TEXTS/REGENf~1.HTM



Sproutie (aka Sprat The QRP Cat) and The Sproutie MK II. Her contribution to the dial calibration of this receiver was carefully knocking the

plug-in coils off the top of the receiver and watching them hit the floor.



Comment by Victor Koren — September 14, 2015 @ 7:06 pm | Reply

Thank you Victor. I really appreciate your kind comments. A SCAF is certainly an option. It would need to installed after

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the AF output stage, as I understand they are relatively noisy. Making and installing all these separate filters was more

work than necessary, I think!

Dave

Comment by AA7EE — September 14, 2015 @ 7:23 pm | Reply

2. Beautiful work as always – just W.O.W. !!

Comment by Alan “W2AEW” Wolke — September 14, 2015 @ 7:47 pm | Reply

Thanks Alan – coming from a real engineer, I really appreciate it. Loving your Drake 2B videos, by the way.

Dave


3. About using the MAX7400 SCAF,assume you put the filter at the detector output.

If a very strong signal will generate at that point a voltage swing of 4Vp-p then the noise floor of the filter would be 90 dB

below that because the data sheet graph shows 80 dB Signal/Distortion+Noise with audio bandwidth of 22 KHz.

If you use a bandwidth of 2.2KHz then noise will be 10 dB lower, getting 90 dB of S/D+N which should be good enough for getting

good dynamic range for the receiver.

Victor – 4Z4ME

Comment by Victor Koren — September 14, 2015 @ 8:46 pm | Reply

Thanks Victor – will bear this in mind. I hadn’t done the calculations but was just going on qualitative reports that

SCAF’s are noisier, rather than attempting any kind of quantitative analysis. One single versatile filter would be a good

solution. Perhaps I’ll attempt a retrofit one of these days,

Dave


4. Really beautifully done Dave. Stunning, so neat an well build as usual. I am still working on an 50W amp for the DSB40

micro you described but when I’ve finished that the Sproutie might just be my next project. Thanks for sharing and please do

the same with your next project. Fokko PH0KKO

Comment by Fokko Veenstra — September 15, 2015 @ 2:07 am | Reply

Fokko – I’ll be interested to know how the Micro 40 works with a 50W amp. Make sure your output is adequately

filtered. Best of luck!

Dave


5. Dave, I am speechless, what a beautiful piece of equipment, you really out did yourself this time. Thanks for sharing those

great videos also. I am envious of your crafting abilities. Please don’t stop.


http://www.youtube.com/w2aew



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http://www.youtube.com/w2aew





Comment by Joel KB6QVI — September 15, 2015 @ 3:29 pm | Reply

Joel – That means a great deal – thank you.


6. I love this construction technique! And this project was so well done! Great Job!

– Steve W4OGM

Comment by Steve Yothment — September 16, 2015 @ 12:35 am | Reply

Thank you Steve!

Dave

Comment by AA7EE — September 16, 2015 @ 12:53 am | Reply

7. Hey, WOW! This is a wonderful peace of art and engineering. Really beautiful and well done and it works great as far as I can

tell seeing the video. Thanks a lot for sharing.

73 de PU1KBD

Comment by Celso — September 16, 2015 @ 1:55 am | Reply

Thank you Celso. It does work quite well. I will try to post more videos and/or recordings in the near future.

Dave


8. Very nice !

Could you tell me where I can find the pads on which you solder the components, please.

Thanks.

Comment by nono — September 16, 2015 @ 4:27 pm | Reply

nono – I thought I had mentioned that in the post, but I see that I had forgotten to include that information. Thanks

for bringing it to my attention. I have corrected it now, but for your info, the pads are from W1REX at QRPMe. The

square ones are MeSQUARES – http://www.qrpme.com/?p=product&id=MES and the ones for the IC’s are MePADS –http://www.qrpme.com/?p=product&id=MEP

I glue them onto the substrate with superglue gel. The liquid form of superglue gets messy and awkward to use. The gel works

a treat.

73,

Dave

AA7EE



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Thanks Pete! Over the relatively limited frequency ranges that the coils cover (often just 1 or 2 MHz), the regeneration

point doesn’t change that much, so at times, it is almost like tuning a superhet. It’s a stable design that works well,

D



Dave


13. Beautiful receiver, Dave! And built like a tank! Appears to be rock solid on the cw and ssb reception. I like how all the

mechanical parts are on the top of the chassis, reminds me of tube construction. I have never used a National dial drive such

as this, it looks to be ultra- smooth.

Comment by Doug — September 19, 2015 @ 8:25 pm | Reply

Thanks Doug. Yes – this National dial and gear drive are very smooth. It makes tuning the receiver a real joy!

73,

Dave

AA7EE


14. Dave, beautiful work…it sounds lovely via YouTube so I can only imagine how it must sound in person. Fantastic!

Comment by Brad WF7T — September 20, 2015 @ 5:21 am | Reply

Thanks Brad – I really appreciate it!

Dave


15. Fabulous receiver, Dave!! I absolutely love the large chassis and wooden cabinet! It’s sort of a throwback to bygone days. All

that’s needed to complete the illusion of a vacuum tube rig is to mount a 115 volt coffeemug warmer (or a 20 watt light

bulb) inside near the top to warm the surface. To a casual observer it would appear to be the genuine article.

It is so kind of you to upload this and share it with like-minded enthusiasts. Keep them coming.

73 de Mick – WB4LSS

Comment by Mick Bradford — September 20, 2015 @ 2:16 pm | Reply

It’s funny you mention the thing about the vacuum tube rig Mick because shortly after completing it, I was wondering if

I should have made it a tube design instead. I do like the power efficiency of solid state though. I keep this receiver

running 24/7 from a small set of gel cells under the desk hooked up to a float charger. They power my K2 as well. The Sproutie

MK II only consumes somewhere in the region of 50mA at 12V on band noise or with the volume low, rising to ~100mA on a

loud station, so I just keep it on all the time!

I am not a member of any local clubs. Although I’m affable in person, I’m a bit of a loner, and not a “joiner” by nature. I do

like to share details of my projects through this blog though. My projects feel complete when I am able to share them. I’m not

trying to give a detailed “step by step” approach, but more of an overall picture, with enough detail to help anyone planning

something similar. One person might just look at the pictures and get an idea for their own project, while someone else might

copy the schematic faithfully. If in some way it helps and gives a reader some ideas, then I think I have contributed in some

small way.




http://gravatar.com/wf7t







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No more big projects for quite a long time though. I’m tuckered out, and need a break! I’m feeling the urge to kick back, listen

to the receiver a bit, put a few more videos online, wind a few more coils, maybe make some tweaks to it, but nothing serious.

The hot weather isn’t helping either!

Thanks for the comments,

Dave


16. Hi Dave!

“C’est beau un homme qui sait ce qu’il fait !” is the first comment that poped up to my mind when I saw your new marvel.

It will push me to upgrade my ugly version of the Sproutie 1 to something with a better look and the new modifications you are

proposing.

All the parts I used came from my junk box but for the version I have in mind I will try to find out better part. I realize how difficult

it is nowadays to find a good variable capacitor, which by the way is one of the most important part of the receiver.

I have a couple of occasions to listen to my receiver in Paris and it is definitively a surprising receiver. My first version was made

during hollidays in a place close from the see and the reception conditions were much better than in the center of Paris.

So, I let you know soon were I am with the new version of the Sproutie.

All the best and 73 from F6GMQ Henri.

Comment by Henri Gérenton — September 21, 2015 @ 2:52 pm | Reply

Henri – I live in a built-up urban area too, and often envy my friends who live in more remote areas, and have much

better shortwave reception. That’s good to know that you tried this little regen away from the city! It might be worth

having an AF output socket on the back, so you can take the AF directly from the detector and use an outboard filter. An

adjustable filter would be fun. There are so many possibilities. If you’re not careful though, you can end up turning a simple

receiver into a very complicated one, which is almost what I did with my Sproutie MK II.

It’s fun to experiment though.

73 to The Sproutie in France from The Sproutie in California!

Dave

AA7EE


17. Hi Dave,

To my surprise I recognise this circuit for the Sproutie Mk2 [Shown in picture 2] – because I happen to be building the same

thing. But mine didn’t come from Sprat – its come from the RSGB Radio and Electronics Cookbook published by Newnes dated 2001.

Virtually everything is the same with the odd component change. Why it interested me is because its listed not as a Regen, but a Q

Multiplier design, with the Q Multiplier being provided by TR2, the bipolar transistor. The original circuit specifies 2N3819 Fets. The

audio filters are not shown nor described in this article while the output stage is given as the usual ‘386 chip – one of the things I

don’t like and don’t want to build into mine, while you’ve chosen a ‘380 which is a better choice.

Meanwhile find your project, craftsmanship and photography simply stunning. A true expression of what homebrew should be about.

Mouth watering









Mouth watering

Comment by Alex — September 28, 2015 @ 5:42 pm | Reply

Alex – I didn’t know this circuit was in the Radio & Electronics Cookbook, but it seems to have popped up in many

places. It is in EMRFD (Experimental Methods in RF Design – an ARRL publication), as well as having been used by

George G3RJV in his column in PW, and in the WBR regen recever. I’m sure it has cropped up in other places too, as it seems

to be reliable and works well.

Oh, yes – those LM386’s. I understand their appeal to constructors who are either looking for an AF output stage with low

quiescent current and/or high gain coupled with circuit simplicity. Some builders like it in the lower gain configuration. I admit

that I have never used it in the lower gain configurations but I understand it is still relatively noisy. I moved to LM380’s

because I wanted to be able to build receivers that I would be comfortable listening to for long periods of time, and which could

put a bit more AF power into a speaker than a ‘386. The LM380 is at this point of course, quite an old chip. I know there are

IC’s out there that are even lower noise. At this point though, in my opinion, any further reduction in noise is mainly academic.

The difference in noise between an LM386 with a 10uF cap between pins 1 and 8, and and LM380 is huge – as you know. I love

having a receiver that is a pleasure to listen to for long periods of time, and all for the price of a chip that costs about a dollar

– a bargain, I think!

I just saw your website. What a multi-faceted site with so many subjects. I will have to take a look later on when I am less

busy.

Thanks for the kind words. The Sproutie MK II was a labor of love, for sure. I am still tweaking the filters, though I am itching

to be able to retire this project and give it a break. You know how it goes though – there is always the desire to make things

work just a bit better. It’s the engineer/tinkerer’s curse!

73,

Dave

AA7EE


18. Dave–

I was upset when you “beat me out” on the HRO dial on ebay—however, seeing the BEAUTIFUL job you’ve done on the finalproduct, my upset has changed to utter pleasure at seeing such an excellent job. Much, much better than I would have done.

Keep the excellent work!!!!

73

Bob Parks

K6AEC

Comment by Bob Parks — September 30, 2015 @ 9:40 pm | Reply

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the sproutie mk ii hf regen receiver _ dave richards aa7ee

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