HB Evaluation of the AMT-5000

"Kit builders today have a variety of transmitters to choose from, including past offerings"

A quote from paragraph 2 of the "Review".

I am a kit builder today.

Can you explain how the variety of transmitters I have to choose from can include past offerings?

Reading more on the AMT-5000 site there, it seems they're saying the 5000 is lacking for not having a low pass filter to cut off audio above 10 khz. Is a filter something that other Part-15 transmitters have? I can't remember that I've seen one before.

Some people think that the low pass filter is a requirement, but not necessarily, it's only a rule that was put into place about 25 years ago for commercial broadcasters to follow, but not necessary for P-15 or even technically.

I read this review a while ago, and I don't believe that there was any testing done on the transmitter, other than that from the AM Transmitter Shootout. And that testing was highly suspect, as it was obvious that the testers didn't know and understand how to properly tune the transmitter (as evidenced from follow on discussions in their Forum - Ermi Roos provided a lot of information on proper tuning). During the Transmitter Shootout, Bill just blamed SSTran for lack of documentation, and included the suspect (i.e., not optimal) data in the final results.

From what I understand, the AMT5000 has the potential to provide the greatest field strength from a 100mw input to the final stage of the transmitter. SSTran provides some general tuning instructions for typical Part 15 grounds, but to ensure that the final stage is operating at peak efficiency, you need to know the ground impedance and use test equipment.

Bill's comments in both the Transmitter shootout and that review demonstrate that he definitely has personal issues with Phil and SSTran. The AMT5000 still awaits a completely unbiased review.

Everything has its cycles of popularity, then decline. TV coming to the mass market was a blow to radio in the 1950s, then again with the move to FM and stereo, and once more with MTV and music videos, media consolidation in the late 1990s, then satellite radio and the rise of the internet and smart phones.

Radio broadcasting has to find the next phase for its being.

I guess you're right about transmitter kits declining in the market, the big loss would be Ramsey Kits, since they sold a couple models. I have two, the VFO tuning stick version and the DIP switch synthesized.

I think the Talking House does work well enough, and has the all important auto tuner, especially good for beginners and sounds important as the gears turn back and forth, peaking up the signal. Getting properly tuned into the ten foot antenna is a prime concern, something that isn't given much consideration in Ramsey literature.

Ramsey's output is low impedance, and just clipping the wire on that will get about 50 feet of signal. They show dimensions for a loading coil, but don't really give enough info on how to peak it to be efficient, an all-important step that all transmitters with 3 meter antennas need to do.

As for pre-emphasis in the AMT-5000, well open ended boost like with FM will cause your modulation to be low, like on CDs with lots of high frequency peaks. The real way stations deal with the treble boost is to have a separate variable gain amp in the processor that acts just on the highs to reduce the peaks a good bit and fit the modulation into the signal tighter, so you don't lose as much volume.

True, if you roll off the frequency lower, then there's less of a high frequency band to deal with, but a good transmitter can take it all the way to 15 khz or higher, and in fact, FM transmitters deal with the same issue of pre-emphasis to 15 khz.

I'd like to see more current reviews, unbiased, except for the final stages that should be biased properly. It seems the other guys didn't have the time or access to the testing range for a long enough time to really get down to business and get some good numbers. I don't think it's really necessary to have a Potomac FIM, even a relative scale in a receiver would work if you self calibrate it to a benchmark level and it's stable. Maybe the Eton? receiver with the db-uv scale would work.

Boomer

Great that you built an AMT-3000, a classic 3-chip PLL with what seems to be a variation on the Wenzel differential amp circuit, if you know that one, with an added audio compressor IC. I like the idea that the designer thought of level control, that's something that the little FM transmitters need, in my opinion. Advanced users would probably want outside processing, but for many, the chip is a good thing, you may not want to do a car cruise and have to bring along a rack mount processor, just hook up a CD or music player full of oldies.

I've seen the schematic of the 5000 that you're talking about, as found on antique radio forums, and it looks like the same PLL scheme, and he's changed the output stage and modulator to FET, using a tapped toroidal output inductor for lower loss, and low loss variable capacitor, more like the Rangemaster. You could probably replace the toroid with an air wound inductor, and the cap with a surplus mini air variable, and of course add the surge protector and bleeder resistor.

Boomer

That looks really good! You're dangerous with electronics.. Good old pine board construction, I like that kind, but it looks like it was a way to mount the circuit before putting it into the re-used case. I always like to save and use old enclosures, so much of that gets wasted or melted down.

I took a look at the schematic for the 3000 again, when I saw the 4th DIP chip and wondered what it was for. I see the designer used a 4 mhz crystal, going to 250 khz, then another divider to 10 khz, which is interesting. You can also use a 10.24 mhz crystal and one 4060 IC to get down to 10 khz for the reference. I'll have to study the 3000 a little more.

Many things considered, it's a good and unique circuit design, with refinements over the basic transmitters we see out there. It has RF chokes on the power and audio inputs, op-amp style modulator/final, and the audio compression chip.

I know what you could build in a tube transmitter, Electronbunker CA's AM stereo. Other than that, he has some well thought out circuits for standard mono tube transmitters that look good. You could make the tube final, and have a PLL driver hybrid circuit.

For the 5000, it does look something like a T-200-2, that probably wouldn't be so lossy at the medium waves. Look at your personal message box here, I sent a bigger picture of the toroid.

I've looked at the Vectronics too, never got around to ordering the board and case yet, and I'm glad it's still actually available.

References:

AM tube transmitter with carrier control:

electronbunker.ca/eb/OneTubeXMTR_2.html

Be sure to follow the link at the bottom for a great rundown on the improved matching method!


AM stereo tube transmitter:

electronbunker.ca/eb/AM_StereoXMTR.html


Vectronics 1290 transmitter:

www.vectronics.com/Product.php?productid=VEC-1290K


The Wenzel circuit for comparison:

www.techlib.com/electronics/amxmit.htm


Boomer