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SSTRAN AMT-3000 AM Broadcast Transmitter

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New review here: www.radiointel.com/review-sstran.htm And Paul Stenning's review here: www.vintage-radio.com/reviews/amt3000.html Posted here with permission from Popular Communications Magazine,
www.popular-communications.com

Join The Part 15 Revolution With The SSTRAN AMT-3000 AM Broadcast Transmitter!

Perhaps you didn’t know, but you can legally own and operate your own AM radio station. FCC Part 15 Rules permit unlicensed transmitter input powers of 100 mW between 510 and 1705 kHz—enough power to span a small portion of many urban neighborhoods!

Micro-broadcasting has many useful applications, including broadcasting your favorite programming over your favorite old radio. Why be at the mercy of ever dwindling AM program variety? Suitable audio sources for your micro-broadcast AM station include cassette, CD or MP3 players, satellite audio channels, or even Internet radio stations. AM radio is going digital, and when the sad day comes when analog AM radio goes dark, you’ll be ready with your own radio station to fill the gap! Here’s how!

The cornerstone for effective microbroadcasting begins with a good quality transmitter, and the SSTRAN AMT-3000 kit is the best we’ve seen to date. The feature- packed transmitter is synthesizer controlled in 10-kHz channel steps (9-kHz for European export models) from 510 kHz to 1710 kHz, easily set via dip-switch entry.

Audio Features

Although this is a monophonic transmitter, the audio input will accommodate two RCA plugs from a stereo source. The stereo signal is summed in the transmitter to prevent loss of right or left channel information. The two inputs can also allow the transmitter to mix multiple mono audio sources for broadcast.

Photo A shows my AMT-3000 with my portable Sony CD player. When the picture was taken the combo was broadcasting ’60s era oldies to a nearby vintage radio. It sounded great! An on-board jumper permits an 8-dB treble boost at 2 kHz; normally the transmitter audio is flat within 1 dB from 20 Hz to 20 kHz. The transmitter is capable of full 100 percent negative modulation (tested and proven in our lab) and also features a frontpanel adjustable 1:1 to 5:1 dB compression ratio. Audio levels exceeding the adjustable limiter level are compressed at a 15:1 ratio. The transmitter audio response is flat from 20 Hz to 20 kHz with low distortion—this is unit is capable of outstanding AM audio quality!

Three front-panel controls for Gain, Modulation, and Compression give the user full control of how the transmitter sounds. The elaborate audio processing (compression and limiting) is handled by an Analog Devices SSM2166 integrated circuit, a powerful level of audio processing that I haven’t seen offered in competitive units. The manual shows how to quickly set these three controls for the best sounding signal and it also gives a more detailed explanation of the Analog Devices’ features than space allows here.

The Kit

Photo B shows what you’ll see when the AMT-3000 is unboxed. Most of the smaller electrical parts are carefully presorted in plastic bags at the factory. The enclosure, knobs, and wall-wart power supply are included in the kit (too often these by Peter J. Bertini, radioconnection@juno.com technology technology showcase new product performance analysis are expensive “options” in competitor kits; everything needed is here, and for one price). The enclosure is robust and professional looking. Considering the quality, complexity, and parts count, this kit is an exceptional bargain.

Starting Assembly

Before picking up a tool or soldering iron, you need to inventory the parts and read the manual first! The kit is moderately difficult to assemble, so the builder should have basic soldering skills and experience with simple kit construction before attempting to assemble the AMT-3000. This is not the kit for inexperienced first-time kit builders. If you’ve never held a soldering iron, you might have a friend do the assembly for you, or first hone your skills on simpler kits.

On the plus side, the manual is clear, concise, and well written. It shows you how to solder and is amply illustrated to aid with construction steps or in parts identification.

The Hard Part

The most difficult task is soldering the SO-14 SMT (surface mount) Analog Devices audio IC with its .05-inch lead spacing! (Unfortunately, the through-hole version of this IC is discontinued.) The IC is shown in Photo C. If you don’t feel up to the task, the IC can be factory installed for a modest $3 fee. One thing I’d suggest is to have a good hand magnifier nearby when building the kit! My near vision isn’t what it used to be, and a reading glass proved to be most useful during assembly when my eyes wouldn’t focus!

Another tricky construction step involves mounting two monolithic capacitors between IC socket pins on the bottom of the pc board. This was a design revision to clean up some artifacts from the synthesizer that were audible on the transmitter signal. The pc board is silkscreened—all of the component legends and body layouts are clearly printed on it! Also, the solder side of the board is masked, which helps prevent unwanted solder bridges across adjacent runs or between component pins. Photo D shows the PC board nearly 50 percent assembled. Note that the larger IC packages mount in IC sockets for easy replacement.

Plan to spend at least a weekend, interspersed with ample rest periods, to build this kit. It’s all too easy to become complacent and rush assembly, and rushing assembly is what leads to errors. My Waterloo was confusing a TO-92 packaged IC with a similarly packaged transistor. Had I read the instructions, I wouldn’t have erred. The assembled pc board, installed in the enclosure, is shown in Photo E.

Hum Reduction

Many in-home AM transmitters are plagued by “tunable hum.” The kit designer felt most of these problems relate to poor power supply design and inadequate RF bypassing. Tunable hum is a loud hum on the receiver audio that’s only present when tuned into a station.

This kit includes an AC wall wart supply, and the diode rectifiers are RF bypassed to prevent incidental 120-Hz modulation of RF currents flowing through the power supply cables. The transmitter also features (jumper removable) RF chokes in the power and audio lines to eliminate stray RF antenna currents from flowing on these cables (if needed) to control tunable hum problems. I’m pleased to report that I never experienced this problem.

Technical Tidbits

The synthesizer uses four 74HC series ICs to produce a crystal- referenced signal between 20 kHz to 2560 kHz (FCC Part 15 rules permit operation between 510 and 1705 kHz, and the actual transmitter operating range is limited to 530 to 1710 kHz by the RF stage tuning. The synthesizer uses a 4-MHz crystal oscillator for the reference.)

The mumbo-jumbo means that, unlike inexpensive competitor models using free-running oscillators, this transmitter’s frequency is locked to a very stable and accurate crystal oscillator; it won’t drift with temperature, changes in humidity, or over time. My frequency counter showed the transmitter was within a few cycles of the frequency it was set to. This avoids audible heterodynes from weak distant AM stations on the same frequency, especially at night. The S4 dip-switch settings set the binary divider count in the synthesizer to determine the transmitter frequency, based on the following formula:

Frequency in kHz = 10 * (S4_value + 1)

Besides setting the transmitter frequency via S4, the transmitter RF stage output is set to the corresponding frequency range using S5, a four-position dip switch. SSTRAN breaks the broadcast band into several different band ranges, and this switch selects the optimal component values for each of those frequency ranges. The transmitter output tuning is peaked (via a ceramic trimmer) for the highest DC voltage at a meter test point on the PC board.

A considerable amount of engineering time went to making the modulation quality relatively immune to improper transmitter tuning. This all sounds more complicated than it really is, since the manual clearly describes the steps for setting the audio controls, transmitter frequency, and tuning.

Antenna Choices

The RF stage uses a tunable Pi-Net output for maximum power transfer to the integral Part 15 antenna system. The Pi- Net also limits harmonic radiation to better than -20 dBc to meet FCC regulations. The supplied antenna is a 118-inch length of white antenna wire and a 72-inch black ground wire counterpoise per Part 15 Regulations.

The manual instructs the builder to solder the supplied antenna and ground wires directly to the pc board to meet FCC Part 15 antenna requirements. However, the RF output is also available through an optional (but supplied) RCA jack should the builder elect to use another antenna system. This runs the risk of not meeting FCC Part 15 compliance if a different antenna system is used. If you live outside the United States, you will need to determine the local laws applying to unlicensed broadcasting.

Getting The Most Range

The SSTRAN antenna yields best efficiency at the high end of the broadcast band; that is, you’ll transmit farther at 1700 kHz than at 530 kHz. Look for a relatively quiet frequency at the high end of the band to get the best range. In many areas of the country this means going into the relatively unoccupied expanded AM band above 1600 kHz, especially at night when distant stations are barreling in across the dial. Many vintage radios won’t tune above 1500 or 1600 kHz; while others will tune to 1700 kHz, which was the old police band used prior to WWII.

How far you can transmit depends on a lot of variables— every location is different. Some Part 15 broadcasters claim they can cover several blocks; others have trouble reaching across a ranch house! Regardless, with everything being done legally, a properly engineered and efficient Part 15 transmitter with good audio and decent modulation depth will have the edge at any location.

Wrapping It Up

Here’s the bottom line: This is a well-engineered product that delivers what it promises. It’s an exceptional value. The transmitter audio is great. You won’t be disappointed with this transmitter. Since a small family-run company produces the kit, I suspect the product is more a labor of love than a profit-motivated venture. The e-mail customer support was good.

My transmitter is in daily use broadcasting old time radios across neighborhood, a topic we’ll be discussing in a future “Wireless Connection” column! Stay tuned for more details and join our micro-broadcast revolution.

Ordering Information

Full kit, AMT-3000, AMT-3000-9K (9-kHz European)

SMT Chip pre-soldered, AMT-3000-SM, AMT-30000-9KSM (9 kHz European)

Contact:

SSTRAN, 3053 Griffith Rd., Norristown, PA 19403;

Web: www.SSTRAN.com

E-mail: info (at) SSTRAN.com
SStran Review

I've built several kits over the years, but this one really impressed me. The instructions are clear and well organized, as is the PC board screening. My eyesight isn't so great, so I really appreciated that. The unit was assembled in just a few hours (I ordered mine with the surface-mount IC already soldered in) and in no time I had it connected to a cassette deck and a 9 ft antenna wire, broadcasting around the house. Since October, my SStran has lived in a waterproof box out in the backyard, beneath a copper pipe vertical & connected by about 80' of shielded cable back to the studio. The sound is great for A.M., and the range is 2 - 3 miles on a good day. The only downside I can find is the cheap feeling controls, but once you set this thing up you never touch the knobs anyway. Summer storms are around the corner here in Carolina, and I expect to lose my little transmitter to nature. But I will definitely replace it with another SSTran. Nice piece!

Marathon Don
1620AM Copperhead Radio,
Lucama NC

My experience with the AMT-3000

I have built many kits and the AMT-3000 kit ranks among the best. The PC board is high quality and the instructions are the best I have ever seen.

The circuit design is "bulletproof" in that it is not critical regarding normal parts tolerances. I have surface mount experience and found it easy to mount the surface mount IC, but if you do not have experience and a professional grade soldering iron, pay the $3 for factory installation.

I have used my unit with both the 3 meter wire and base loaded antenna. The only problem I had was with the base loaded antenna. At resonance, the audio distorts due to the high Q and I found it necessary to leave R18 installed. This reduces the field strength but vastly improves the audio. [Edit for updated information] After writing this paragraph and after more experience and experiments I found the problem was not the antenna Q but rather that I did not have it tuned properly. Once carefully tuned according to the instructions the audio was clean. [End edit]

The only hum problem I saw was due to a ground loop between the transmitter and the audio input grounds. One solution is to use an audio ground isolator in the audio line (Radio Shack has them). I did not use this, but others who have claim this works.

The only thing I would change is to add a power on/off switch on the front panel but that is just because I don't want to be on the air without audio. It is not illegal, just a personal preference.

The audio processing setup can be a bit confusing and after you set it according to the instructions, some further experimenting may result in audio more to your taste.

In my opinion this is a very good part 15 AM transmitter.

Neil

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