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Hi Ben! Thanks very much. I've never tried using a single DPDT relay in this kind of circuit (well ... almost never!) but my main reason is to keep as much physical distance between the input and output to try and prevent unwanted coupling etc. Most of this comes from (bad) experience of switching RF amplifiers in and out of circuit. It's amazing how easy it is to build an oscillator when you really just want an amplifier! For small signals through a filter you might be able to manage it though.Thanks again, Best wishes, Nick

Hi there! Good question. It was an arbitary choice. The proper way to establish the quiescent collector current is by Load Line Analysis which I really didn't want to get into in the video. In truth, my choice of 10mA is a bit higher than it should be. My own analysis shows that for my design, then 8.5mA is just about ideal. It'll still work with 10 though. If you are interested check out this excellent 4-part series on Load Line Analysis: ruclips.net/video/dOv_nsBvYTM/видео.html. Thanks for watching and for your question. Best Wishes, Nick

Thanks very much. Best wishes, Nick

Thanks Nate. Yes, I'll be starting work on the next one soon. Thanks for watching and commenting. Best wishes, Nick

Hi Peter, well done for persevering! The current through the voltage divider just needs to be at least 10 times the base current. If you model it with LTSpice you can calculate what LTSpice thinks the Beta is (about 308 in my case). If you want the numbers to match up then you'll need to use this figure (not your measured one) in your calculations for R2 and R1. Real circuits with real components will also have some variations because of tolerance etc. This stuff can honestly take a while to get your head around. But once you've got it ...73, Nick

Thank you very much Ronald. Best wishes, Nick

Hi Frank, thanks for watching and commenting. Yes, I quite enjoy having a look at some of the homebrew creations that have gone before. There is always something you can learn I find. Best wishes, Nick

This particular design is an audio amplifier. Although you can put music through it I will only be using it in a Direct Conversion Radio Receiver so the range of interest will be 300 Hz - 3 kHz. You can of course calculate the capacitor values for other frequencies. If you are going to use it for RF though just be aware that you will need some impedance matching. Best wishes, Nick

Because we know we need 1.9V on the base (comprising VBE (0.7V) + VE (1.2V)). So that's why we've dropped 1.9V over R2. This means there is only Vcc - 1.9V remaining (i.e. 12 - 1.9 = 10.1V) and so this must all drop over R1. This is to satisfy Kirchhoff's Voltage Law about the the sum of all the voltage differences around a closed loop must be zero. Hope this helps. Best Wishes, Nick

@@M0NTVHomebrewing it helps thanks

Thank you very much. Best wishes, Nick

You are welcome Simon. Best wishes, Nick

Thanks very much Dylon. I'm glad it made sense. Best wishes, Nick

Thanks Ed! Glad it was useful. Hope you're doing OK. Best wishes, Nick

Cheers Andy! Best wishes, Nick

@@M0NTVHomebrewing :)

Hi there, the input power is basically just the voltage * current and the same for the output. It is a very low powered amplifier. As I stated - forget HiFi! It was an experiment into how simple I could make it. If you want more output power that will comfortably drive an 8Ω speaker then you probably want to consider one of the other amps I showcased in the video. But ... it works - at least for me. Best Wishes, Nick

Sure, I get your design goals. My comment is motivated by the various amplifier videos I’ve watched on RUclips; they all take a similar approach: ‘let’s make the collector current this, and therefore the gain is this based on the Beta’, or ‘let’s make a gain of 10’. But it seems to me that an ideal system has a gain to fully drive the speaker at max volume, and no more. In that way, you get lower distortion, minimal current draw. Compared with a gain that overdrives the speaker and isn’t useful. I’m just curious about this approach!

Regardless, I’m going to join you in building a homebrew direct conversion receiver! Looking forward to the rest of the series….

When it comes to the gain you can bypass the emitter resistor with a cap - but you can also put a smaller resistor in series with the cap to get more precise control over the gain. I'm going to tackle this in the next video when we add the AC signal into the mix.

Thanks. Good to have you on board!

Thanks very much. Glad it was helpful. Best wishes, Nick

Hi Mike, thanks very much for watching and for sharing your thoughts. Best wishes, Nick

Your notation is odd to me please explain further.

​@@andymouseWith such resistor values, V of the base will be 2.05V (1.826 V LT Spice). This means that a signal with an amplitude higher than 2.05-0.7=1.35V will be cut off.

​​@@andymouseIn the scheme of the emitter repeater, the voltage at the base of the transistor is V=6.746 (LT Spice). Such transistors do not exist in nature. Greetings from Ukraine.

@@mikepaul2831 Thanks :)

Thanks very much. I'm running LTSpice (on a Mac). It's free and there is a huge user base which means plenty of people to ask when you get stuck! You can download it here: www.analog.com/en/resources/design-tools-and-calculators/ltspice-simulator.html?ADICID=PDSR_EMEA_Power-Management-LTSpice-NB_Google_Mult_202410&gad_source=1&gbraid=0AAAAACxqTx93ynCBN7P6ldtCDjBRBLK1q&gclid=Cj0KCQjwjY64BhCaARIsAIfc7YaCGr6t1mqlAvB6tgAb2VrqlOZJROvEP4T6aBsGwbSzZyeJz50zHy4aAkR4EALw_wcB

Thank you very much Heinz. 73, Nick

Cheers Peter. Thanks for watching and commenting. 73, Nick

Hi Dean, thanks for watching and commenting. Yes, you can do (Vcc-Ve)/2. But when am I ever going to pass up the chance to go the scenic route?!! 73, Nick

Hi! I've not had any issues with stability over lower current levels. You do need to watch higher currents though. It might be necessary to increase the supply voltage in order to achieve some higher currents. The most critical thing in the current source configuration I've found is actually the voltage levels. You need the sufficient headroom for the circuit to perform properly. Having said all this most of my use case is around 10mA - which is about right for 1N5711s in a diode ring mixer. As always, your mileage may vary! Thanks for watching and commenting. 73, Nick

You are very welcome. Thank you for watching and commenting. 73, Nick

Thank you very much. Glad it was of some interest. 73, Nick

Cheers Andy. Yes, it's true that sometimes the simple rule of thumb is more than sufficient! Thanks for watching and commenting. 73, Nick

Hi William, thanks very much for your kind words. I'm glad it made some sense! Look after yourself. 73, Nick

What makes you said that he did better job than your physics teacher? From the Philippines...

Hi Joao, thank you very much. I'm pleased that the video helped and very well done on breadboarding the amplifier. Take care. 73, Nick

Thank you Louis. I have to admit that it has taken me a while to really get my head around all this stuff myself ... and I'm still learning I'm pleased to say. Glad it helped. 73, Nick

Thanks so much as always Ace. I'm pleased it was useful. Look after yourself. 73, Nick

Hi Markus, Thanks very much indeed for your kind words. Thanks also for filling me in on the Licensing changes in Germany - I wasn't aware this was happening. Hopefully it will encourage a few more folk to give ham radio a try. Please don't apologise for your English - it is MUCH better than my German! Nochmals vielen Dank. Machs gut, mein Freund. 73, Nick

Thank you very much indeed Chris. I'm certainly not adverse to using an op-amp or two myself but I find there is something strangely empowering in understanding how the humble transistor amplifier works. Thanks again. 73, Nick

​@@M0NTVHomebrewingCan you please explain why the Collector junction will be out of phase by 180 degrees?

Good question. It is because of the way the transistor operates. The phase inversion happens because an increase in input signal causes a corresponding decrease in the output signal, and vice versa, due to the interaction between the base-emitter current and the collector voltage across the load resistor. When the input signal increases and makes the base-emitter junction conduct more (increasing Ic ), the voltage at the collector decreases. Conversely, when the input signal decreases, the base-emitter junction conducts less (decreasing Ic ), and the voltage at the collector increases. This relationship results in a 180-degree phase shift between the input signal at the base and the output signal at the collector. Hope this helps. Best wishes, Nick

Thanks very much Jacob. 73, Nick

Cheers Mike. Glad it was some help. 73, Nick

Thanks very much Alfredo! 73, Nick

Thanks very much. All the very best with your own projects. 73, Nick

I know! I had to buy those tins without even getting any mints inside. It's never right!

Hi! By all means send me an email (address on QRZ.com). Just be aware that I don't have a lot of time but I'll do my best! 73, Nick

Hi there, apologies that the schematic is a bit pixelated and probably not too clear. What looks like "20Ω" is actually "ZO" i.e. impedance output. In my case I designed it for 50Ω but you could actually build it for different impedances. All of my resistors are 51Ω. The bifilar wire is about 22 - 24 SWG. As long as you can get the turns comfortably on the toroid then it's not too critical. Thanks for watching. 73, Nick

@@M0NTVHomebrewing Thanks for all the Info 👍👍.. Because, I very much plan to build one.. (Could be) as you say.. Could be Handy. Thanks Again and 73s

You are welcome! 73, Nick

Thank you very much for your kind words. I'm really pleased you found the video helpful. Best wishes, Nick

Thanks for watching! 73, Nick

Hi Ben, there is no provision in Jason Mildrum's Calibration software to change the test frequency. According to his comments in the README that accompany the Etherkit_Si5351 library as long as you get the calibration right for 10MHz then you are good to go. I guess you could play around with it yourself. It would be interesting to know how well the calibration holds up for higher frequencies. I've only ever done the 10MHz test and the Si5351 has been fine for me up to lower VHF frequencies (up to about 100MHz). As always your mileage may vary! Thanks for the interesting question. 73, Nick

Hi there. I guess it depends on how much current you are pulling through those TIP transistors. I've not noticed mine getting so hot that I've had to resort to heat sinking but you are right that this would be a good idea. If by distortion you are referring to crossover distortion then this is largely avoided by running the transistors in class AB with some DC bias. Both of the amps with push pull configurations that I show in the video are like this. I have to say I'm quite interested in some of the higher switch mode amplifier classes like E. I've never delved into these before but the efficiency is wonderful and ideal for battery operation. Something else for me to explore ... Thanks for watching and commenting. 73, Nick

Thanks very much. Yes, you are absolutely right about the speaker. Other (enclosed) speakers I've tried give me more lower frequencies. Once it is in an enclosure it will probably do for the 300 Hz - 3 kHz range I'll be using it for. As I say, not remotely Hi-Fi but good enough! 73, Nick

Thanks very much! 73, Nick

Hi Barry, when you say "wrapping the toroid" what do you mean?

You put 16 loops around the toroid. Those loops wrap 80% around the ring. That counts as another 0,8 wrap, if I remember my EE course correctly. It was a long time ago. Might explain why you always have to remove at least 1 wrap.

Thanks Barry, I've not come across this before. Something to check out. 73, Nick

Thanks for sharing. That sounds great! I think the class AB push pull is a winning combo really - if you want a discrete transistor amp that actually generates some power. In truth I was going to design one of these (and I did) but I wanted to see how simple I could make an amplifier and it still be practically useable (at least for me). Thanks again and well done on your own design. 73. Nick

Thanks very much Ace! Hope you are doing OK. Let me know how you get on if you do have a go at one of the designs. All power to your elbow! 73, Nick