That's a very good little AM radio. A typical AM portable radio these days costs less than your mixer module, and none have such a high quality tuning section, crystal filters, or good bandskirts as you have on what pass for their filters, assuming they have any at all. Your receiver has excellent, natural sound as near as I can tell from the sound coming out of my laptop. No squeals, no hums, no intermod. You're building the good stuff. As a teen in the 1960's I had a good transistorized AM-FM tuner kit I built. I was very proud of it. Good sensitivity, good selectivity, good AGC, good audio. But I noticed that a crystal radio I'd built that I was running through the same amplifier and speakers as the expensive tuner kit actually had better, more natural sounding audio, more dynamic range, simply more 'real' sounding. The kit tuner sounded like a very good kit tuner. The crystal radio sounded like a person in the room. That was when I realized that the more parts you use, the worse a radio actually sounds. Every circuit, and certainly every active circuit adds in some subtle degree of nonlinearity, phase distortion, something not quite right. The high end sophisticated ones just add in less. They can never actually make the sound 'better', more real sounding. So when I see your AM radio design I see a circuit designed with just the minimum of parts, but very high quality parts, and I like it. A few years ago I did a refurb of a very expensive old Marantz FM receiver, and I was impressed with the 'real, live, in the room' sound of the audio. After going through the schematics and the components I realized that it got that sound from using numerous expensive to make, passive LC circuits, not a bunch of active devices, and then well shielding all of those low-level signals. There were just enough active devices to bring up the signal level to what was needed to produce the required output. there were not active gain stages and feedback loops sprinkled around everywhere in the radio. And that was the secret to the natural-sounding audio that made it so valuable, which was a couple thousand dollars, even after so many years. They didn't try to compensate for nonlinearities. They avoided producing them.
Thank you very much for the feedback. I'm certainly very happy with how it is sounding so far. No hiss or other noise between the stations. Hopefully, I can maintain that when I look at the AF amplifier. 73.
@@CharlieMorrisZL2CTM, I don't recall AM radios ever having the sort of inter-station hiss that FM stations do. A little noise, but not that uniform loud Gaussian sort of hiss. I think it's related to the sort of modulation used. FM always has the amplitude of the carriers always cranked up. AM only needs to do that for weak stations. If we had a decent AM station in town I might copy your radio for myself. But all we have here is garbage. Clearly you have a station there that you like. Do they also stream online?
I would welcome extra rf-gain. For years we, in the Netherlands, have been deprived of the big broadcast transmitters. What's left is an ever changing offer of LPAM stations. (between 1 and 100 Watt).This requiers close proximity and/or a very good radio. Absolutely love your radio. Scrambling for parts now. PE1LLA.
Anither great radio..the very neat and artistic work...the oscillator capacitor is a piece of art with that gear arrangements..radiofun242 also made experiment once by varrying gains in different sections ..in rf and if...most am radio have tuned fromt end and 1970 80 radios advertised turned rf stage..you completely skipped tuning the rf ...the second capacitor of gang capacitor could be used for rf front end tank circuit...i saw 3 trf stages in a russian ishim radio...the radio you built sound amazing without any of those...i wander if you may need turned front end for weak am stations and dxing....you vedios helps us a lot and its always fun to watch and listen tou doing these stuff...
I am a bit confused by the fact that you put a 910 ohm resistor to ground on the input of the crystal filter. I think you really want to make the output impedance of the Amp driving the input to the filter 910 ohms. What you've done here is make the input of the filter see 910 ohms in parallel with whatever the output impedance of the driver Amp is.
If he does a test with a RF generator with variable AM modulation frequency, then it may show up any possible issues such as ripple in the frequency response.
I'm afraid you're right. The input impedance of the used rf-amplifiers is about 2300 Ohm. The output about 1400 Ohm. So the 910 Ohm resistors are being shunted by these values. As a result the input 910 Ohm resistor will be brought down to much less than 600 Ohms. But hey, it works and it sounds great !
Beautiful work Charlie. I'm guessing that that variable cap has the rotating vanes (and thus the panel contol) grounded, with the stator vanes going to the circuit. That may be why you can touch it with no frequency shift. FB on NOT having shielding around the VFO. Too often dogma and lore has us putting unnecessary metal boxes around this stage. 73 Bill N2CQR
Thanks Bill. Yes, the rotating vanes are grounded, which adds to the stability. As you know, I prefer not to have enclosures arounds the circuits as I enjoy looking at them and thinking about what each component is doing. 73.
How did you decide on the inductance of the coil in the collector load? Why is a coil beter than a resonance tank or maybe a simple resistor? Also, I think your attempt to match the impedance of the filter is incorrect - if the input filter impedance is 910 ohm, this should be the output impedance of the common emitter amplifier stage. Since for CE config it's the collector load, your collector resistor should be ~910 ohm or equivalent. Shunting with a resistor makes it worse, since now you have 910 ohm in parallel with ~25uH of the coil which is roughly 1600 ohm at 10.7MHz.
Yes, I took a few shortcuts on this particular build. In the past I have made the effort to better impedance match crystal filters with the output/input impedance of the IF amps. While not ideal, it's working well for this particular radio.
@CharlieMorrisZL2CTM Of course, working radio >> perfect theoretical design. On the other hand, this could be a starting point to pretty interesting experiments on fine tuning and squeezing the max performance out of it. If you have a signal generator and basic measurement tools like a decent audio interface and software, you could do a couple of follow-up videos on those. Would be interesting to see the effect of impedance matching on the distortion introduced by the filter stage. Cheers!
