SolderSmoke Direct Conversion Receiver Challenge: A Short Video from Dean about the Virtues of Building the AF Amplifier ONE STAGE AT A TIME

 Dean will soon post his longer video on how to build the fourth and final board of the SolderSmoke Direct Conversion Challenge receiver, but we know many builders are chomping at the bit, and are going ahead with the building of the AF amp board. We hope that this short preliminary video will help. Here Dean descibes the benefit of building the three stage amp, ONE STAGE AT A TIME.

Response to the challenge has been great. There are at least 30 receivers under construction around the world. Many are already inhaling RF. All of the problems that arise with true homebrew are being identified and fixed. Dean's full AF amp video will be out shortly. Thanks Dean!

More Background on the Mythbuster II Rig (and a short video)

Above is a short clip of me hearing an old friend on the new receiver.  TRGHS. 

A couple of guys on Facebook asked for a schematic for this rig.  I don't really have one -- as you will see it is a collection of different circuits from lots of different sources.  I was also thinking that if you need a schematic for a rig like this, you probably shouldn't try to build a rig like this.  But in an effort to be nice, I supply here some background info on some of the circuitry and parts sources.  And no, I don't have BOMs nor Gerber files.   

More info on the Yaesu VFO is here:

https://soldersmoke.blogspot.com/2021/07/mythbuster-video-3-using-vfo-from-yaesu.html

The VFO is kind of high in frequency. But it is very stable. I can't really take credit for this -- the credit goes to Yaesu. But I like the circuit because it is all analog and all discrete component. It even has a split stator temperature compensation variable cap! At Pete's recommendation, I originally bought these boxes thinking that I'd just get the anti-backlash gears and the reduction drives. But the sellers always sent the entire VFO boxes. They even come with a "clarifier" circuit which I now use as an "up 5-10" feature that lets me work the DX-peditions on 20.

As for the tape, I got mine from Amazon. It has conductive adhesive:
https://www.amazon.com/gp/product/B07QQJ4MX1/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

The boards for the IF Termination Insensitive Amplifiers come from Todd K7 TFC at Mostly DIY RF:  https://mostlydiyrf.com/tia/

The little LM386 boards that I have been using are available here: https://www.amazon.com/dp/B00LNACGTY?ref=ppx_yo2ov_dt_b_fed_asin_title  I precede it with one stage of AF amplification from the original BITX 20 sechematic.  I put a 10K pot between this stage and the LM386 board. 

Here is the Amazon source for the 1/8 inch plywood that I will use for the case (I have used up all the pandemic treadmill packing material):  

https://www.amazon.com/dp/B0D5VCBBP4?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1

Bill N2CQR Builds Yet Another Mythbuster Transceiver

 This one is for 20 meters (no need for 75) and will go to the Dominican Republic.

6 crystal filter at 5.2 MHz. VFO from old Yaesu FT-101 Termination Insensitive IF amplifiers using boards from Mostly DIY RF No RF amp ahead of the mixer. First mixer is homebrew diode ring. Bandpass filter has 4 LC circuits. Steep skirts. Low insertion loss. Bal Mod/Product detector has two diodes (singly balanced) Carrier osc is crystal controlled and homebrew. Audio amp starts with a 2N3904 amplifier followed by an LM386 board. Transmitter portion will be done next.

The crystal filter as seen on the Antuino

Filter on the blank board. 

Bandpass filter (-20 db = 0)

VFO box, carrier osc, Bal Mod/Product Detector, AF amps

The Antuino looks at the Crystal Filter

"The Art of Electronics" #8 -- Why Not a Simple Emitter Follower as The AF Output Circuit?

Click on the image for an easier read

Back when Dean KK4DAS and I were trying to find a suitable AF amplifier circuit for our High School Direct Conversion receiver project, we were debating what to use as the final.  One option was the standard NPN-PNP push-push amplifier (like in Figure 2.53 above)  -- an advantage with this one was that it would not require an AF transformer.  But we decided that this circuit would add complexity to a project that we were hoping to keep very simple. 

Another option was a simple common emitter amplifier with a transformer in the collector circuit.  This worked, and was simpler.  We ordered the transformers.  

In the midst of all this, at the local radio club hams asked us why we just didn't put a single emitter follower at the output to handle the impedance transformation to an 8 ohm speaker (sort of as in Figure 2.52 above).  They were convinced this would work.  I was not so convinced and pointed out that we had never seen such a circuit in any of the ham radio literature.  If this could be done, why hadn't the likes of Doug DeMaw and others used this circuit in their many, many rigs?  

This discussion kind of ended there (we opted for the common emitter transformer circuit), but I have thought about it from time to time.  A couple of weeks ago, when I got the second edition of The Art of Electronics, I found the above discussion of the use of this kind of emitter follower circuit.  You can see why this circuit has not been used.  Just to be sure, I built one in LTSpice.  Sure enough, it takes way too much current.   

Thank you, Horowitz and Hill! 

SolderBaloney and SolderTruth

This is so nuts that it actually had me checking to see if this was possibly an April 1 joke.  But alas, it was put out on March 30 and is apparently quite serious.  Rogier PA1ZZ sent me this.  I think his reaction was the same as mine.   My message to Rogier: 

Ha!  He misses the key point:  HOW MANY OXYGEN MOLECULES ARE IN THE SOLDER?   Here at the SolderSmoke Laboratories we only use oxygen-free solder.  And wire.  Sounds MUCH better.  Just kidding.  I think this is audio-fool  hogwash.  I'm sure my friend Ron WA6YOU (a real solder expert) will agree.    How dare this guy in the video besmirch the name of good ole 60/40 Kester!   And even at the local high school, Dean and I were pleased to find that all the students and their teacher had already REJECTED lead-free solder.  Also, this guy seems to think that the lead has been replaced by silver.  Dude, that would be some expensive solder!   His explanation of rosin is, I think, kind of shaky.  Thanks for sharing Rogier.   73  Bill 

Next I suppose we'll have recommendations on how different solder mixes can produce different audio effects:  "For a more mellow sound, go with 60/38 and 2 percent copper!"  

I immediately sent this to my friend Ron WA6YOU, a noted expert on this subject: 

Ron's response: 

OMG!!!!!  I don't know where to start and I haven't even had breakfast yet.  First off Sn60/Pb40 contains NO silver and melts at 361 F and yes I do have my irons in class set to 800 F.  Today in my classes I use Sn63/Pb37 since it is Eclectic....it is either solid or liquid and no in between.  For the cables I build for the satelittes (center pin of SMA male connectors), I use KESTOR Sn62/Pb36/Ag02 which is only 2% silver and called silver bearing.  You can still buy it on  Amazon.  As for flux, I use Kester RMA186 liquid and contrary to Paul, it does not "make the solder melt"....it basically de-oxidizes and provides a path for the solder to flow.  The flux inside the solder melts at 347 F.....BEFORE the solder melts at 361 F thus making the path for flow.  Remember when we were kids and had the little round can of Kester flux that you would dip into?

  

FYI:  To solder the pin of an SMA connector I do NOT rely on the flux inside the solder....Prior to inserting the center conductor of the RG-316/DS (mil-spec double shield teflon)  I apply a "dropllet" of RMA186 on the 7 stranded wire tip and insert it into the pin.....next I apply another "droplet" into the inspection hole (not called a solder hole).  Then I put a droplet of silver bearing solder on the PIN TIP of my iron and apply it directly to the hole in the pin.....magnification is definitely needed.  The solder flows into the hole and I tell my students that if you don't see an outline of the inspection hole, then you used too much solder and the pin will not insert into the into the body of the SMA.  I can make 6 to 8 cables an hour and all are within 1 (ONE!!!) MM tolerance in lengths.  If they want a cable 138 mm long from connector front to front, then it can't be 137 or 139 mm.....yep I can do that and I would love to show you all sometime.  And yes I crimp the ferrule.  I have 8 birds in space and the last three have 80 cables each of mine made in my basement.  

I am just waiting for the day when some USG safety officer comes into my class and has me surrender all "lead solder" and have to replace it with SAC305.  See my handouts below from my lesson plan.  Let me demo this for you all sometime.  It is said that I can put a Type-N male connector on a garden hose......HI HI.

  

Thanks for the link and remember.....The US is one of the few countries still using lead.  Hoard your lead solder.....the day is coming.

73 de Ron, WA6YOU

Solder and flux in use at N2CQR

By Jeff Murray K1NSS (2014)

Update on the High-School Direct Conversion Receiver Project: Board #4 Completed

One of the receivers. (l to r) AF, PTO, Mixer, BP filter

The students continue to make great progress on their direct conversion receivers.  Yesterday they were enthusiastically sitting down to workbenches, building Manhattan-style circuits, working directly from schematic diagrams.  It was really impressive.  They are building receivers. 

This week we had two sessions of about two hours each. We introduced the final board:  the audio amplifier.  Many of the students began work on this amplifier; others were catching up with work on boards presented earlier. 

The AF amp is their most challenging board:  It used 14 Manhattan pads and about 26 components.  We warned the students that amplifiers often aspire to be oscillators.  We told them to pay attention to layout, and to keep their leads short.  

One of my builds, and a Manhattan board pattern

At first, the students just built the first stage on the AF amp board. They tested this, then moved on to build the other two stages.   

By the end of Friday, two groups had completed the build of the AF amplifier board.  

We think there are about 13 receivers in production.  Some are near completion, others will need more work.  

On Thursday of next week those teams that have completed all four boards will put the circuits together and will test the entire system.  They will then add all needed front and back panels and socketry. 

We really want the students to complete as many of these receivers as possible.  Exam season and the end of the school year is approaching, so we have to get this done.  We will remind students that they don't want to that person who ALMOST finished a project!  We will urge them to GET IT DONE!   They can tweak it and mod it later.  This kind of tweaking and modification is part of the homebrew experience. 

We have been presenting awards to the students who are first to complete each stage:  The winners of the PTO board competition got a copy of SolderSmoke:  Global Adventures in Wireless Electronics.   Those who won the mixer competition got a W1REX Hamfest Buddy transmitter.  Thanks Rex!  And this week we presented an award to the students who were the first to complete their bandpass filter.  You've heard of the Tony, the Emmy and the Grammy?  Well, we presented "The Torry"  (from Toroidal).  The trophy was made from a toroidal winding tool made in Alaska by KL7FLR.  I explained to the students who had made it.  Thanks Paul! 

"The Torry"

A Homebrew LM386 -- Does Anyone Want to Build It? Help Save Us All from the Indignity of ICs!

 

I recently commented that I was building a discrete LM386 for a Direct Conversion receiver.  I think I was exaggerating my project.  In fact I just built a very ordinary push-pull amplifier using a 2N3904 and a 2N3906 as a complementary pair AF amplifier.  This is part of an LM386, but there is much more inside that little chip.  Dave went much further.  His schematic is above.  

Here is link to a higher definition image: 

http://soldersmoke.com/homebrewLM386.jpg

 

Dave writes: 

Hello Bill -

Please find enclosed the LT spice circuit for the small audio amp that I developed.  I have included numerous notes on the schematic as to component function, suggested values to tweak, etc. 

My intention was to come up with a relatively low parts-count design while adhering to the classic three-stage topology [diff pair / voltage amp / voltage follower] that has been used as the basis for so many audio power amplifiers for decades.  I'm sure some of the parts could be eliminated at the risk of possible stability issues, but a lot of that also depends on proper layout, length of speaker leads, speaker load impedance, etc.

In any case I thought it would be great if someone wanted to build up this design to see if it works in real life or whether any serious mods are needed to get it to behave (I have no illusion that the spice models are entirely accurate, nor is my analysis thorough).

It should be capable of at least 500mW into an 8 ohm load at 9VDC supply, and over a watt at 12VDC.  It should handle a 4 ohm load although at reduced voltage swing on the output, and with increased output transistor heat dissipation requirements. 

The output stage is a complimentary compound ("sziklai") pair which should, in theory, maximize voltage swing from a low supply voltage (as compared to the more traditional complimentary Darlington configuration. 

The bias current is set by a fixed pair of resistors and could be quite different from the simulated value, so I would be careful on initial power-up to monitor the quiescent current draw.  I would guess it need not be more than a few mA or so, and definitely less than 40mA or so.  This can be made adjustable if desired.  

Regards,

Dave

-----------------

So, does anyone out there want to give this a try?   You could be helping to save generations of homebrewers from the indignity of using an integrated circuit! 

The 17 & 12 SSB Transceiver -- Circuit and Build Info -- Video #4

I REALLY LIKE THIS RIG. IT IS LIKE A MAGIC CARPET THAT CARRIES MY VOICE ACROSS THE SEAS.

Cutting Display Hole sets off smoke alarm. Reverse Polarity Protection. IF and Crystal Filter at 21.470 MHz 50 ohms! TRGHS! Amp for VXO Carrier Oscillator/BFO. Mic Amp from uBITX. Transmit/Receive switching from mic connector. VFO: NO DIE CAST BOXES! HT-37 Variable Cap, Frequency Shift. BP filters from QRP LABS designs (G0UPL). TIA amp boards from K7TFC. Needed RF amp to hear band noise. BITX40 PA design, but RD006HHF1 instead of IRF510. Should I run receiver input through LP filter? Frequency Readout Story: How to use one San Jian counter on two bands.

Save $2970! Build this $30 DIY Microphone!

There are a number of good ideas in this video.  We've known for a long time that many expensive mics are really simple, cheap mic capsules in fancy packaging.  This video takes a $12 mic capsule (which looks a bit like the ones we are using for the podcast), adds come amplification, a USB interface, and some really cool looking bronze packaging to come up with a very nice microphone. 

I liked his use of solder wick as a shield for the DIY mic cable.  Who knew the wick was hollow!  I also like the cooking torch for use in soldering the brass.  I need one of those. The little Murata DC voltage booster with positive and negative output seems useful. 

It seems that anyone working on this kind of project quickly gets pulled into the use of "audio speak." Late in the video Matt says his mic has nice low-end "presence."   A quick look at the comments section shows one person saying that one or the other of the mics sounds a bit "moist."   

There are many other similarly interesting projects on Matt's YouTube channel:

https://www.youtube.com/c/DIYPerks/featured

Trying to Repair Some Old Gear, He Got Hit with a Dose of LSD!

Oh no, here's something else for us to worry about when working on old gear.  As if the treat of electrocution or radioactive poisoning were not enough, now we have to worry about being hit with a dose of  the 1960's drug culture. That could be one bad trip indeed.  Imagine if you were having a hard time troubleshooting the Buchla Model 100.  All of a sudden things start getting weird and your test gear starts dancing on the bench.  

Fortunately, this is not likely to happen with a rig like the DX-100.  With rigs like that the only similar danger is nicotine poisoning. 

https://allthatsinteresting.com/engineer-accidentally-takes-lsd?fbclid=IwAR2KzZl3qoL6oqq5SqWYquKpHR2mGxk2GN2Kk0O-ytUb9MNXmVNX9XnyeaU

Thanks to Stephen Walters for finding this groovy story. 

VBE Multiplier Makes KLH Receiver "Cool Running"

Yesterday I turned to the SolderSmoke wizards for advice on how to fix my KLH Model Twenty-one II FM receiver.   I had finally gotten the thing working -- it wasn't the speaker, it was the AF amp, probably one of the final transistors was blown.  I replaced the finals and the driver. For the finals I used a TIP29C and a TIP30C.  For the driver a 2N3906.    With this fix the receiver was sounding good, but the heat sink on the AF amplifiers was way too hot. 

If you look at the comments in yesterday's post, you will see some great suggestions on how to fix this problem.  The comments and Google led me to Alan W2AEW's YouTube channel and his video on a circuit called the VBE multiplier.  Voltage Base-Emitter multiplier.  I'd never used this circuit before.  It allows you to adjust the bias on the bases of the two transistors in a push-pull amplifier.  

This morning I built the circuit on a small piece of PC board.  There were just two components: a 10k trimmer pot and a 2N3904 transistor.  

With the little board installed, I adjusted the pot for a 1.2 volt difference between the bases of Q6 and Q7. I ended up with base voltage values almost identical to those called for in the KLH schematic. 

The receiver sounds very nice now, and is no longer on the verge of bursting into flames.  I even made up my own version of the pillow that KLH claimed was necessary for proper acoustic suspension.  

Sometimes it is nice to be able to listen to something other than the chatter on the ham bands. And it is fun to do so with a receiver that you have worked on.  

I even used some Desitin as a substitute for heat sink compound. 

Thanks to Rogier for the receiver, to ZL2DEX, K0EET, W2AEW and David McNeill for the good advice. And to Dale K9NN who sent me a box of parts from which emerged the 10k pot I used in this project.  Thanks guys. 73  

An Understandable Chip: The LM386

Over the years I have made my distaste for integrated circuits very clear.  I've presented them as mysterious little black boxes that don't really belong in our homebrew rigs,  rigs that we are building, after all,  because we want to avoid the use of MYSTERIOUS BLACK BOXES.  

So, I don't use voltage regulator chips -- I use Zener diodes.   I prefer analog LC oscillators to AD9850s or Si5351s.  And I have repeatedly built discrete component audio amplifiers when most normal people just put an IC AF amplifier in the circuit. 

When I built the Q-31 Shortwave AM receiver, I kind of ran out of gas at the end.  I wanted to get the receiver going and I didn't want to build yet another discrete AF amp.  So I used an LM386.  

I rationalized this deviation from cherished values by noting that the discrete AF amplifier circuitry that I was using was remarkably similar to what exists inside the LM386.  So that little chip is just as understandable as my discrete component creations.  It wasn't REALLY a mysterious black box... 

Around this time I found a web site that made me feel mush better about all this.  It explains very nicely how the little LM386 does such a great job.  It really packs a lot of amplification into a very small package.   Here is the web site: 

https://www.electrosmash.com/lm386-analysis

Shortly after my transgression,  Jenny List over at Hack-A-Day posted a nice piece looking at the inner workings of Op-Amps.  Who knows, I may seen be using 741 chips too! 

Here is the Hack-A-Day piece: 

https://hackaday.com/2020/05/09/an-op-amp-from-the-ground-up/

Building the Ceramic Discrete Direct Conversion Receiver - Part 3 -- The Audio Amplifier

Once you have achieved JOVO and have the oscillator covering the frequency range you desire, it will be time to build the audio amplifier stage.  Now, I'm sure many of you will be tempted to just throw an LM386 or 741 op amp in there.  But don't do that!   You want to build the WHOLE receiver.  Don't let that IC manufacturer rob you of the FULL HB EXPERIENCE.

The DISCRETE amplifier circuit we will used is based on a design that several of us used to replace the LM386 in the BITX20 receiver. 

OM Arv presents one version here: https://sites.google.com/site/arvidevans/discretecomponentafamplifier

And here is a tutorial that does a GREAT job of explaining how this circuit works.  I strongly recommend you study the tutorial carefully.  This site will allow you to really understand how your amplifier works.  

http://www.electronics-tutorials.ws/amplifier/class-ab-amplifier.html 

It is not that complicated.  But here you have to take care to make sure that your amplifier does not turn into an oscillator.  Keep the outputs away from the inputs.  Keep the leads short.  Some planning is needed here.  Click on the picture below for layout ideas.  The right 1/3 of the board contains the AF amp circuitry. 

After you build the circuit, TEST IT!  If you have an AF signal source and a 'scope, great.  But if not, just hook some earbuds or headphones to the output, connect a 9 V battery and PLACE YOUR FINGER on the input of the first AF amplifier.  If you hear a noticeable increase in hum when your finger makes contact, congratulations, your AF amplifier is amplifying. 

The Complete Schematic

When you get this stage and the oscillator working, you are 3/4 of the way to completion.  Next we will built the mixer, and then the front end filter and amplifier. 

Voltage Regulators as Audio Amplifiers - Who Knew!?

Mike KC7IT pointed out that the AF amplifier chip in my Sawdust regen receiver is actually a TL431 voltage regulator. Mike writes: "It's using the voltage reference input as audio in, and the voltage being regulated as the audio out."  It works great as an audio amplifier, and with just three terminals it is a lot easier to use than our normal LM386 or 741 chips. I'd never seen a regulator used this way.

Here's the data sheet:  http://www.ti.com/lit/gpn/tl431

And here are a couple of sites that discuss this very interesting repurposing:

http://theradioboard.com/rb/viewtopic.php?t=6733

http://techlib.com/electronics/audioamps.html#TL431

When Bypass Caps are Not Enough: Active Decoupling

I was having a noise problem with my NE602 Si5351 OLED display receiver.   There was an annoying high pitched whine in the audio output.  The source was easy to identify:  If I reached in and unplugged the OLED display, the noise disappeared.  

Next I had to find out how the OLED noise was getting into the rest of the receiver.  It could have been through the SCL SDA or even the ground lines.  It could have been just through capacitive or inductive coupling from the display board itself.  A big clue came when I tried powering the display from a completely separate power supply: BINGO! The noise disappeared. So I knew the noise was going into the rest of the receiver through the Vc line that powered the OLED. 

I had been powering the OLED from the 5V regulator on the Arduino Uno. In an effort to isolate the noise, I put a separate 5V regulator in the circuit for the OLED.  No joy -- noise still there.  I then tried putting an RC low pass filter between the OLED and the 5V regulator.  Still had the noise.  Finally I remembered something from the AF AMP circuits of Roy Lewallen, Rick Campbell and Roger Hayward. ( I think Roy was the pioneer on this one.)  They all used an "active decoupler" between the first AF amp and the power supply line.   I confirmed that it was my first AF amp that was picking up the OLED noise.   I built the active decoupler (just three parts!) and the noise disappeared.  GONE! 

There are only three parts, but the way this circuit works is kind of complicated and not very intuitive.   There is a good discussion of how it works here:

www.facstaff.bucknell.edu/dkelley/elec351/Lab/elec351lab5_sp04.doc

Roy, Rick and Roger were using this circuit to knock down 60 Hz AC hum, but I found that my OLED noise was at around 200 Hz -- I figured (correctly) that the active decoupler would take care of this as well.   I think this little circuit can be useful in dealing with the kind of noise generated by the digi displays that many of us are now using.    

David Rowe has a really interesting analysis of this circuit here:
http://www.rowetel.com/?p=4781

Great Hackaday Article on the Venerable LM386

I'm not crazy about chips, but I've come to like the LM386.  It is not really a little black mystery box -- as the article points out, the internal circuitry is simple and completely understandable.  So you shouldn't feel any appliance-op angst when you use one of these. 

http://hackaday.com/2016/12/07/you-can-have-my-lm386s-when-you-pry-them-from-my-cold-dead-hands/