SolderSmoke Challenge -- Direct-Conversion Receiver Video #5 -- Building the Audio Frequency Amplifier

 SolderSmoke Challenge – Direct Conversion Receiver – the Audio Amplifier Build and Receiver Demonstration

 In this episode Dean, KK4DAS wraps up the initial build of the SolderSmoke Challenge DCR.  He takes us through the audio amplifier stage and demonstrates the newly built receiver just moments after connecting the audio module to the mixer and hooking up a speaker.  The audio module is conceptually simple – three nearly identical stock-standard common emitter audio amplifiers which provide the 80-100 dB of amplification you need to go from microvolts of RF to volts of audio to drive a speaker.  The challenge with all that amplification in a small board is to keep it amplifying but to stop it from oscillating and as you’ll see in the video, good construction technique is critical to good performance.

 

And wow!   We now have more than 30 builders working on the SolderSmoke Challenge, with more builders completing the challenge every day.  And those are just the ones who are active on tour Discord server.   If you have completed the DCR or even if you have just started building it, we want to hear from you.  Send a picture or better yet a video – make it a selfie and you can join the SolderSmoke challenge hall of fame!

 

And if you are not yet building it, you must ask yourself one question: 

 

What are you waiting for?

  

Join the discussion - SolderSmoke Discord Server:

https://discord.gg/Fu6B7yGxx2

 

Documentation on Hackaday:

https://hackaday.io/project/190327-high-schoolers-build-a-radio-receiver

 

SolderSmoke YouTube channel:

https://www.youtube.com/@soldersmoke

 

SolderSmoke blog DCR posts:

https://soldersmoke.blogspot.com/search/label/TJ%20DC%20RX

 

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!

Mr. Carlson Restores a BC-348 -- But 40 Meters Sounds Very Weak. Why?

Mr. Carlson mentions that he is a ham radio AMer.  He is also on SSB, but AM is, he says, his preferred mode.  FB. 

I got a real chuckle about the MVC switch position on the front panel.  "Manual Volume Control"  like "Manual Gain Control."  I thought I had invented this term.  You know, "real hams control their gain manually."  This is why Farhan never put AGC in the BITX20. But it seems the Army Air Corps was way ahead of me with the BC-348.    

Mr. Carlson makes it sound (!) like the Dynamotors are a thing of the past.  Not true.  Every Saturday morning I listen to the Old Miltary Radio Net and hear a number of Dynamoters spinning in the background.  One belongs to Buzz W3EMB who uses a BC-348.  Buzz has an SDR attached to the IF of his BC-348, which I think is an admirable mix of the old and the new.  Those BC-348s are, after all, quite old.  WWII old -- like 85 years old.  And still working.  

Paul has a good discussion of the importance of short lead length, and of mounting shielded capacitors properly, and of the usefullness of a good groundplane under the capacitor.  Go Manhattan boards!  Paul's presentation on how to identify the outside foil of a capacitor was very good, but I was wondering if you could also find out by using a file to remove some of the yellow insulation, then test with a DMM to see which terminal is connected to the foil. 

"Lots of times you have to add solder to remove solder."  Indeed. 

The importance of testing for BOTH capacitance and leakage.  Yes. 

But why bother with "period correct" internal wiring harnesses, when you have already put a bunch of modern caps in there?   I mean, I'm in favor of the re-capping, but this seems inconsistent with the need for "period correct" internal wiring harnesses. 

When Paul first fired up the receiver, I was hoping he would disconnect the antenna to see how much band noise was getting through. 

When Paul got to the IF alignment, he spoke of the dangers of working on energized high-voltage gear.  I had been thinking about getting a BC-348 myelf, but Paul's comments reminded me of why this is probably not a good idea for me.  Paul's comments about "knowing where your hands are" is on target.  "One hand behind the back, " is a good rule for this kind of energized testing -- this will help prevent current from a mishap from flowing through your chest.  

I may have more comments later.  Off to the beach now.  

Back from the beach:   

"Contrary to popular belief, the simpler the receiver, the better they hear."  Amen Mr. C. 

Paul's heroic cleaning of the 915 kc crystal made me feel like a wimp for not having tried to do this with some "bad" 455 kc rocks I was given while trying to build the Mate for the Mighty Midget receiver.  I may go back to those crystals and try to clean them as Mr. C did. 

The grand finale of this 2.5 hour video was, as expected, a demonstration of how good it sounds.  But unfortunately it did not sound good.  Paul tuned through the 40 meter band and I heard NO CW signals.  I didn't even hear FT8.   There were a few anemic SSB signals and, as he reached the upper portion of the band some very weak AM broadcast signals.  I didn't even hear CHU Canada's time signal.  Huh?  Why?   Our very simple homebrew Direct Conversion receiver sounds a LOT better than that.  I mean look at the sweep of 40 meters that I did using only a simple dipole:  https://soldersmoke.blogspot.com/2024/12/an-evening-bandscan-on-40-meters-using.html  Why is there such a big difference in performance? 

Could it be the antenna that Paul was using?   He was on what he calls the 369 antenna.   Could there be a problem in the receiver?  Could it be band conditions?   

SolderSmoke Challenge – Direct Conversion Receiver – the Band Pass Filter

 The SolderSmoke DCR challenge is going well.  Our Discord server is bustling with activity and we are impressed that several intrepid homebrewers have already completed the receiver.  So,  we know that you can too!  In this episode Dean, KK4DAS walks us through the design and construction of third of our four boards, the 40-meter band pass filter.  The band pass filter ensures that the only signals that get through the receiver to the speaker are those that are in the 40-meter band.  Dean also  gives an update on the Challenge  and discusses some recent improvements we've made to the circuits based on feedback from our builders. 

 

 

Join the discussion - SolderSmoke Discord Server:

https://discord.gg/Fu6B7yGxx2

 

Documentation on Hackaday:

https://hackaday.io/project/190327-high-schoolers-build-a-radio-receiver

 

SolderSmoke YouTube channel:

https://www.youtube.com/@soldersmoke

 

SolderSmoke blog DCR posts:

https://soldersmoke.blogspot.com/search/label/TJ%20DC%20RX

USAF Video: Why SSB is Superior to Ordinary AM

OK, the title of this blog post has an element of click bait in it.  Just this morning I was listening to the Old Military Radio Net on 3885 AM.  I'm sure some of those guys would disagree with the assertion that SSB is better than AM.  But I think this old Air Traffic Command video lays it out very well.  

I kind wish they had dicussed Double Sideband, Suppressed Carrier transmission.  

I wonder want the year of production of this video was.  Does anybody know?

For the record:  I am the owner of a DX-40, a DX-60, and a DX-100.  I also am on 10 meter AM with a converted CB rig.   So I have transmitted both sidebands, and the carrier.  

 

Update from KK4DAS on the SolderSmoke Direct Conversion Challenge: BP Filter Added to Circuitry -- Full Video on BP Filter Soon

Dean now has the PTO, the Diode Ring, and now the 40 meter bandpass filter.  Dean fires up his signal generator and, using his 'scope we see audio coming out of the mixer.  FB.  

Dean says he will have the complete build video for this stage soon.  Thanks Dean. 

 

Radio Antenna Fundamentals Part 1 -- 1947 USAF Film

Lots of good info in this 1947 film.  But... At the end they say that an impedance mismatch between the line and the antenna will result in the loss of radiated power.  Is that really true?  This seems to lead to the obsession that many hams have with an SWR of 1:1.  If some power is reflected due to the mismatch, where does that power go?  And what if you put a transmatch at the junction of the transmitter and, say, an open wire feedline.  You could match the antenna system perfectly to the 50 ohms that your transmitter is looking for.  But the mismatch between antenna and feedline would remain, right?   

Video of images and Data Stream: Huygens Probe Parachuting onto Titan

This happened back in 2005, but I saw this video for the first time this morning.  I really like the display.  I frequently see Titan next to Saturn with the 6 inch reflector that Elisa got me. 

And I like parachutes. 

Diode Ring VFO Part II: How Much LO into a Diode Ring?

Last week we were trying to determine how much LO injection we really need in the SolderSmoke Direct-Conversion receiver.  The answer seemed to be "enough to turn the diodes in the diode ring on and off."  Ok, but this brought us to the question of how far we should go with this.  Does it make sense to go for more LO signal? If so, why? And how much more?   Todd VE7BPO offered a very thoughtful comment.  He pointed out that for a simple receiver like this, turning the diodes on and off would probably be sufficient.  Sometimes we hear 7 dbm, others say 10 dbm, or even 0 dbm.  But what is the logic that underpins these figures?   Solid State Design for the Radio Amateur (SSDRA) provides the answer on page 120.  See above.  

With a diode ring (or other switching mixer) you want the LO (VFO or PTO) to be the signal that is switching the diodes  You do not want the incoming RF signal to also be strong enough to switch the diodes.  Having the RF do this would result in something of a mess at the output.  

If you have a weak LO signal going into the mixer, it might on peaks reach the level of turning the diodes on.  You will get some mixing action.  But as the SSDRA paragraph indicates, during much of the LO cycle the diodes will not be switched on.  And they won't be firmly turned off either.   A strong RF signal could come in, add to the LO voltage, and switch the diodes.  That would not be good. 

So if you put a strong LO signal in there, on half the cycle that signal will be turning two of the diodes on.  But on the other half of the signal, that same LO signal will bereversed in polarity,  turning those same diodes off.  Hard off.  Definitively off.  It would take one very strong RF signal to overcome the reverse bias signal put on those two diodes by that LO voltage.  That is the advantage of a stronger LO signal.  

 

Other Workshops: Making The World's Smallest Motor

When I watched this very cool video, I thought of all those hams who complain so bitterly about how difficult it is to wind a toriod on a T-50 core. 

Thanks to Hack-A-Day for alerting us to this. 

 

First 10 Meter AM Contact from Hi7/N2CQR

I threw this in the bag on the way to the DR, almost as an afterthought.  But I am glad I did.  Today, watching reports on my 10 meter CW beacon (Mike WN2A provided the keyer),  I noticed that my signal was unusually strong.  At W3POA I was 7 db above the noise.  At DF2CK I was 4 db above the noise.  So I switched to the 10 meter AM rig (which is a converted GE brand CB rig) and worked John G3YPZ (see video above).   I was running about 5 watts to a quarter wave vertical.  But the signal reached the UK. 

Here are some details on my rig: 

https://soldersmoke.blogspot.com/2025/01/looking-for-contacts-on-10-meter-am.html

And https://soldersmoke.blogspot.com/2023/11/bill-n2cqr-goes-citizen-band.html

Thanks John!  

Fireball (Meteor) Over the Dominican Republic

Click on the link below to see the video

https://www.tiktok.com/@william.de.jesus01/video/7464040455185993002

I had been telling Mike WN2A how I had put a little red LED on my beacon (he gave me the keying hardware) and that I was hoping to put the LED in the window of my 7th floor shack.  I want it to serve as a kind of CW "lighthouse." We are, after all, close to the ocean.  I thought it would be cool.  Last night, soon after sunset, Elisa and I went out to walk the dog and to look a my little light.  

Well, as expected my little light was too small.  But HOLY COW,  it had us looking in exactly the right direction at exactly the right moment.  At 7:09 pm on January 25, 2025 a very bright, green, long-lasting fireball streaked across the sky from North to South (the guy in the clip above got the direction wrong).  It was throwing off fragments.  One kid in the neighborhood saw it.  And soon Instagram lit up (!) with reports and videos.  I filed a report with the International Meteor Organization: https://fireball.imo.net/members/imo_view/event/2025/516  It was all very cool.  The tiktok clip (above) has a collection of some of the best shots.  Just click on the link below the picture.  

I hadn't seen one of these since March 1995 (I was in the Dominican Republic then too!):  

7 MARCH 95 EVENING: POSTED ON CIS:

Also observed a pretty spectacular fireball in the North (near Polaris) at about 2330. So bright I thought it was a skyrocket. Very slow moving looking like pieces falling off it.