How can I improve this cheap AM transmitter? - Page 1
Forum section: electronics / rf-microwave

author: MaximRecoil
author_rank: Regular Contributor
timestamp: March 29, 2023, 11:58:56 pm
content: It's very low-powered; it only has a range of about a foot. I use it to listen to old radio shows, which are mostly talking, on an AM radio. I feed it an audio signal from my PC's line-level speaker output jack and power it with a 6 VDC, 2-amp "wall wart" type AC adapter power supply.With the transmitter's volume/gain pot turned up all the way, and with my PC's Windows volume control slider at 50%, it has a good signal-to-noise ratio (I can barely hear any static when it's at a comfortable listening volume), and talking sounds okay, but when music plays there's noticeable distortion. If I turn the PC's volume control down to 25% (and turn up the AM radio's volume control to compensate), there's less distortion, but a worse signal-to-noise ratio. If I also turn the transmitter's volume/gain pot down to about 50%, distortion pretty much disappears, but the signal-to-noise ratio gets really bad.As far as I can tell, the transmitter only has 3 active components, i.e., the 3 transistors (they are all S9018, at locations BG1, BG2, and BG3). The lower PCB is just for the optional batteries which I'm not currently using; it doesn't do anything else). Are there better transistors that I could use and/or anything else I could do to improve the amount of low-noise volume I can get before it starts to audibly distort?

author: Kim Christensen
author_rank: Super Contributor
timestamp: March 30, 2023, 12:08:46 am
content: You may be able to compress the audio using the application on your PC. That way you could have the volume set to an optimal level for all situations.I assume you've oriented the radio relative to the transmitter for max signal? (turned the loopsticks for max coupling)Do you have a schematic for this circuit?

author: MaximRecoil
author_rank: Regular Contributor
timestamp: March 30, 2023, 12:49:21 am
content: Quote from: Kim Christensen on March 30, 2023, 12:08:46 amWhat do you mean by "compress"? If you mean file size compression, the stuff I was just listening to is already highly compressed (~16 kbps Opus). If you mean compress the dynamic range, since this stuff was recorded from radio broadcasts to begin with, it already has a compressed dynamic range.QuoteYeah, reception isn't a problem. One of my radios has a signal strength meter, and it shows pretty much maximum strength. Like I said, I can get it to a good listening volume and barely hear any noise/static at all; the problem is the distortion when music plays (the distortion isn't really noticeable when it's just people talking though). It's the same type of distortion you get when you turn anything up beyond its clean amplification capabilities, which is what makes me wonder if different transistors would help.QuoteSurprisingly, I do (along with a PCB layout diagram). I didn't think I did, else I would have included it in my original post:

author: Kim Christensen
author_rank: Super Contributor
timestamp: March 30, 2023, 03:56:17 am
content: QuoteYes, that's what I meant.QuoteQuoteThese two statements are very contradictory to me... Sounds like it doesn't output much power at all, even for something like this. I did notice that the PCB has a connection for an external antenna. You might want to connect a longish wire to the "monitor" connection. Loudness of AM reception is very dependent upon received signal strength, so you may just be overdriving it to compensate.QuoteIt's a very simple circuit. The fidelity of the design won't be that great. Was this a kit or did it come pre-assembled?

author: MaximRecoil
author_rank: Regular Contributor
timestamp: March 30, 2023, 04:56:46 am
content: Quote from: Kim Christensen on March 30, 2023, 03:56:17 amWhen it's within range the reception is great, which is why there's hardly any static at all; much stronger reception than any actual AM radio station here. I have it sitting just a few inches from the radio's internal ferrite rod antenna (or external one in the case of my radio that has the signal strength meter).QuoteI didn't know it had a connection for an external antenna. I wonder why it's called "monitor." I'll definitely try that though.QuoteThe sound is pretty good for spoken word, and even with some types of music there's no noticeable distortion. I first noticed the distortion with some music that had prominent bass in it; the bass notes were very noticeably distorted. Then I noticed it again while listening to a recording of an old Art Bell radio broadcast, during his "bumper music" ("Right Back Where We Started From" by Maxine Nightingale), and it wasn't bass that was distorting this time, but rather, the higher pitched instrumentals at the start of the song.It was a kit that I assembled. I used all the parts that came with it except for the off-brand electrolytic capacitors. I had some new Nichicon capacitors of the right values here already so I used those instead.Aren't the transistors amplifying the signal? If so, could they be replaced with more capable transistors? They wouldn't need to be all that much more powerful.

author: Kim Christensen
author_rank: Super Contributor
timestamp: March 30, 2023, 05:24:29 am
content: AM is supposed to have a limited bandwidth. I noticed that the circuit doesn't have an audio bandpass filter on the input, so this may be the source of the problem with different music vs voice. Audio is normally limited to 5 kHz for AM broadcast. It should probably have a lower audio frequency cutoff as well.ie: By restricting the bandwidth, it would reduce the overall audio level of the music while leaving the audio level of voice pretty much the same.

author: MaximRecoil
author_rank: Regular Contributor
timestamp: March 30, 2023, 06:42:46 am
content: Quote from: Kim Christensen on March 30, 2023, 05:24:29 amLike I mentioned in my first post, the distortion goes away, even on the problematic music, if I turn the signal down enough, but that decreases the signal-to-noise ratio to the point that you get a lot of "static" that you don't want to hear along with the audio that you want to hear. It needs a little more clean amplification than it has, maybe 10% more.Connecting a wire (about 10 feet long) to the "monitor" through-hole didn't do anything that I could notice; didn't increase the reception distance or anything.I did notice that I get a little less distortion (along with a pretty good signal-to-noise ratio) if I turn up the volume in Windows and decrease the volume pot on the AM transmitter, for example, Windows volume at 100% and transmitter volume at about 40% (which is about the opposite of how I normally have it). It seems to do better with a hotter input signal that it doesn't have to amplify as much, than it does with a weaker input signal that it has to amplify more.

author: 807
author_rank: Frequent Contributor
timestamp: April 01, 2023, 10:45:04 pm
content: That circuit modulates the free running oscillator, so will probably introduce a lot of FM. This could make the audio sound distorted.Do you have a scope that you could connect to the monitor output? If you also have an audio generator, you could look at the modulation depth to see if you are getting anywhere near 100%.

author: MaximRecoil
author_rank: Regular Contributor
timestamp: April 02, 2023, 03:49:19 pm
content: Quote from: 807 on April 01, 2023, 10:45:04 pmI don't have an oscilloscope, but clipping seems to be the cause of the audible distortion.I have two ways to control the input signal level: the Windows volume control slider and the pot on the transmitter. I can make the audible distortion disappear if I set the pot to 50% and the Windows slider to 25%, but that results in a lot of static when I turn the radio up loud enough to hear well because the signal is too weak.If I set the pot to 100% and the Windows slider to 25%, everything sounds good and there's hardly any static, except for some distortion in certain types of music. I can also make everything sound very distorted, including talking, by having both the pot and the Windows slider set to 100%. I don't have enough power to reach a good listening volume while having both low-noise and no audible distortion.Is there anything in this transmitter that amplifies the input signal? If there's no amplification going on, that means I only have a line-level signal being converted to radio waves, which is pretty weak.

author: radiolistener
author_rank: Super Contributor
timestamp: April 02, 2023, 06:01:35 pm
content: For a medium wave transmiter large antenna is mandatory required. Your mistake is that you dont' connect antenna to two terminals marked as "Monitor", this is why you have weak signal on receiver.According to this schematic:your transmitter is designed for low impedance output, just because output coil of transformer has low impedance. This means that your transmitter is designed for low impedance antenna, such as magnetic loop. So, in order to get better efficiency you're needs to connect magnetic loop antenna instead of dipole. The simple way is just to connect large wire loop to "Monitor" terminals of your transmitter. Note, that wire loop should have enough cross-section of the loop, this is very important parameter for magnetic loop antenna.As alternative you can wound 1-2 loop around your receiver with a wire and connect that coil to "Monitor" terminals of transmitter. In such way you can maximize reception signal level for your receiver and minimize for other devices which is not covered with such wire loop.PS: I also notice, that there is connection between output coil and GND terminal of the power supply connector. This connection is intended to use power supply cable as a part of antenna. But this is not a good idea, I would suggest to disconnect it and use separate antenna antenna wire connected to both "Monitor" terminal. But don't expect much from such modification, it just allows you to avoid "strange behavior" of your transmitter which depends on power supply cable length in current implementation.Another thing which needs to be taken into account is power supply. For AM transmitter it should be linear power supply with a good filtering. If you use switching power supply such as a phone charger or other SMPS, it can flood your music with a weird noise.

author: MaximRecoil
author_rank: Regular Contributor
timestamp: April 02, 2023, 07:42:36 pm
content: Quote from: radiolistener on April 02, 2023, 06:01:35 pmYou say "monitor terminals" but I only see one terminal that's marked "monitor," so I'm not sure how I'm supposed to connect a loop of wire. Do I connect both ends of the wire loop to the monitor terminal, or do I connect one end to the monitor terminal and the other end to the ground (GND) terminal that's beside it? I tried it both ways. With both ends of the wire loop connected to the monitor terminal, there was no difference at all. With one end connected to the monitor terminal and the other to the GND terminal, it didn't work at all (zero reception). When I removed it reception was instantly restored.I don't think it's a reception problem to begin with. The included ferrite rod transmitting antenna is only a couple inches from the radio's ferrite rod receiving antenna, and there's hardly any static. The audio comes through loud and clear, that is, until certain notes of certain songs play and you hear distortion.The problem seems to be lack of volume in the line-level signal from my PC. Line-level signals aren't meant to be used as-is, they are meant to be amplified, and if this transmitter doesn't amplify the signal (I can't seem to get an answer to the question of whether it does or not, so I'll just assume it doesn't), then that's a problem. In order to get sufficient volume it has to be over-driven, resulting in clipping/distortion.QuoteI'm using a switching power supply that gives the same results as powering it with batteries. I have another switching power supply that I tried too, but it introduced a constant buzzing sound so I ditched that one. I was going to order a linear power supply, but before I did I tried that switching one that I already had on hand, and since it sounded the same as the batteries I didn't bother ordering a linear power supply.

author: radiolistener
author_rank: Super Contributor
timestamp: April 02, 2023, 10:59:46 pm
content: Quote from: MaximRecoil on April 02, 2023, 07:42:36 pmThere are two terminals for antenna on your PCB, they marked as GND and Monitor. I suggest you to break wire between power supply GND and GND terminal of antenna. It minimize effect of power supply cable on your transmitter efficiency.

author: radiolistener
author_rank: Super Contributor
timestamp: April 02, 2023, 11:09:46 pm
content: Quote from: MaximRecoil on April 02, 2023, 07:42:36 pmNoQuote from: MaximRecoil on April 02, 2023, 07:42:36 pmYes. These both terminals are intended for antenna connection. Note that antenna always needs two terminals.Quote from: MaximRecoil on April 02, 2023, 07:42:36 pmAntenna wires should be connected to both - Monitor and GND terminals. One side of wire to Monitor terminal, second side of wire to GND terminal. You can experiment with length of wire and winding count for better reception. Magnetic loop needs to use at least 10-20 meters length of wire and loop needs to be at least 1x1 meter or more.As I remember, resonant frequency of 10 meter piece of wire used as magnetic loop with 12-400 pF variable capacitor is about 3.5-7 MHz. For MW band it needs at least twice longer wire. So, don't expect that it will work good with 1-2 meters of wire like it works for VHF band.If it stops to work, it means that your loop impedance is too small for that circuit. Use more long loop with more windings. If it stops to work, it may be due to too small impedance of your loop - try more longest wire and more winding in a loop. You can also try to use two long wires as a dipole, but it needs much more size than magnetic loop antenna (about 150-200 meters of straight wire).Keep your receiver within transmitter antenna loop, it maximize energy transfer through near field.Also note that magnetic loop should be large enough, for example it can be wire around your room. At medium wave good efficiency antenna needs to have dimension about 150-200 meters.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 03, 2023, 12:27:32 am
content: Quote from: MaximRecoil on March 30, 2023, 12:49:21 amTry shorting all 3 terminals of BG3 to effectively bypass that amplification stage, you'll then have to turn up the volume to compensate.

author: MaximRecoil
author_rank: Regular Contributor
timestamp: April 03, 2023, 03:26:15 am
content: Quote from: radiolistener on April 02, 2023, 11:09:46 pmLike I mentioned in my previous post, I get the exact same results powering the transmitter with the AC adapter power supply as I do with four AA batteries (which takes the power supply and its cable completely out of the equation).Antenna wires should be connected to both - Monitor and GND terminals. One side of wire to Monitor terminal, second side of wire to GND terminal. You can experiment with length of wire and winding count for better reception. Magnetic loop needs to use at least 10-20 meters length of wire and loop needs to be at least 1x1 meter or more.Thanks. I don't have enough wire here to experiment with large loop antennas, and I definitely can't make a magnetic loop antenna, for more reasons than just a lack of wire.Quote from: NiHaoMike on April 03, 2023, 12:27:32 amSo the BG3 transistor is functioning as an amplifier? If so, is there a type of transistor with more amplification that I could replace it with, that would work as-is in the circuit? I don't need the amplification to be all that much stronger, since the way it is right now sounds good for everything I listen to through it aside from certain notes in certain music.By the way, what do you mean by shorting all three terminals? Do you mean to short them to each other, like would happen if you wrapped a single piece of wire around all three of the transistor's legs with it still in circuit?

author: radiolistener
author_rank: Super Contributor
timestamp: April 03, 2023, 03:56:39 am
content: Quote from: MaximRecoil on April 03, 2023, 03:26:15 amany cable or wire affects RF fields, and you cannot predict how it can affect. It can be better or worse, just unpredictable, this is why I recommend to break connection between GND terminal of antenna and GND terminal of power supply. In such way you can eliminate random effect of the power supply wire and concentrate on antenna experiments.

author: jwet
author_rank: Frequent Contributor
timestamp: April 03, 2023, 06:21:03 am
content: The circuit is kind of a minimalist AM transmitter- and not terrible. I don't know why it only transmits a foot- it should cover a few hundred feet even with a poor antenna. The two upper transistors are an oscillator operating in a somewhat balanced way- this isn't bad. The third transistor pulls on this pair to increase current, this is AM modulation- it shouldn't affect frequency too much at least first order. The 3.3K resistor in the emitter circuit of Q3 sets the modulation "depth"- it sounds like you're getting decent modulation. Decreasing the value of the 3.3K would increase modulation- you could play with this. The way the circuit is designed, it would also increase output power. This kind of puts you in a box- it would be nice to set RF power output separate from modulator depth. You could experiment with putting something like a 1K resistor to ground from the emitter of the two upper transistors- this would get them making a carrier at some higher power. You would then use the BG3 circuit as is with the 3.3k to further pull on two upper emitters. So I guess the change would be to bridge the lower Q and R6 with a 1K and see if that let's you have a strong carrier with decent modulation. I hope this makes sense. The pot setting stuff is not important, open this wide open and use the PC volume control. You said you don't have a scope, if you have voltmeter, it might be fruitful to probe around the terminals of the transistor and make sure that there all operating in their linear range. If you get the bias values for the nodes, post them and we can have a look. I played with little am transmitters as a kid- I made room bugs and other gadgets in the 60's. Good luck.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 03, 2023, 06:53:29 am
content: That's a Peltz oscillator. If R6 is too big, it might stop.I suspect the oscillator might stop intermittently when modulated with music, and such the audio distortions.- First, connect an antenna, a big loop of wire between the pads 'Monitor' and 'GND', as told before.- Then, if you still have distortions, I guess it's because the oscillator doesn't oscillate continuously. Try reducing R6, for example put another 2...3k in parallel to reduce R6 value, so to increase the current and make the Peltz oscillator more stable.

author: MaximRecoil
author_rank: Regular Contributor
timestamp: April 03, 2023, 07:36:46 am
content: Quote from: jwet on April 03, 2023, 06:21:03 amReception falls off rapidly and drastically if you move the transmitter more than about a foot from the radio. Before I ordered this thing I watched a YouTube review of one, and his had the same ~1 foot range, so I didn't expect anything more than that to begin with.QuoteThanks for the detailed information. I might try swapping out the 3.3K resistor for a lower value one first, since that would be the easiest thing to try, and I might already have some resistors on-hand to try.QuoteI do have a voltmeter, but I don't know what I'd be looking for when probing around the terminals of the transistor with it. I don't know what "bias values for the nodes" means either.I made some direct-feed recordings from my radio's headphone jack into my PC's soundcard's line-in jack while it was playing broadcasts from my transmitter (which I fed from a different PC):https://app.box.com/s/uzhdxkmmeea60seaa0twxp3ct26ro0q1File #1 is a 20-second clip from the song Tom's Diner; Windows volume is at 100% and the transmitter's pot is at 100%. I included this to show that there isn't a reception problem, since there's hardly any static and it sounds almost as good as the source audio which is from a CD (the source audio sounds "brighter" for lack of a better term, due to containing higher frequencies than the recording from the AM transmitter), which I included for comparison (file #5). I chose it because it's just talking/singing, no musical instruments, and the source is high quality.File #2 is an extreme example of the type of distortion I've been talking about (100% Windows volume, 100% pot). The source audio (which I also included; file #6) is someone's recording off the radio, so it's not very clean audio to begin with, but it's typical of what I usually listen to through this transmitter.File #3 is the same audio clip, but with the input signal levels set to where I usually have them (100% Windows volume, 50% pot), because it gives me enough volume with low noise, talking sounds fine, but there is still some distortion in the music.File #4 is the same audio clip, but with the input signal levels turned down enough to eliminate audible distortion in the music (25% Windows, 50% pot). This results in a poor signal-to-noise ratio, i.e., prominent static.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 03, 2023, 08:23:33 am
content: Seems like the distorts happen only when the audio signal swings too much. Try making R6 twice as small, or twice as big, and see if there is any difference. Maybe, try powering the transmitter from another wall-adapter, or from batteries. Anything between 5-12V should be close enough voltage to power the transmitter.Were your posted records made with an antenna attached to the transmitter, or without?

author: MaximRecoil
author_rank: Regular Contributor
timestamp: April 03, 2023, 08:41:52 am
content: Quote from: RoGeorge on April 03, 2023, 08:23:33 amI just tried it with a 1K resistor in place of the 3.3K resistor at R6, and that didn't make a difference, so I removed the 1K and put the 3.3K back in there. I also tested the 3.3K resistor with a meter (out of circuit) and it's close enough to its stated value (3.27K). I don't have a resistor that's twice as big; the biggest I have is 5.1K. I guess I could try that.QuoteI've already tried it with batteries and a different AC adapter; no difference (aside from the different AC adapter introducing a constant buzzing sound in the speakers, so I didn't continue using it).QuoteJust with the onboard ferrite rod antenna which is positioned a couple inches from the radio's ferrite rod receiving antenna, and perpendicular to it. I don't have an antenna to attach to the monitor/GND terminals. As I mentioned in a previous post, I tried a loop of 18 gauge insulated wire that was about 10 feet long but it made it so there was no reception at all when it was attached. Removing it instantly restored reception. I don't have enough wire here to experiment with big loops.

author: PA0PBZ
author_rank: Super Contributor
timestamp: April 03, 2023, 09:25:50 am
content: What frequency is this transmitter supposed to transmit at? I'm beginning to wonder if you are listening to a harmonic frequency of the transmitter and not the fundamental frequency (harmonic meaning 2 or 3 times the frequency of the transmitter) or maybe even at a mirror of the fundamental frequency. Do you have any means of measuring the output frequency of the transmitter? Did you tune the radio over the full band to see if there is any other spot where you can hear the transmitter?

author: MaximRecoil
author_rank: Regular Contributor
timestamp: April 03, 2023, 10:15:02 am
content: Quote from: PA0PBZ on April 03, 2023, 09:25:50 amIts variable capacitor corresponds with the variable capacitor in a typical AM radio, i.e., if you turn its tuning dial all the way to the left, it corresponds with all the way to the left on the AM radio (about 530 kHz). I currently have the transmitter set to about 570 kHz, and the radio can also just barely pick it up when tuned all the way to the right (about 1710 kHz). It's mostly static/noise when the radio is tuned to 1710 kHz, but you can just barely hear the audio signal.

author: jonovid
author_rank: Super Contributor
timestamp: April 03, 2023, 03:37:18 pm
content: looking at your the circuit. it seems like an odd way to modulate AMbetter have one transistor as your dedicated oscillator circuit and the other transistor as amp/ or buffer output modulation tank circuit.from a audio transformer secondary on its power input.my experience with AM MW Medium-wave bugs is the harmonics are horrendous popping up all over the Medium-wave band at just in the same room.as the radio receiver.

author: radiolistener
author_rank: Super Contributor
timestamp: April 03, 2023, 06:36:44 pm
content: Forgot to say, this transmitter doesn't have frequency stabilization with crystal oscillator and buffering, so it's frequency depends on impedance of your transmitting antenna.What this means - if you connect some transmitting antenna and signal disappears from your receiver, it doesn't means that transmitter stopped to work due to overload, it may just change frequency. So, you're needs to check if transmitter works on different frequency before conclusion that it stops to work. Just scan entire AM band on your receiver if signal disappears after some change within circuit or antenna.The good practice is to check signal with a small wire loop (RF sensor) connected to oscilloscope. But if you don't have oscilloscope, you can use your receiver, just scan entire AM band to find the carrier of your transmitter.

author: MaximRecoil
author_rank: Regular Contributor
timestamp: April 03, 2023, 07:17:23 pm
content: Quote from: radiolistener on April 03, 2023, 06:36:44 pmThanks; that explains it. I tried my loop of wire again, and as before, it instantly killed reception when connected, but like you said, it only changed the transmitting frequency (in this case it changed it from about 570 kHz to about 640 kHz). Using the wire loop antenna sounded exactly the same as using the onboard ferrite rod antenna though, so it didn't fix the distortion.I'm glad that I now know how to make an external antenna work with it, which allows me to increase the range well beyond a foot.

author: radiolistener
author_rank: Super Contributor
timestamp: April 03, 2023, 07:53:09 pm
content: Quote from: MaximRecoil on April 03, 2023, 07:17:23 pmAntenna allows to improve signal level, it allows to improve signal-to-noise ratio quality and increase reception distance. But it cannot fix distortions. If you have distortions, you're needs to add low pass filter on audio input of your transmitter to limit audio signal bandwidth to about 9-10 kHz, also you can add equalizer - it can help to compensate frequency response of your modulator. If this didn't fix distortions, you're needs to improve schematic of your transmitter.

author: radiolistener
author_rank: Super Contributor
timestamp: April 03, 2023, 07:58:30 pm
content: Quote from: MaximRecoil on April 03, 2023, 07:17:23 pmLarge external antenna can help, but you cannot increase distance a lot with this transmitter. This is because it has too low output power. In order to cover long distance, you're needs more serious transmitter with RF power amplifier, about 10-100W, but note that such transmitter may needs a license and has more requirements for signal quality, it's out of band emission should be suppressed with filters for at least -40..-60 dBc.

author: MaximRecoil
author_rank: Regular Contributor
timestamp: April 03, 2023, 08:38:55 pm
content: Quote from: radiolistener on April 03, 2023, 07:53:09 pmIt didn't do that in this case. Like I said, it sounded exactly the same.QuoteFixing the distortion is the only thing this thread is about. If you don't think it can fix the distortion, then why suggest adding an antenna?QuoteIt doesn't have anything to do with the signal bandwidth, because as I've mentioned before (and as I've demonstrated in the recordings that I linked to in reply #18), turning down the input signal level (with the Windows volume control slider and/or the transmitter's potentiometer) eliminates the distortion, and turning it up increases the distortion. Turning the signal level up or down doesn't change the signal's bandwidth.QuoteThat's what I've been asking about right from the start. I believe that it needs a little more clean amplification of the signal. I've asked multiple times if there's a different transistor I can use for example, but so far no one has addressed that question. Some transistors can amplify drastically more than others can, all the way up to transistor-based "chip amps" that can output hundreds of watts from a line-level signal. It seems like there would be a type of transistor (perhaps combined with replacing some other parts) out there that would work in this circuit and give me a few extra milliwatts of amplification.QuoteI'm not looking to increase it by a lot in the grand scheme of things. I can increase it enough so that I can place the transmitter in a different part of the room than the radio, rather than having to place it a few inches from the radio.

author: 807
author_rank: Frequent Contributor
timestamp: April 03, 2023, 09:38:03 pm
content: I suspect that the AM is clipping before it gets near 100% modulation. At low audio input levels the audio sounds good, but quiet on the radio. At higher audio input levels, the sound on the radio will be higher, but distorted, as the signal will clip before reaching 100%. That's where a scope is really handy. You will see exactly what is happening.I don't think replacing the transistors will help. It's a series modulated circuit. BG3 modulates the RF oscillator with the audio signal. I still don't think that modulating a free running oscillator does the fidelity any good, as it introduces FM.The usual method of series modulation is to have half the voltage across the RF section & half across the AF section. What is the voltage at the collector of BG3?I wonder if the audio could be improved by doubling the supply voltage? It could give the audio modulator (BG3) more of it's linear range to play with.How bright is the LED? R9 a very high value resistor for a 6v supply.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 03, 2023, 10:32:18 pm
content: Quote from: MaximRecoil on April 03, 2023, 08:38:55 pmMost probably you don't need more amplification, the signal is so big already that it distort. Instead of modifying the schematic, I would rather try to find in what point the signal gets deteriorated.You say you have another PC with a line-in, for recording with the soundcard, right? If so, then probe the audio signal with the soundcard (or listen directly with another PC/amplifier, not with the radio). Probe the audio before and after C1, before and after C2, and most importantly at point marked I3, in the emitter of BG3. The amplitude doesn't matter, but it should be without distortions.Another test you can do, is to measure the DC values without any audio. Between GND and the base of BG3 it should be about 8.2/(51+8.2)*6V = 0.83VOutch, that's too small! It means only 0.2V in I3, which means the max audio-in level with the W potemtiometer at maximum (100%) would be of only 0.2V.The hour is 1AM here, I hope I didn't messed up anything, but there is no way for that schematic to work with 1V audio in and the W at 100%. You'll have to either increase the value of R4, or decrease R3 until the base of BG3 will be 2...3V (the audio has to be smaller than the DC voltage on R6, and the DC voltage at R6 will be the base voltage minus 0.6V the drop on BE junction of the BG3). The line out of a PC usually outputs about 1V audio with vol at 100%.For 6V power supply I would start at half that, so I would replace R3 and R4 with two resistors of the same value. Any value between 10k...50k would do it, it's not critical. It's the ratio between them that matters. The idea is to lift the DC voltage at point I3 above 1V. With 3V in base of BG3 (when R3=R4), I3 should be about 2.3V.That is if you want to be able to feed the circuit with audio at 100% from PC.If the I3 voltage is raised, then R7/R8 ratio should be changed too, the simplest way would be to switch places between R7 and R8, put the 15k to the upper side and the 43k to the lower side.You may want to try a simpler test before starting to modify anything.- tune the radio to the transmitter- put the PC volume to 100%- dial down the W potentiometer until the distorts go away- record the received sound- pause the music and record the noise received by the radio when there is no audio in- disconnect the audio cable- dial down the W potentiometer to zero- record the received noise againThat will tell the signal to noise ratio, and might give an idea about the AM modulation factor in the transmitter.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 03, 2023, 10:52:16 pm
content: One more thing, if you want to broadcast correctly, you'll need to do audiopreemphasiscorrection (raise the high pitch sounds) for AM modulation, because any radio-receiver will dodeemphasiscorrection (will lower the high frequencies) after demodulation.The correction curve is given by the AM specs as in fig.1 here:https://www.nrscstandards.org/standards-and-guidelines/documents/archive/nrsc-1-1988.pdfIIRC there were some small changes from one continent to another, and over the decades, but not much. You can use that one by passing the sound (inside PC) through a equalizer, so to raise the volume for > 2kHz before feeding the audio to the modulator.

author: MaximRecoil
author_rank: Regular Contributor
timestamp: April 04, 2023, 06:51:31 am
content: Quote from: 807 on April 03, 2023, 09:38:03 pm1.05 VDC with the 6 VDC power supply (5.5 VDC power supply actually; I'm not using the 6 VDC one anymore because it produced faint buzzing in the speakers; nothing to do with the voltage; it's just a crappy power supply), and that remains a constant regardless of the Windows volume level, regardless of the onboard potentiometer level, regardless of what type of audio is playing, and regardless of whether audio is even playing or not.QuoteI tried a 12 VDC power supply and it sounded exactly the same. The voltage at the collector of BG3 was 4.66 VDC with the 12 VDC power supply.The LED is normal brightness with the 6 VDC power supply, and it didn't look any brighter with the 12 VDC one.I don't think that whatever FM is being introduced is causing a problem, because this transmitter is capable of very good fidelity under certain circumstances (listen to my file #1 that I linked to in reply #18 for example). If FM was causing a problem, it seems like you would never get good fidelity from it because it would always be interfering.Quote from: RoGeorge on April 03, 2023, 10:32:18 pmA big signal doesn't inherently distort. Commercial AM radio stations for example, put out massive signals (tens of thousands of watts in some cases) without clipping/distortion. Clipping happens when you don't have enough clean amplification for the level of sound (volume/decibels) that you want to achieve. That's why audio power amplifiers which are rated honestly by their manufacturers include a THD (total harmonic distortion) figure along with a power (wattage) rating. An amp that's rated at 100 watts per channel at 10% THD is a lot less powerful than one that's rated at 100 watts per channel at 1% THD, which in turn is a lot less powerful than one that's rated at 100 watts per channel at 0.1% THD, and so on. All of them can output at least 100 watts per channel, but the 10% THD one will sound terrible/distorted at that power level, while the 1% THD one will sound pretty clean (but right on the borderline of being able to hear distortion), and the 0.1% THD one will sound perfectly clean, and you could probably drive it to a few/several hundred watts of output per channel before you'd notice distortion.QuoteI did measure voltage at BG3, but I did it at the collector, not the base. It's 1.05 VDC with a 5.5 VDC power supply, and 4.66 VDC with a 12 VDC power supply (there are more details above in my reply to "807"). I can measure it at the base later.Edit:It's 0.75 VDC at the base of BG3 with the 5.5 VDC power supply, and 1.61 VDC with the 12 VDC power supply.QuoteI'd actually rather not have the PC audio level at 100%. Ideally I could set the Windows/PC volume slider somewhere between 25 and 50%, the onboard pot to 100%, and get enough clean volume from the radio like that. That would give me a single volume control (always leaving the transmitter's onboard pot set to 100%, and always leaving the radio's volume control at the same setting) and plenty of headroom to adjust the volume for various audio sources that are recorded at different levels.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 04, 2023, 07:04:14 am
content: Seems like nothing will improve the situation, you made me curious.I've found a variable capacitor and a ferrite bar with coils, leftovers from a vintage miniature pocket-radio. And still have a working AM radio. Will use the AM ferrite and the capacitor to build that modulator today afternoon, for the fun of it.This will take less time than poking at hypotheses then writing long posts about each.

author: 807
author_rank: Frequent Contributor
timestamp: April 04, 2023, 12:17:32 pm
content: Quote from: MaximRecoil on April 04, 2023, 06:51:31 amI think the point that RoGeorge was making, was that your audio signal is already big enough for that little transmitter, and turning it up to maximum is what's causing the transmitter itself to distort, as it's overmodulating. I don't think he meant that the transmitter power is so big that it distorts.Just to recap, what is actually wrong with recordings #3 & #4? I can hear that they are quieter than #2, but then #2 is either clipping the RF before reaching near 100%, or is overmodulating.Also, if you are comparing your undistorted signal to that of a broadcast transmission, then a broadcast transmitter will always sound louder as they use various techniques to get the maximum "loudness" from their transmitters.Just one more point about FM. As you increase the audio level, the FM deviation will also increase, so will be less noticable at lower settings. It may be that the deviation is still within the passband of the receiver, so may be unnoticable, but still not a good design IMO. Not that you can do anything about it of course, but just something to note.

author: radiolistener
author_rank: Super Contributor
timestamp: April 04, 2023, 03:02:00 pm
content: Quote from: MaximRecoil on April 03, 2023, 08:38:55 pmit is depends on signal bandwidth, because signal power depends on signal bandwidth. If you don't limit signal bandwidth, it will lead to saturation at lower audio level.It means that missing low pass filter to limit audio bandwidth leads to signal distortions at low level of audio volume.Quote from: MaximRecoil on April 03, 2023, 08:38:55 pmTurning signal level up or down leads to change power level. At some power level saturation happens and it leads to distortion.Limiting signal bandwidth allows to reduce power (by cut-off high frequency power which is useless for AM), as result it allows you to setup higher signal level without distortions.

author: radiolistener
author_rank: Super Contributor
timestamp: April 04, 2023, 03:08:37 pm
content: Quote from: RoGeorge on April 03, 2023, 10:52:16 pmthanks, nice paper

author: MaximRecoil
author_rank: Regular Contributor
timestamp: April 04, 2023, 04:22:17 pm
content: Quote from: 807 on April 04, 2023, 12:17:32 pmMore  amplification would make it a bigger transmitter, and you wouldn't have to turn the signal up as much to reach the volume you want, resulting in less, or no, audible distortion.Quote#3 has distortion in the music; not nearly as much as in #2 of course, but I only included #2 as an extreme example to clearly illustrate the type of distortion I've been talking about. Its massively distorted, brick-walled volume isn't relevant, because I don't ever listen to it like that.#4 doesn't have any distortion that I can hear, but because of the very low levels, there is a poor signal-to-noise ratio, meaning you hear more background static/noise than in #3. I increased the volume of the #4 file after recording it to more or less match the volume of the #3 file, which simulates having to turn up the radio's volume more to compensate for when the input levels are that low. The raw recording of #4 was about 1/3 or 1/4 the volume of #3 raw recording.QuoteThey use dynamic range compression, which is the same thing they do to make CDs louder. It's a result of the so-called "loudness wars," which started in radio and eventually crept into CDs.QuoteThe #1 recording was very clean and it was at maximum input levels (100% Windows volume, 100% pot), though the source recording itself (which was from a CD) had a lower volume (and little-to-no DRC) than the over-the-air radio recording source I used for the other examples.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 04, 2023, 06:15:31 pm
content: Before anything else, please try the following test- first, tune to an AM broadcast (from a local radio station) and set the desired audio volume in the receiver (to have a reference sound/volume)- then, tune back to your transmitter- in the PC, make a correction curve like the one shown in the attached picture- preferably use the Winamp, so you can load the EQ settings from the attached .eqf file (unzip it first), if not redo the same corrections in any preferred EQ- make sure the EQ is on (in windows it will sound having no bass when the EQ is on)- connect the audio your transmitter and set the Windows volume to 100%- dial down the W potentiometer in the transmitter until you have in the radio speakers about the same volume as it was when you were receiving the air broadcast from the local radio station (and no distortions, of course)Preferably use the Tx antenna, and keep the radio close to it, too, during the tests.This is about the max signal/noise ratio you can get without building another AM modulator and/or an RF amplifier.- Is the sound good enough now, without distortions and with a decent signal to noise ratio?If you have a radio that can tell the strength of the RF signal (magic eye/tunning indicatorhttps://en.wikipedia.org/wiki/Magic_eye_tube), that would be great to see how much RF signal your Tx can give in comparison with a local radio station.

author: MaximRecoil
author_rank: Regular Contributor
timestamp: April 05, 2023, 07:00:50 pm
content: Quote from: RoGeorge on April 04, 2023, 06:15:31 pmI can't do that because there are no AM radio stations that come in good here, not even close to good. The best ones just sound like static/noise with a very faint, barely intelligible signal. It has always been like that here, as far back as I can remember (late 1970s).QuoteI haven't used Winamp since the mid '00s, but I made a similar EQ curve in foobar2000:That does just about eliminate audible distortion (because it's attenuating the frequencies that distort the most), though it makes the audio sound rather thin. I can see it being useful for certain types of audio though.

author: A.Z.
author_rank: Super Contributor
timestamp: April 05, 2023, 07:50:07 pm
content: Quote from: radiolistener on April 03, 2023, 06:36:44 pmit depends...http://techlib.com/electronics/amxmit.htmhttp://techlib.com/electronics/amxmit.htm#Unfair%20Radio%20Transmitter

author: RoGeorge
author_rank: Super Contributor
timestamp: April 10, 2023, 09:09:19 pm
content: Made yesterday the circuit, and fiddle with it this evening, while probing with an oscilloscope. Doesn't look good.- asymmetric carrier waveform- strong parasitic FM modulation- the AM modulation factor is too small- at 6V supply the modulation factor is bigger, but it can only take about 0.1V audio, or else the oscillator will stop during the negative swing of the audio- at 12V works a little better, but then the modulation factor is even smaller, so more hiss and noises at reception for the same audio volume- frequency shifts a lot with the power supply voltageFirst 3 pics are at 6Vdc power and 0.775Vrms/1kHz (0dB audio). Normal zoom, then zoomed-in to see the RF carrier during peak/troughs. Next 3 are the same thing for only 250mVrms, to get a smaller modulation factor. All pics look very bad to me.Will try to squeeze more from that schematic, but I'm afraid that schematic as it is would never work decent.