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Now that I own both the AMT3000 and the AMT5000, I have to say, the audio on the AMT5000 is nowhere near as clean as the AMT3000.
Now, I’ve had some tuning issues with the AMT5000, but Phil Bolyn has been helpful in that regard. I had sent the transmitter to him to have a look at and he is taking good care of me, however, no fault at all has been found with the unit.
So, I just wanted to hear from some of you out there to find out if it’s something that has come up for you? The audio distorts badly on the mid to upper highs but is noticeable whether you’re listening on a narrow band or wide band receiver. Removing the pre-emphasis helps alleviate the symptom somewhat, but not completely and without S22 jumpered, the audio sounds dull and muffled. And yes, I have checked my audio levels at both sides, the TX gain and the input gain. I’ve even tried some more aggressive audio processing in my audio chain, and it’s still the same. It’s like the audio just crashes at a certain point, no matter what the gian or modulation levels are at…it happoens even at very low volumes.
Unfortunately, I do not own a scope at this time, but I certainly wish I did because I’d really like to see what is going on there.
Just wanted to know what your experiences were in terms of the audio response?
Perhaps I’m just missing something and any help would be fantastic. Thanks guys!
Also, just to let you know, I did use the search function prior to starting this thread and could not find anything specifically related to this issue.
Total posts : 45366Were the two transmitters’ audio compared using the same receiver? If so, then let’s await comments from those here who have the AMT-5000 (I don’t).
There is a tremendous difference among the various radios I use. Some sound crisp and clear while my car radio sounds dull and flat on audio. I think one big difference is whether the receiver is designed for the relatively new NRSC compression used on AM broadcast.
Here’s a link to a rather long discussion on this topic.
Neil
Total posts : 45366The reason I tell you that “there is hope” is that I operate both an AMT 3000 and AMT5000 everyday being fed from the same audio, and for me the transmitters sound exactly alike, and both are exceptionally clean audio without the pre-emphasis jumper.
Well, I am slightly processing the audio with a Stereotool Plugin from the computer audio path. Besides a strong level boost by limiting/compressing, I have the bandpass filter set for 125-10,240 Hz to control the splash on the amplitude modulated carrier.
The audio quality of both transmitters is indistinquishable from each other, and either as good as other AM stations, and in some cases better, since we have stations in the area with poor feeds from limited phone-lines or whatever, and one that runs church material from audio cassettes.
If you will, please describe your audio chain and the type aurio (i.e., speech, genre of music). Maybe we can find something there to explore.
Total posts : 45366Hey Guys,
First of all, a big thank-you for chiming in and offering to help!
I use several radios for monitoring the feed. All produce the same result in regards to this problem. All the receivers are solid state. For monitoring I have a Carver TX-11a that hides absolutely nothing. As a matter of fact, the audio from the AMT3000 is as clean, crisp and actually a bit brighter than any FM station on the dial. I hear the full audio range with that receiver and it allows me to hear every detail that is coming out of that little transmitter. I was told a long time ago, that if you’re going to do AM, that is one tool I should have for monitoring the off air signal.
I also use two different Grundig field radios, a cheap Radio Shack portable, a Sony stereo receiver, a Denon component tuner, a Yamaha receiver, a little Sony Walkman, a clock radio, a Panasonic boom box/ghetto blaster, a Honda car stereo, a Nissan car stereo…and I’m pretty sure I’m forgetting one or two. Basically, every type of radio you may find in your average household today. It doesn’t seem to matter. I hear the same result in terms of the audio distortion.
Now…as for the audio chain. Well, it can be as simple as just connecting an iPod to the back of the transmitter or my studio feed. This also doesn’t change the result in regards to this issue. The studio feed, without getting into too much detail, is fed from a 16 channel Allen & Heath board going through a DBX compressor/limiter, to the audio card, processed through Stereo Tool and back out a second audio card out to the transmitter.
The format of the station is an Oldies/Classic Hits hybrid with a lean towards a classic soul/urban sound, but anything that charted after 1940 is fair game.
However, I have a tendency to think that when all was perfectly fine with the AMT3000 and the only change to the station was the transmitter and now there exists a problem, then the problem most likely lies within the new/changed component.
Another small detail to add but may not matter in regards to this problem is, I built the AMT3000 myself, having ordered the kit through SStran, digging out my soldering tools and what not and putting it together over the course of two evenings. Whereas the AMT5000 was built by a professional builder who is endorsed by the manufacturer. 🙂
Perhaps it may have to do with some fine tuning?
Total posts : 453661580, there is no doubt you have covered every base in trying to explain the audio distortion on the AMT5000. If no one else could find it, surely PhilB would have spotted a problem if it resides in the transmitter itself.
Your radio collection is way more than most of us have, and your knowledge of the audio sources speaks for itself.
The last thing I can think of is that somehow the RF from the antenna is mixing with the audio…
Or, and this just popped into my mind, perhaps there is a very strong RF source in your vicinity, such as a full power tower, being detected by your own transmitting antenna and back modulating into the RF signal. That is a situation that can badly interfere with audio modulation.
Total posts : 45366“The last thing I can think of is that somehow the RF from the antenna is mixing with the audio…”
Hmm….That could be. Like I said, Phil has been helping me with some tuning issues (up to now I really didn’t think they were related, but now I have to wonder) and I am certain we will get to the bottom of it. I was just wondering if any of you guys had run into this little audio snag? Tomorrow I am getting a different multi-meter (just to be sure) and I will continue to work with Phil on this issue.
I also have no issue whatsoever with the possibility that I may be doing something incorrectly. The AMT3000 and the AMT5000 may look similar and sport similar features, but they are two different beasts. So what may work for one, may not work for another. I can accept that. 🙂
I sincerely appreciate the help thus far.
Total posts : 45366When I first began playing with AMT3000 in 2006, I tried a frequency about 940kHz, and when the antenna was resonant at that frequency, it received a lot of RF from 850 AM, a 5kW licensed station less than a mile away. I could even hear their audio very frazzled winside of my audio envelope.
It’s also true that a 3-meter antenna is at full wavelength in the FM band or thereabouts, which means the antenna might be very receptive to a nearby FM or TV tower.
Total posts : 45366Only thing I can add is that my audio (on both the sstran amt3000 and the T.H. 5) can sound tinny, metallic and kind of scratchy when my loading coil is not properly peaked.
Total posts : 45366Does it have a pot that changes the RF output? If so, turn the power way down and monitor the sound..
I tried cranking my Rangemaster up and the audio got real “flat” with no highs. Just an idea..
Total posts : 45366Hi 1580:
I am thinking about the audio line that feeds the input to your AMT5000.
Is it balanced or unbalanced?
Is it shielded?
What is the line level of that audio feed (i.e., -10dB, 0dB, +4dB)?
Try removing the AMT5000 and attaching an audio amp/speaker to check the audio quality in that feed line.
Total posts : 45366Hey Guys,
Sorry for the delayed response. I’ve had a bit of life thrown my way and got very busy with stuff.
“Does it have a pot that changes the RF output? If so, turn the power way down and monitor the sound..”
I have toyed with the RF out setting and it doesn’t seem to make much of a difference in terms of the audio distortion.
As for the audio…
The line inputs are unbalanced using standard RCA connectors/cables. Cables to connect are of high quality and are well shielded.
I’m suspecting that this problem is being caused by the transmitter not being peaked optimally. I still need to get that sorted out with PhilB when I have a chance. There are inconstancies between his measurements and what my tools are telling me and I’m inclined to think I may be getting false readings from my multi-meter. The strange thing there is that my multi-meter seems to be spot on accurate with other stuff and I’ve never had any real issues with the AMT3000, but perhaps the voltages measured for trimming C1 are too low for accuracy. I’m not sure that makes any sense or not as I’ve never really encountered such a problem.
I know enough to solder components together and understand the basic fundamentals, but I’m not quite an engineer….not yet anyways. 😉
More to follow. I’m certain it’s something on my end. But I do know for sure that my audio is very clean as I am quite particular about audio quality. Whenever I’ve demonstrated my station to those through the Carver tuner, I always get the same reaction…the shaking of their heads in disbelief at how good AM radio can sound. 🙂
Thanks for all the great suggestions so far!
Total posts : 45366Another thing to consider is that there might be misplaced components in your transmitter. Though not likely sometimes the colors red and orange on resistors are hard to distinguish for example. An ohmmeter check of resistors with the power off and the meter not in diode mode can help. This is just something to keep in mind if all else fails.
Related story, I built a KnightKit VTVM many years ago and the 3V AC setting never did work. I checked, rechecked, and checked my checking and did not find the problem. About a year ago I was inspired (or bored) and checked it again and found that I had soldered one of the resistors to the wrong switch terminal. I hope it doesn’t take you this long to find the problem.
Neil
Total posts : 45366What Radio8Z said about mis-reading the colors on a resistor happened to me, when I thought I had a 1k resistor in the right place (brown, black, red), yet I was getting strange results.
Then the lucky moment arrived when I noticed my resistor was brown, red, black, which I think is a 12-ohm resistor.
Also, 1580, you said something that needs to be double-checked… you said that changing the RF output power control had no effect on modulation, and the fact is it SHOULD have a very strong effect on modulation.
There is a correct balance that always needs to be set between modulation level and power level.
If you increase the RF power the modulation decreases and needs to be raised.
When the RF power is at minimum the audio modulation must be reduced or it will strongly over-modulate.
Keep working on it. What you learn will pay off as part of your technical education.
Total posts : 45366Any Class E system can have AM envelope distortion at higher modulation frequencies if the loaded Q of the output tuned circuit is high. I tried, but failed, to contact 1580 to find out if he has an unusually good antenna system with ground radials. The envelope distortion should not be noticeable in a typical Part 15 AM setup because the antenna system loss resistance is usually fairly high; but a low-loss antenna system can cause noticeable distortion. A quarter-wave vertical monopole would not produce significant distortion because the radiation resistance is high compared to that for an electrically short monopole, and the loaded Q is not very high. However, a Part 15 AM antenna has very low radiation resistance, and if the loading coil has high unloaded Q, and the ground loss is low because of a good ground screen, the loaded Q of the output tuning circuit of the class E amplifier can be high.
Envelope distortion is produced in a Class E system by the fact that the tuning circuit is deliberately tuned below resonance, and this causes the upper and lower sidebands to have different amplitudes and phases. This distortion of the sidebands causes envelope distortion.
One thing that 1580 can try is tune for peak output power (not peak DC current into the final amplifier), which can be done with a relative-reading field strength meter (which is very cheap compared to a calibrated FIM). This is not class E tuning, but should nevertheless give deceent efficiency. If the envelpe distortion reported by 1580 is due to High Q of the antenna system, peak output power tuning should cause the distortion to disappear.
The reference for envelope distortion in an amplitude modulated Class E amplifier is Kazimierczuk, IEEE Transactions on Circuits and Systems, June, 1984, pp. 543-549.
Total posts : 45366Carl Blare has posted that he uses a limited-space antenna system built into a window frame. Such an antenna system is likely to have poor efficiency, which means low Q, and it is not surprising that Carl reported in this thread that he gets the same audio quality with his AMT-5000 as he gets with is AMT-3000. As I stated in my previous post, a low-loss, high Q, antenna is likely to cause envelope distortion in a Class E system. It would be very helpful if 1580 reports the kind of antenna system that he is using to show whether or not it is a high-quality antenna system that is causing his audio distortion.
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