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! 

Watching Shortwave Broadcast Stations on the TinySA Spectrum Analyser

November 18, 2022 1244 UTC. I was using a TinySA spectrum analyzer to look at noise levels on the 40 meter ham radio band. I also wanted to take a look slightly above the band (in frequency) to see Radio Marti at 7355 kHz. As I was doing this I remembered that Vatican Radio was on the air at 7305 kHz from 1230 UTC to 1245 UTC. So was just going to catch the last moments of that day's transmissions. Sure enough, I caught it, and watched it disappear from the TinySA screen. See the video above.

Radio Marti continued on. In the morning we can hear the rooster recordings from that station. We are using it to test how well our homebrew Direct Conversion receivers avoid AM detection. In the video I mistakenly said these two transmitters were on the air with 250 megawatts. The correct power is 250 kilowatts. Both transmit from Greenville NC. I think the signal from Vatican Radio is stronger here because they are using a different antenna pattern -- Radio Marti is aimed at Cuba.

This reminds me of a cool project I have not yet done:  modifying the TinySA to allow the user to listen to the station: https://soldersmoke.blogspot.com/2021/10/how-to-listen-with-your-tinysa.html  I notice that Dean KK4DAS (my colleague in DC receiver design) was the only commenter on the blog post describing the TinySA mod. TRGHS.  We need to to do this. 

Here are the reports showing when Vatican Radio and Radio Marti were on the air on November 18, 2022: 

A 1966 73 Magazine Article on a Homebrew Permeability Tuned Oscillator (PTO)

In April 1966, Lewis Fitch W4VRV of Columbus, Ohio built a Permeability Tuned Oscillator.  It is remarkably similar to the devices we are building today.  

Lewis opened his article with this: 

Clearly, this guy was one of us! 

His article is filled with good practical advice on VFO construction, with a special focus on PTOs.  I was intrigued by the way his PTO mechanism allowed for the use of a reduction drive.  This would help us avoid the indignity of attaching a digital frequency counter to such a quintessentially analog device.   

Check it out: 
https://worldradiohistory.com/Archive-DX/73-magazine/73-magazine-1966/73-magazine-04-april-1966.pdf

Page 30.  

Thanks to Michael (VE2BVW ?) for suggesting that I dig up some old 73 Magazine articles on PTOs.  A quick search revealed that there weren't many.  If anyone out there knows of good PTO articles in the ham or EE literature, please let me know. 

The 2Q -- A Homebrew Solid-State Drake 2-B from 1967

This is really an amazing project.  Way back in 1967 (that's 55 years ago) John Aggers W5ETT of Ponca City, Oklahoma decided to homebrew a solid state version of our beloved Drake 2-B receiver.  Triple conversion.  No crystal filters.  Twenty two discrete transistors and no ICs. Tuned circuits at 50 kHz to provide most of the selectivity.  And he did it.  Just look at the picture above.  It even LOOKS like a Drake 2-B.  

I sent this to our friend Dale Parfitt, W4OP who more recently built a receiver like this.  He too was amazed by this project.  

The article by John Aggers is very clear and provides a lot of good information on how he designed and built this receiver using the technology of 1967 and junk box parts.  I was struck by the lack of diode ring mixers. And I was somewhat taken aback by his use of plug-in socketed transistors.  The AF amplifier is our still-familiar transformer-less push-pull complementary pair design. John did a wonderful job on the mechanical tuning and slide rule mechanism. 

Three cheers for John Aggers W5ETT.  This article is a reminder of the great benefit to the hobby of writing up a project and putting out there in the world.  Here we are, more than half a century later, reading John's article and learning from it.  FB OM.   

https://worldradiohistory.com/Archive-DX/73-magazine/73-magazine-1967/73-magazine-10-october-1967.pdf

Page 8

SDR Direct Sampling: The End of Homebrewing (as we know it)

I sometimes hear hams claim that our efforts to build simple direct conversion receivers are "very relevant" to modern technology and are "directly applicable" to today's communications techniques.  These hams will say that direct conversion receivers are at the heart of modern rigs. 

That's a nice thought, and it might have been true in the past, but I don't think it is true anymore.  

I think the future is what you see written on the black box (!) that encloses the receiver in the above video: "DIRECT SAMPLING RECEIVER."   In the recent past we did have two direct conversion receivers in the front end of SDR receiving systems.  These receivers produced I and Q signals that were fed into the computer (often via the sound card).  That was nice.  

But the writing has been on the wall for a long time.  There is no longer a need for all that direct conversion and I and Q.  Just put a fast Analog-to-Digital converter chip at the front end, convert the entire HF spectrum to a digital stream, and send that stream to your computer.  Or to another part of your "rig."  As in the ubiquitous 7300. 

I don't mean to be a Luddite here.  That big waterfall is very nice.  The receiver sounds great.  But I am a homebrewer and I prefer to build my own gear.  Ordering this black box on my phone,  having it delivered by Bezos to my front step, and then updating the driver, is not what I consider homebrew radio.  

A couple of things I spotted:  The Si5351 chip in the box -- at least one part was recognizable.  And the completely vertical skirts on all the SSB signals -- lots of 7300s out there.  

Hey, to each his own, YMMV, whatever floats your boat.  Just don't kid yourself into thinking that our beloved DC receivers are still somehow being used in these modern black boxes. 

Amazingly Cool MONTV Video on Direct Conversion Receivers with Glue Stick PTOs

I think this is one of Nick's best videos. And he has made a lot of good ones. 

https://www.youtube.com/watch?v=xl9dnajkYO0

This is a really excellent description of how a Direct Conversion receiver works. But more importantly Nick really captures the joy of building one of these receivers using discrete, analog components, including a Permeability Tuned Oscillator made from our beloved Glue Sticks.  

Extra mojo comes in the form of a mixer designed by Pete Juliano using J310s to simulate a 40673 dual gate MOSFET.  Fantastic.  Icing on the cake comes from a W8DIZ AF amp out of SPRAT magazine. 

There is a grand finale.  I won't spoil it.  Watch the video. Suffice it to say that Farhan would be pleased with this.  

Great stuff.  Thanks Nick!  

A Treasure Trove of Permeability Tuned Oscillator (PTO) Info and Links (Plus Info on Direct Conversion Receivers)

There is really great info on this page, and even more in the links at the bottom of it.  While the page is about PTOs, the links often discuss their use in Direct Conversion Receivers. I really liked the Tin Ear receiver.   And it was great to again come across the work of Alan Yates VK2ZAY.   Alan very admirably admits that laziness caused him to use an LM386 audio amplifier in place of a more virtuous discrete transistor design. 

 https://qrpbuilder.com/pto_mechanism

I bought one of the qrpbuilder PTO kits and I will soon put it together.  I have been having good results with a Glue Stick PTO and with a brass screw PTO form designed by Farhan and 3D printed for me by Dean KK4DAS. 

LET'S GO PTO! 

Dhaka Jack Moves to France

Jack Welch AI4SV has been an important member of the SolderSmoke community for many years.  I remember fondly our Straight Key Night CW contact in which he told me that my HT-37 had "presence" even on CW.  His thoughtful (!) piece on time crystals was also quite memorable.  Jack has finally settled down (a bit) after a string of foreign assignments.  He has landed happily in France, in a villa, on a vineyard, surrounded by wild boar and hunters.  FB OM. 

Hi Bill & Pete,

I've packed up the shack and moved from Cyprus to France, so no more 5B4APL. To obtain a French callsign, you have to submit proof that you've lived in France for three months, so I'm F/AI4SV until December and then we'll see. 

I'm not sure how long we will be here, but probably a few years at least. Since we know next to nothing about French real estate, we are renting for the first couple years -- a château on the outskirts of Bordeaux. Before you think that I've come down with delusions of grandeur, I should point out that in that area, château means an old, stone house that is hard to heat in the winter -- and particularly difficult to run wiring around. Antennas and grounding are going to be particularly challenging. The selling point for the house was not so much my hobby as its location in wine country. In fact, there is a Sauterne my house's name on it (although I have nothing to do with production of the wine, that's in professional hands).

Back in the early days of Soldersmoke, Bill used to occasionally mention the dreaded Italian wild boar, the cinghiale. I didn't think that would ever be terribly relevant to me, but it is. A couple days after arriving in the Bordeaux suburbs, a sanglier (French cousin of the cinghiale) strolled across a road as I came around a bend. We almost had a month-long supply of bacon, but I managed to steer around him.

Since it will be a while before all our belongings arrive and even longer to set up a proper station, I have focused on operating QRP in the field and activating SOTA summits. That has gone well, but I aborted my most recent attempt when I ran into a bunch of orange-clad rifle-toting hunters who were combing the mountain in search of sanglier. Apparently it's a big thing here. I decided to survive to activate the peak on another day.

Finally, I have attached a journal article, which at first glance doesn't seem to have a lot to do with radio, but kind of does. It turns out that both the human ear and violins have non-linear characteristics that cause them to function as audio frequency mixers. Looking through the article, you'll find some familiar looking formulas about mixing products, harmonics and resonance. If Bill wants to get away from ICs, perhaps his next rig could include a 17th Century Italian violin as a mixing stage.

Cheers & 73,

Jack
F/AI4SV

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

Hello Jack:  

Great to hear from you.  Wow, France!  You are rivaling my string of nice-to-go assignments.  FB OM.  Have fun.  

Yes, the Cingales.   Hunting season was always a bit of an uneasy time.  We used to dress the kids up in reflective vests.   One time we found a very drunk Italian hunter wandering around with a shotgun (that was kind of scary).  We would know when hunting season started by the sound of gunfire in the morning.    Kind of reminded me of other places!  

In retirement I have gotten back into VWS.  We are having a lot of fun.  Just yesterday 30 students at the Thomas Jefferson High School got their Technician licenses. They will soon build Direct Conversion receivers. 

As for mixing, what you sent reminded me of my early confusion on this subject.  In the SS book I describe the "Terzo Suono"  -- it is really just an additive heterodyne.  I confused it 

with a true mixing product.  But it was an educational confusion. 

Please keep in touch and let us know how things are going at the Villa!  

73  Bill 

Direct Conversion Receiver -- Simple 2-Diode Mixer Defeats Radio Marti, but Diode Ring is the Best

 Here is another update on Direct Conversion receiver construction. In Northern Virginia we get very strong signals from the Radio Marti transmitter in Greenville NC. During the morning hours it is just above the 40 meter band at 7335 kHz. In the evening it is a bit higher in frequency at 7435 KHz. (in the video above I mistakenly give the morning frequency, when in fact they were on the higher evening frequency). In either case, Radio Marti has been a big source of unwanted AM breakthrough in our simple DC receivers. It now serves as something of a test of our bandpass filters and mixers.

In this video I try out the simple mixer described in detail here: https://soldersmoke.blogspot.com/2022...

The following morning, I tested the mixer with Radio Marti (in fact) on 7335 kHz. By adjusting the VFO signal input to the minimum value needed to turn on the diodes, I was able to bring Radio Marti AM breakthrough to minimal levels. But I could still hear it (weakly) in the background. Putting a very simple diplexer at the audio output of the mixer (just a .1uF capacitor in series with a 47 ohm resistor to ground) helped a lot.

I could also hear break through from Spanish-language broadcasts from Vatican Radio on 7305 kHz (using the 250 kW transmitter in Greenville NC) from 11:30-11:45. Perhaps most surprisingly, I was also getting AM breakthrough from 40 meter FT8!

Here is a short video showing the simple two-diode mixer in action during the morning hours:

I also tried out the more common two diode mixer with trifilar toroid. (In this one, the VFO turns both diodes on, then turns both of them off). The results were similar to what I got with the other two diode mixer.

We are trying to develop four circuits -- bandpass filter, mixer, variable frequency oscillator, and audio amplifier -- that will be simple enough for construction by high school students, but not so simple as to compromise performance. We want the receiver to work well.

So far, my conclusion is that the best results come from the diode rig mixer with two trifilar toroids. Here is a short video showing the diode ring in action on the morning of November 9, 2022:

Understanding a Very Simple Two-Diode Mixer

 

Take a look at the simple little mixer above.  I think I first saw it in SPRAT.  Thinking that it was really just a simplified version of the two diode Doug DeMaw mixer that I had been using for years, I couple of years ago I built it into a little Direct Conversion receiver.  It worked great.  But later, I began to have doubts about it.  In the words of young James Clerk Maxwell, I started to wonder about "the particular go of it." 

You see, the way the DeMaw mixer is set up,  both of the diodes are simultaneously on and off.  This has the effect of "chopping up" the incoming RF at a rate set by the VFO frequency.  Boom.  Fournier.  Mixing.  Great.  

But look at the mixer at the top of this post.  Here the VFO signal is coming in on the wiper of the 1k pot. The same signal is hitting both diodes at the same time.  The diodes are not being fed differentially.  So D1 and D2 are NOT both simultaneously tuning on and off.  Instead, when the wiper goes positive, D2 turns on while D1 is off.  On negative swings of the voltage at the wiper, D1 turns on while D2 is off.  For me, this made it a "mystery mixer." 

This reminded me of the sub-harmonic DC receiver I built earlier in the year:  The VFO runs at half the operating frequency, but the diodes are set up to switch on and sample the RF TWICE each VFO cycle.  This is the equivalent of having the VFO at the operating frequency.  

Could it be that this was just a sub-harmonic mixer with the VFO at the operating frequency? (I should note that Doug DeMaw published a design that actually made this mistake.  See:  https://soldersmoke.blogspot.com/2011/07/doug-demay-and-polyakov.html ) I knew that this would sort of work, but it would not work very well.  And the mystery mixer seemed to work very well.  Hmmm. 

I was loaning the DC receiver with the mystery mixer in it to a local high school.  I worried that I was loaning them something that I didn't really understand. I remembered that I'd been trying to figure out this mixer since early 2021:  https://soldersmoke.blogspot.com/2021/02/some-thoughts-on-singly-balanced-mixers.html  

Our beloved book, Solid State Design for the Radio Amateur (SSDRA) has an explanation of this circuit on page 74.  But this explanation didn't seen to work for me.  Check it out. YMMV. 

Bottom line:  I still couldn't figure this circuit out, so left it alone for while.  

The other day I woke up and looked at it with fresh eyes.  Suddenly it hit me.  Although the VFO was hitting the diodes in the same non-differential way as is done in the sub-harmonic mixer,  the RF (signal) is entering the mixer in a differential way.  This means that the two diodes are taking turns sampling the upper side of L2, then bottom side of L2, via L1 and L2.  This results in a complex repeating waveform that is similar to that of diode ring mixer.  Within that complex repeating waveform, there are sum and difference frequencies. I did some noodling on this: 

The key difference between this mixer and the sub-harmonic mixer is the way L2 is positioned:  In the sub-harmonic mixer, there is no differential feed of the RF.  Both diodes get the same polarity of RF.  The VFO switches on D1, then D2.  The RF is sampled at twice the VFO frequency.    But in the mystery mixer that had me scratching my head, the RF is fed to the diodes in differential form.  So while the diodes here are -- as in the sub-harmonic mixer -- being switched on and off sequentially, they are taking turns sampling the top and the bottom of L2.  That provides the complex repeating waveform that we need to get the sum and difference frequencies.  In a DC receiver the difference frequency is audio. 

What do you guys think?  Do I have this right?  How would you characterize this mixer:  Is it multiplying by 1 and 0?  Or is it multiplying by 1 and -1? 

This would be good mixer for a school project.  It is simpler than a mixer with a tri-filar toroid. 

SolderSmoke Podcast #241 Mars, Direct Conversion, PTOs and Glue Sticks, Anniversary of the BITX20, Multus Proficio SDR, Boatanchor Station, MAILBAG

The board I use to test DC RX circuits 

SolderSmoke Podcast #241 is available

Audio (podcast):  http://soldersmoke.com/soldersmoke241.mp3

Video (YouTube): (215) SolderSmoke Podcast #241 October 28, 2022 - YouTube

Introduction:

Back on Mars.  Opposition approaching.  I have a Mars filter.  And (like T.O.M.) a Mars globe. 

N2CQR DXCC done

SolderSmoke in the WayBack Machine

Sticker news

PARTS CANDY -- Don't Scrimp with a Crimp! 

Bill's Bench

School DC RX projects -- in Hyderabad and Northern Virginia. 

Direct Conversion Receivers -- Keeping it Simple, Learning a Lot.  A step beyond the Michigan Mighty Mite. Do we really need 100db?  Do we really need to shield VFOs?  Farhan's super-simple and stable Colpitts PTO.  Audio amps, 1000-8 transformers and rolling your own LM386

PTOs and Glue Stick PTOs.  Paul Clark WA1MAC. Brass vs. Steel bolts.  #20 thread vs. #28 thread.  Backlash Blues. The best Glue Sticks. 

2 meters and the VWS.  Bill has a Baofeng. 

SHAMELESS COMMERCE:  MOSTLY DIY RF

Pete's Bench

20th Anniversary of the BITX20   Pete's early BITX rigs. 

Computer Woes

The Multus Proficio SDR rig

Simple SSB in China  BA7LNN

Things of beauty: Tempo One, NCX-3 and a SBE-33

MAILBAG

-- NS7V is listening.

-- Graham G3MFJ sent SPRAT on a stick.

-- Nick M0NTV  FB Glue Stick and 17 Shelf videos.

-- Dino KL0S HP8640 Junior

-- Mark AA7TA   Read the SolderSmoke Book

-- Steve EI5DD Connaught (Ireland) Regional News

-- Dave K8WPE  Planting the seeds of ham radio interest

-- Peter VK3YE Ruler idea on PTO frequency readout

-- Michael AG5VG Glue Stick PTO

-- Tobias  A polymath with UK and Italy connections.  And cool tattoos.

-- Alain F4EIT   French DC receiver

-- Michael S.  was in USMC, working on PCM/TDM gear

-- Alan Yates writes up Amazon transformer problem

-- Todd VE7BPO,  Dale W4OP, Wes W7ZOI

-- Farhan VU2ESE sent me an sBITX

-- Todd K7TFC   The Revenge of Analog

-- Jim Olds    Building QRP HB gear  

The Multus SDR rig Pete discussed

The older rigs Pete mentioned

My version of DC RX that Farhan is working on

My PTO with VK3YE's ruler frequency readout

DC Receiver: 100db Gain? Diplexer? VFO in a box?

Here is a progress report on Direct Conversion Receiver developments. Dual Tuned Circuit, Diode Ring with Diplexer, PTO VFO from Farhan's Daylight rig, two stage 49 db BJT AF amp with a transformer. It works very well. I discuss: Shielding of the VFO -- necessary or not? Why brass in the PTO? Do we really need 100db in a receiver, especially with ear buds? Sourcing the AF amplifier's transformer. Using W7EL's diplexer. (I think it has solved my Radio Marti breakthrough problem). Developing a DC RX circuit that can be built by students. I end with a bandsweep of 40 meters that includes CW, FT-8, SSB, and AM