Half Of The ZL2BMI DSB Transceiver – A Simple 80M Direct Conversion Receiver :-)

A few weeks ago, Jason NT7S mentioned the ZL2BMI DSB transceiver as a rig I might be interested in building. He was right – I had seen it in SPRAT but for some reason hadn’t seriously considered making it into a project. The mention from him somehow got me to take another look at it and, well, it was such a simple design, it didn’t seem as if I had anything to lose by giving it the old college try.

The great thing about building a transceiver is that if the transmitter part doesn’t work, you’ve still got a receiver. That’s what happened to me (or at least, until I figure out my problems with the transmitter.) Everything was working fine up to and including the building of the driver stage. Once I added the BD139-16 final, I started experiencing problems with a constant residual carrier being transmitted when no modulation was present.

However…….once I had finished the first part of the build, which consisted of building the receiver, I took a little time to enjoy listening to the receiver and generally being surprised that such a small collection of parts would allow me to listen to the ragchewing on 75M. Little things like touching a wire to the antenna terminal and hearing atmospheric noise coming out of the speaker always give me a kick.

Here’s the schematic for my (only ever so slightly different) version of the receiver part of the ZL2BMI DSB transceiver:


There are no RF or AF gain controls in this schematic.  The circuit is still on a board, not yet in an enclosure, and in the experimental stage. If I ever get it into a box, I’ll add an RF attenuator pot in the antenna input circuit. This is even easier to build than a receiver with a VFO, as there are fewer toroids to wind. In fact, there is only only one – the single antenna input inductor.  Coverage was about 3911 – 4009KHz, so I didn’t bother winding a rubbering inductor, figuring that 100KHz of coverage was already pretty good for such a circuit.

The frequency drift wasn’t very encouraging. I was expecting a little better from a ceramic resonator VXO, being around 200 – 300Hz/hour upward drift after an initial 15 minute warm-up period.  The free-running VFO in my 40M NE602 CW DC RX had a better stability – on 7MHz! This board wasn’t in an enclosure though, whereas the 40M receiver was.  I wonder if that could have made the difference?

However, the receiver sounded pretty good, and there wasn’t much to it:

Here’s another view:

Looking at the schematic, you’ll see that as well as the 0.1uF coupling capacitor from pin 5 of the NE602 to pin 3 of the LM386, there are 2 x 0.1uF decoupling capacitors – one from pin 5 of the NE602 to ground, and the other from pin 3 of the LM386 to ground. I saw a wonderful looking version of the ZL2BMI transceiver built by a ham in the Czech Republic, but looking at his schematic, saw that he left out these 2 decoupling capacitors. There was a 0.1uF coupling cap from the output of the NE602 to the input of the LM386 and that was it. I thought that perhaps he was onto something, so I also left out these 2 caps. Well, they are quite important. The 3 capacitors between them form a kind of simple diplexer – as far as I can tell from my limited knowledge. With just the coupling cap, I was hearing stations, but was also hearing breakthrough from nearby strong in-band signals. Adding the 0.1uF from the output of the NE602 to ground cut out the breakthrough as well as cutting down on some of the higher-frequencies in the audio. Adding the second 0.1uF cap from the input of the LM386 to ground helped shape the audio a bit more and cut down on some more of the higher frequencies, making the receiver more pleasant to listen to for long periods.

I shouldn’t admit this in public, but my first thought on seeing the 3-capacitor network that connects the two chips was “How can a 0.1uF cap bypass RF to ground, when the same value is also coupling audio to the input of the next circuit? If the 0.1uF coupling cap passes audio to the next stage, why doesn’t the 0.1uF bypass cap short all the audio to ground?”

After a bit of thinking, I realized that the capacitors form RC filters with the circuit impedances which determine which frequencies they pass and which they don’t. Imagine you’re an audio signal coming out of the NE602 and heading towards the coupling cap for the input of the LM386. You are going to see that 0.1uF capacitance as well as the input impedance of the LM386, which is about 10K. The 3dB cut-off frequency of this high-pass filter is given by:

But what about those 0.1uF caps to ground? Well, they also form high-pass filters, and the impedance in this case is the impedance of the connection to ground which, if the capacitor is connected properly to the ground plane, should be very low. Therefore, the cut-off frequency of this simple filter is much higher. Audio frequencies are blocked and subsequently passed on to the LM386 AF amp, while RF is bypassed to ground.

While pondering the really nice-looking ZL2BMI rig that had been built by the the Czech ham, I decided to do some testing of my own.  Maybe he had a reason for leaving out those 2 coupling caps?  I decided to replicate this in my circuit and found very quickly that without at least one of those 2 bypass caps, the circuit experiences breakthrough from strong stations on nearby frequencies.  I made a recording with and without one of the bypass caps, and here are the results:

If I were going to build this as a simple little receiver to listen to the ragchewing and general chat on 75 and 80M, instead of using a ceramic resonator, I would use a varactor-tuned free-running VFO as with my Hi-Per-Mite DC RX so that I could cover a wider portion of the band. I’d also use a double-tuned bandpass filter for the antenna input and include an RF attenuation pot as well as possibly an AF gain pot.

Naturally, these simple receivers have their limitations, and it doesn’t stop me from dreaming about owning a Ten-Tec RX-340, but I get a real kick out of receiving good-sounding signals from a handful of parts.


63 thoughts on “Half Of The ZL2BMI DSB Transceiver – A Simple 80M Direct Conversion Receiver :-)

    1. Thank you Rogier. I think that part of the appeal is the close-up nature of the pictures. To be honest, my projects don’t look quite as nice in real life, but I’ll happily accept your compliment!

    1. Anil – Indeed I have seen the mods by G3XIZ. In fact, I incorporated his microphone pre-amp into the TX section of my ZL2BMI transceiver. I also tried feeding the driver through a capacitor the way that he does, instead of the transformer arrangement that ZL2BMI uses, but reverted back to the original arrangement. I’ll decide which method to use when I get the TX section working. Thanks for your suggestion.

  1. What a beauty Dave! It’s such a treat to see the circuit laid out like you’ve done, manhattan-style. Everything is visible and the routing is easily seen, easily worked on. I’ve put this project on my list of things to do.

    1. poweredbynature – I think the appeal is the simplicity of the circuit – and it does work, as you can hear from the video. Let me know how it goes when you build it please!

  2. 22uH for the choke of the driver transistor is too much. May be 2,2 uH. If realimentation persist, shunt a 1 K resistor in paralell with this choke.
    Regards. Edgardo Maffia LU1AR – Buenos Aires – Argentina

    1. Thank you Edgardo. This project has been sitting on a shelf for the last few months, but I will try your ideas when I start working on it again. 73!

  3. I am presently building this transceiver. I have gotten the receiver to work pretty well and I have got the transmitter to give some output but it was self oscillation. Still working to debug it. I have been using a carbon mic like the original but it is not working out too well either. Think I will try the mic circuit you show.
    Regr=ards, Steve N4IOR

    1. I had the same experience as you Steve – got the receiver part working well, but found that my transmitter was self-oscillating. I shelved the project but really need to get back to it and attempt to solve the problem. There are quite a few working ZL2BMI transceivers around, so the issue can certainly be resolved!


  4. Dear Dave: You are’n takin advantage of the balanced nature of the NE602 and LM386 IC’s. If you put both ends of the input coil to 1 & 2 pins and decouple to ground one of them tru a 100 nf capacitor and connect pin 4 of 602 to pin 2 of the 386 using same scheme of the pins 5 and 3, you gain 20 dB of common mode regection to AM powerfull stations.
    On the other hand, you can balance better the remaining carrier adding a 500 k trimpot with the wipper grounded, thru a couple of 100 K resistors to the pins 1 & 2 of the 602. This don’t affect in eception. Disbalancing one pin 1 or 2 of the 602 to ground thru 22 K resistor, you generate full power carrier, very usefull to adjust. You mus decouple the end of the resistor with 10uF electrolytic to avoid hum in the carrier.
    Don’t forget the rejection ratio is only 30 dB. In SSB the carrier is in the side of the filter and this is not very trouble. In DSB you must play with the polarization of the driver stage, to be non linear for small signals. Like a squelch. Small signal no output. A bit more give you full power.
    I hope this will be usefull.
    I enjoy making small QRP equipment. Look at my site http://www.lu1ar.blogspot.com.ar
    Edgardo Maffia LU1AR
    Buenos Aires – Argentina

    1. Great input Edgardo – thank you. I love your blog too – that 50MHz transceiver looks very interesting!


    1. You can use any varactor that has a fairly wide capacitance swing Dan. Look at this webpage for test data on the MV1662 – http://www.hamelectronicsmagazine.com/MV1662.html It looks like it has a capacitance of about 430pF at 2V and 180pF at 8V. If you can’t find a suitable varactor, try a variable capacitor instead. That would make the circuit simpler, as you could leave out the 100K resistor, the 0.1uF capacitor and the 10-turn pot (as well as the varactor of course). The maximum operating voltage of the NE602 is 8V, so the regulator should probably be for a lower voltage – say, 6V.

  5. the support for the coil cant be any tip, or specific t 50 2 type. i will try to use one from a power suply for start until i locate the one needed

      1. when it is don i am going to send you some pictures of my small project the idea is that i want to us it for field work so it is going to run of a 12 v 20 A battery , on the part of the audio amp i decided to repaced with a TDA 2822 it is stereo runs from 1,8v to 12 V

  6. sory for these questions i know they sound stupid but is the first time i built a receiver of these tip i am used building other tips that don t include integrated circuits

      1. Yes you can Daniel. They are interchangeable. It’s a fun little receiver. If you want to build it for 40M, hamshop.cz have 7.2MHz ceramic resonators. You’d just have to wind the input coil for 40M instead.



  7. I am having big problems the rig it is built but it does not work . The amp part works very well but no signal from the NE602. There are on the market a number of pint tips for the NE602?? If it is a yes then please send to me the models you know.

    1. Dan – I’m sorry but I don’t understand your question. I had the exact opposite problem – everything in my rig worked except for the driver and PA. Actually, they did work but were suffering from RF feedbakc/parasitics which I think was due to the driver and PA not being sufficiently shielded from the rest of the rig.

      Is your receiver working? Getting the receiver to work should be the first thing. If it’s not working, check to see if the oscillator works (by listening for it on another receiver). Let me know how it goes. I’m not that great at troubleshooting though, I’m afraid


  8. I am thinking I did the connections all wrong on the NE602 but that is a maybe that is why I am asking if there are other pin configurations to the NE602 . I am thinking to put a standard 4 MHz crystal in place of the ceramic resist I am having big problems locating on of 4 MHz only one of 4,5 MHz

    1. There are not any other pin configurations for the NE602/SA612 chips that I know of.

      Are you in the US Dan? Mouser have 4MHz ceramic resonators on this page – http://www.mouser.com/Passive-Components/Frequency-Control-Timing-Devices/Resonators/_/N-6zu9h?P=1z0wo7jZ1z0z5h6&Ns=Pricing|0

      This one looks good – http://www.mouser.com/ProductDetail/ABRACON/HWZT-400MD/?qs=sGAEpiMZZMsBj6bBr9Q9aZwSgYMCcq7%2fWM3hRdgxV%252bE%3d

      Mouser will ship via first class mail if you ask them, to keep the shipping costs down. They also have thru-hole resonators for 3.88 and 3.84MHz, though not in stock, so who knows when they will get them back. Worth keeping an eye open for though.


  9. well I managed to get a varicap diod from an old tuner bloc I had it is a BA and it should do the job I managed to do something whit the coil I wound it on a carcass with ferrite bed from the tuner I found I added 28 turns to the secondery I will keep you updated of what I am modifying and maybe you can aply them to your tuner if they are good updates 2 this project

    1. Eric – I have learned from my experience with this build and realized that I need to provide better isolation between the driver/PA and earlier stages of the TX. Thank you so much for the design! I am currently working on VK3YE’s Micro 40, which incorporates elements of your design too (as you know). I have an idea to rebuild yours at some stage as a xtal-controlled rig on a single freq for my daily net on 40M.

      I’m looking forward to seeing news of your developments.



  10. good news it is starting to work but not just yet I changed the ferit ring with a ferit rod also I found a ceramic resist that i order. I stead o radio amator band a lot of local radios I changed the diod again with 1N4148 it is inly 4 pF so 2 or maybe 3 in paralele a have to do a lot of modifying until it will work until then I will use my old trusty tuner wit 2 tranzistors for 80 m Yo10dan out

  11. KISS at its finest! Sorry for being late to the party, but this is the best most simple design I’ve seen for receiving. A few questions: How does the audio sound in comparision to your Hi Per Mite build for CW? I know that is a great filter, but KISS tells me leave that for a later improvement!

    Do you have a function generator? Let’s say you have one, and want to test the receiver on different frequencies – How would you replace the oscillator section with a function generator? One recommendation was to just connect the function generator to pin 6 via a decoupling (.1uf) capacitor)

    Lastly, Have you tried using axial inductors in an input filter circuit before the NE602? I’ve seen examples of non-toroid (axial inductor) circuits that output 50 ohm impedance, but I’m not sure how to match the 50 ohm output of a filter to the 1.5k input of the NE602. WITHOUT a transforming toroid. I know I shouldn’t be afraid of toroids, but remember, I’m shooting for KISS and toroids are not KISS!

    Thanks for reading my babble,

    Jeff W8ZLW

    1. P.S. I’m very into making printed circuit boards and would love to colaborate. I’m planning make this simple receiver via open source. Of course I will send you copies of boards to you for free for you to play with!

    2. Jeff – sorry for the delay in replying but I have been busy with a new project, and it is taking up a lot of space in my head.

      The audio in this receiver is a lot more open than that in The Rugster, due to the very basic audio filtering. It is virtually wide open. If you’re used to hearing more complex receivers with narrow filtering, it takes a bit of getting used to. I have come to like it, but not everyone does.

      I have not used an external oscillator to drive one of these receivers, but your suggestion sounds about right. If you find you have too much drive, you can reduce the value of the coupling capacitor.

      I have not used molded axial inductors in place of the higher-Q toroidal coils for the input filter. It is a very broadband filter anyway, so they would probably work fine. As for matching, I don’t think this circuit does much of a job of matching the antenna to the 1.5K input impedance of the 602. If your version provides a mismatch, you may not notice the difference anyway!

      PS – Have you tried using a crochet hook to pull the wire through the core when winding toroids? I use one about 2mm in size, and it helps greatly in pulling the wire snug against the core.

      73 for now,


  12. I want to build a simple direct conversion receiver by replicating your circuit using a low cost DDS as local ocillator. However, I do not know how to make the NE602 to output the upper sideband component of a given signal so I can make it to work on 15m and above amateur bands. Can you please help? Thanks in advance!

    1. David – direct conversion receivers are not single-signal receivers. As you tune through an SSB signal on a DC receiver, you place your local oscillator on both sides of the signal – and hear the result. On one side, the speech frequencies are inverted and the signal will be unintelligible. Whatever band you are listening to, you simply tune to the side of the signal that produces an intelligible signal.

      I hope this helps.



      1. This makes perfect sense! Thanks so much for the super fast response. I really appreciate it. 73!

      2. You’re welcome David. A simple direct conversion receiver like this is capable of receiving both LSB and USB (as well as CW) on any frequency that it is tuned to, so go for it! If you plan to use it over a wide range of frequencies, it might also be an idea to include some kind of preselector circuit to at least provide some protection from strong out of band signals. If you just want to use it in specific bands, such as ham bands, then a series of switched input filters would work. It sounds like a fun idea for experimentation!


  13. If I were to use a variable capacitor instead of a varactor diode where do I connect it? A potentiometer has 3 connections. By the way
    Love your layout,very neat indeed.

    Kindest regards


    1. You’d connect it in place of the varactor, Simon. As the pot is only used to supply a control voltage to the varactor, you don’t need it if you use a variable capacitor. You also don’t need the 100K resistor or 0.1uF cap. Remember that physical stabiity is important if you use a variable capacitor, so make the wires to it short and stiff. Good luck!

      1. Thankyou very much for the information most appreciated and very helpful…I have built the receiver and it works although I only hear mostly faint Morse signals I have yet to hear any voice there seems to be more static hiss than signals lol.i. Was wondering what guage wire you used for the antenna input torroid coil? I have used what I had to hand it’s quite thin guage ,I’m thinking would it make a difference if I was to use thicker wire?

        Sorry for the questions I’m relatively new to the field.

        Kindest regards


  14. Simon – the gauge of the wire you use is not that important. With toroids, the gauge of the wire has very little effect on the inductance of the coil. The more important considerations are that if you use too thick a gauge, you won’t fit all of the turns on the core and if you use too thin a gauge, the toroid won’t be self-supporting. So don’t worry about your wire gauge too much.

    What frequency is the resonator you used? That determines the part of the band you are on. Also – what country are you in? The 80M band is big in the US, covering 3.5 – 4MHz, but in other parts of the world, it’s smaller. In the UK, it’s 3.5 – 3.8MHz. This could affect what you are hearing too. FYI, this little receiver was quite loud into a small speaker when I built it.

    PS – apologies for not replying in the same thread but, for some reason, WordPress only allows 2 replies in the same thread. Also – absolutely no need to apologize for the questions. Asking questions is how you get answers, and how you learn. I do it all the time – in fact, sometimes, I think that all I do is ask questions!

    1. Hi dave.
      I live in the UK,and using a 4mhz ceramic resonator..I do hear signals all of which are morse.the audio is quite loud.I built a rudimentary morse transmitter using just 5 components and a 4mhz ceramic resonator and connected the battery and the receiver belted out a very loud tone.so there is definitely nothing wrong with the audio.it must be that 80meters is more popular in the US.
      I will buy some more crystals and experiment more.i find this all rather exciting building your own gear,there is nothing more rewarding.my overall goal is to build a qrp transceiver and learn morse and start qso’ing preferably on a frequency that don’t mind newbies.
      Thanks again dave for you’re input,it’s really helped me out.

      1. Simon – the UK amateur band doesn’t go as high as the US one. In the UK, it’s 3.5 – 3.8MHz, which is why you’re not hearing much. I don’t know if they’re still available, but there used to be 3.68MHz resonators. If you can find one of those, that should give you coverage from very roughly 3.6 – 3.7 MHz, which is the bottom part of the phone portion of the UK 80M band. You should hear more then. Let me know if you’re successful in tracking down a 3.68MHz resonator. Feel free to e-mail me. My e-mail is on my QRZ page (you need to have a log-in to view it).

        PS – really glad you’re having fun building little receivers. They are great fun. I grew up in a small village in your part of the UK. We could QSO on 80M with our simple homebrew gear if I still lived there!

  15. Hi Richard,
    We have built the Micro 40. The receiver is great, but we are not getting much out of the transmitter. Can you tell us what was the maximum power you were getting out of yours.
    Best wishes
    Walden and George

    1. George –

      I never measured it exactly, but it was less than a watt – probably somewhere in the region of 0.75W. Glad your receiver is working well. I find building receivers much more fun than transmitters!



      1. Hi Dave,
        Thanks for coming back to me. That’s about the power we were getting, and we thought it was a bit low, but it will be fine for us as we don’t live many miles away. I wish I could keep my boards as clean as yours, do you spray the lacquer on before you mount the components.
        Regards George

      2. Yes George, lacquer sprayed on before mounting pads and components. I scrape the lacquer off with a sharp blade when mounting pads, and also scrape the underside of the pad, to help the superglue work it’s magic. I used to scrape the board before soldering to it, but then found out that it’s not necessary to do that.

        I can’t help wondering if there is a relatively easy fix to get more power out of this rig. A little more would be good.

        73 for now,


  16. Hello, your style of building receivers is one that i adore, the manhaton is fantastic way to go if you want to keep the over all project cost down, i have used this style and the island style too, your 75 m receiver i built to and here in france living in the limosin region next to the massive-central i get just about all four corners of the world passing through the few discreet components on the board, its just so incredible, my next project is to build 40 meter usb/lsb receiver, and a switched transmiter, just like the old days, i enjoyed your project and you have inspired me to get back into my hobby. thanks!!…

  17. could someone explain to me the capacitor network on the output pin 5 of the mixer?

    I assume the first cap is to shunt RF and plus component to ground. Second cap I expected to be a larger value so as to be low impedance to the AF…
    if it’s a pi network the values still don’t make sense to me.

    1. I’m a little busy right now McGurkryan and don’t have time to type a complete explanation. Those capacitors form high pass filters, so you also have to take into consideration the resistance/impedance in the network. The 0.1uF capacitor that is connected to pin 5 of the NE602 goes to ground. The impedance of the connection to ground is very low, placing the filter cut-off in the RF range. However, the 0.1uF coupling capacitor is grounded via the input impedance present at pin 3 of the LM386, which puts the cut-off the simple filter network in the AF region.

      Hope that helps!

      73 for now,


  18. I’m in the process of building this circuit, learning a ton! I was wondering how you backed into the calculations for the input filter windings of 4t and 25t for the t50-2. How can I use the tool found at http://toroids.info/T50-2.php to learn how to modify the primary and secondary windings for the different bands?

    1. James – it’s been 8 years since I built this little receiver. I don’t remember how I came up with the numbers of turns. It’s possible that I got them from a similar design without questioning them but, at this point, I don’t remember, I’m afraid.

      To figure out the values for a different band, let’s use 40M as an example. 7.15MHz puts you in the middle of the US 40M band (sorrry, I don’t know where you are). Let’s try a 220pF capacitor. Entering 220pF and 7.15MHz into Diz’ calculator gives 21 turns. If you wanted to use a 270pF capacitor instead, you’d use 19 turns.

      As for the number of turns on the primary, I don’t think there were any impedance transformation calculations involved. I’m pretty sure it was simply a case of winding a few turns on the primary, without too much thought. There could be room for improvement here. If you were ever to get serious about further developing this circuit, it would be nice to have a double-tuned bandpass filter feeding the NE602, to better protect the mixer from out-of-band signals. However, it’s a nice, simple circuit that works quite well, and there is some virtue in keeping it that way.

      Hope that helps a little.

      73 for now


  19. Thanks Dave, I’ll do some more research on the number of turns, appreciate the help. Yes I may try building a bandpass filter, for reasons you’ll see below.

    I tried breadboarding the circuit but I’m having a ton of trouble, you can see pictures of my work here: https://imgur.com/a/3ZH8myY. I followed your circuit minus two things: 1) I’m using a JDS6600 DDS to inject a sine wave in the 3900-4000KHz range into pin 6 of the NE602 with a voltage of 300mV, and I’m using a 1N5234B 6.2V zener diode for ping 6 of the LM386, as that is all I have on hand. For antenna, I’m using a MFJ end fed half wave 10-80M antenna that works great with my other radios.

    The audio quality has a ton of static and pops, and I’m only able to hear a very powerful shortwave radio station no matter what frequency I set my DDS to. My question, is this a circuit that I should put together on a piece of copper and not a breadboard? Would the bandpass filter help as well?

    Wondering where I went wrong. Any advice would be greatly appreciated!


  20. James – I am so sorry for waiting far too long to reply. It has been a while since I did any building, and I have been really dropping the ball on replying to questions.

    The few times I have breadboarded my RF projects, I have found that they don’t work as well as when I build them over a ground plane. The other thought I’m having is that the NE602 does tend to overload quite easily. The antenna input circuit of this circuit is extremely unselective, and with the antenna you’re using, it’s possible you’re overloading the NE602, causing it to pickup strong stations on other frequencies.

    73 for now


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