The AMT3000 is designed and supplied by SSTRAN in the USA. It is a 100mW unit for compliance with the FCC Part 15 regulations in the USA, and includes an audio compressor and limiter as well as a selectable treble-boost circuit. The RF oscillator is crystal controlled for stability and precision.
The kit comes complete with a case, knobs, PCB and all components. Unlike other kits, you don't have to pay extra for the case! A USA mains adaptor is also included, which is of no use in the UK, but again illustrates the completeness of this kit compared to others like the Vectronics VEC-1920K. I purchased a suitable UK mains adaptor from CPC for about £3.
The price is $89.95 (or $92.95, see later) plus
$15.70 for shipping to the UK. Payment is by PayPal and when I ordered (26th
September 2004) it came to £62 in total. Delivery took about a week, which
is fair enough. I did not have to pay any customs charges, however I understand
some buyers have been charged around £12 for duty/VAT etc.
A 3.6MHz crystal oscillator and divider circuit is used to provide a 9kHz reference. Note that 9kHz is the AM frequency spacing in most of the world apart from the USA and Canada (the USA/Canada version has a 4MHz crystal giving a 10kHz reference). A phase-locked loop oscillator and programmable divider circuit is locked to the reference frequency. So if the frequency required was 900kHz, the divider would be set to divide by 100, so that when the oscillator was running at the correct frequency the output of the divider would be 9kHz. The phase-locked loop compares this output with the reference frequency and adjusts the oscillator frequency so that they match. So by adjusting the divider, the oscillator can be set to any desired frequency in 9kHz steps. The divider is set by means of 8 DIP switches.
Left and right audio inputs are combined by two resistors. An Analog Devices SSM2166P IC carries out the audio processing functions, first compressing the audio by an adjustable amount between 1:1 and 5:1. This basically makes quieter sounds louder, giving the more consistent sound level effect we are familiar with on AM music stations. This is followed (in the same IC) by a 15:1 limiter which prevents over-modulation and the resulting distortion. Normally the levels would be set to the point just before the limiter comes into operation. A selectable treble-boost circuit is included between the limiter and the modulator.
The RF power amplifier is a long-tail pair circuit designed to give 100mW with zero modulation, as required by the FCC Part 15 regulations. The modulator works by varying the long-tail pair current source, thus varying the RF output power. The output of the RF amplifier passes through a tuning circuit consisting of four selectable inductors (selected by 4 DIP switches) and a trimmer capacitor. This is adjusted to give optimum matching to the antenna. A diode detector circuit on the antenna output allows a test meter to be connected when peaking the antenna matching.
Separate power supply regulators are used for
the oscillator and audio stages (5V) and the modulator and RF stages (15V).
The bridge rectifier diodes are bypassed with capacitors to give a low impedance
RF path when reverse biased. Selectable inductors are included in the power
feed and audio ground leads. These can be added into the circuit by removing
jumpers if necessary to minimise hum.
The kit is nicely packed in a good quality shipping box with ample bubble-wrap packing. The electronic components are packed in separate bags according to type - capacitors, resistors, ICs, inductors etc. Within the capacitors bag, all the 0.1uF capacitors are in a separate bag. All bags are labelled. Compared to some kits where everything is mixed in one or two bags, this is a real help to constructors.
The SSM2166P IC is a surface mount device with 0.05" pin spacing. This is half the pitch of normal ICs. With a fine tip on your soldering iron and good lighting (and your reading glasses, if you wear them) it is possible to fit this device yourself. However the kit is available with this device already fitted to the PCB for an extra $3, and I chose this version to save that fiddly task.
The manual gives clear well-written assembly instructions. Tick-boxes are included so that you can mark off each part as it is fitted. The assembly sequence is generally logical, although I would have left the pots and sockets until later. All the parts fit the PCB correctly apart from one capacitor (C32) which is a smaller pitch so needs the leads forming to suit (I understand this will be corrected on the next batch of boards).
The complete PCB assembly stage took me about 90 minutes, however I have a fair amount of experience of building PCBs at work (until recently I was an electronic design engineer). You may want to allow yourself several hours to complete the PCB, perhaps spread over a few evenings.
It is best to insert all the parts of the same type (such as all the resistors), then solder them all in one go. This way, if you make a mistake by fitting a part in the wrong place, you are likely to discover it before it is soldered in place.
The instructions state that the aerial and earth wires must be soldered directly to the PCB for Part 15 compliance, although a suitable plug and socket are included should the constructor wish to use them. Since the unit cannot be legally used in the UK either way, I fitted the plug and socket as it makes for a tidier job.
Case and power adaptor
The kit includes a white plastic case complete with drilled and printed front and rear panels. Everything fits together very neatly and easily.
The knobs are not quite the right size for the pot shafts so they rotate slightly eccentric, but this is not really a problem. Case assembly took me about 15 minutes.
The kit is supplied with a 120V AC to 18V AC power adaptor with an American 2-pin plug. This is of course useless in the UK, so I ordered a 15V AC UK adaptor from CPC for about £3. The unit will work from any adaptor providing 15V to 20V AC or 20V to 24V DC at 300mA or more, and fitted with a 2.1mm power plug
It would of course be possible to buy a 230V to 120V transformer to use with the supplied adaptor, but this would cost more than just buying a new adaptor.
Following my suggestion to the manufacturers, the kit is now available without the power adaptor. This gives a saving of $4 on the kit price and $2.40 on the shipping - a worthwhile saving for something we would never use anyway!
Testing and using
Unlike some other transmitters I have reviewed, this unit worked perfectly at the first test. The testing instructions are clear and quickly prove that the unit is working correctly. The unit is clearly designed for consistency and reliability.
The oscillator frequency is set by 8 DIP switches, following a table in the manual. I set it to 531kHz, which being at the bottom end of the band is normally quiet in this area. The antenna matching inductors are then selected by a further 4 DIP switches following another table in the manual. With power applied and the antenna positioned vertically, a multimeter (set to DC voltage range) is connected to test pads T1 and T2 then the antenna trimmer capacitor is adjusted for maximum voltage reading.
The operation of the three front panel controls is perhaps slightly unclear from their labels, however once you have read the manual and played with the unit a bit, it becomes clear enough.
GAIN is actually the audio input level to the compressor/limiter circuit. It's best to start with this turned right up. Once the modulation and compression are set, the gain should be turned down to the point where the volume decreases slightly. At this point the limiter is only limiting the peaks.
MODULATION sets the level from the limiter stage and should be adjusted to the point just below that where distortion is audible. On my unit this was at about the mid position.
COMPRESSION sets the amount of compression applied to the audio. Fully anti-clockwise is no compression; fully clockwise is 5:1 compression. I found that for rebroadcasting FM radio (Saga 105.7FM, naturally) on AM, turning it around one fifth of the way up sounded good. The setting will obviously vary depending on the type of programme you are broadcasting, the radio you are listening on and your preference. For CDs, MP3s and digital radio you may want a bit more compression than for FM radio, since FM is already compressed to some extent.
An internal jumper link allows the inclusion of a treble-boost circuit between the compressor/limiter and the modulator. This compensates for the roll-off of higher frequencies within the radio that would otherwise cause the sound to be a bit "muddy". The treble boost is just the right amount, as it sounds correct (compared to AM broadcast stations) and not excessive.
Internal jumper links may be removed to add inductors to the AC supply input and the audio ground if needed to minimise hum. I had no hum problems with the links fitted.
From my tests the range of the unit is very similar to the Vectronics kit. With the antenna positioned vertically upstairs and the earth wire connected to the electrical earth, I had usable reception throughout the house (a modern 3-bedroom semi) and a little way out into the garden. Out on the road the signal becomes buried in the background noise. This test was done at 531kHz, where the range will inevitably be lower. At higher frequencies the range will be greater, so for our use we will probably need to detune the antenna trimmer to keep the range to within our own homes.
This AMT3000 is the fifth AM transmitter unit I have tested, the others being the AM02 (no longer available), AM03, Gizmo and Vectronics VEC-1290K. The AMT3000 is by far the best. Indeed I really cannot fault it at all. It is straightforward to assemble, works exactly as described and sounds really good.
It looks smart and professional too - people seeing the completed unit wouldn't realise it was built from a kit. Although it is more expensive than the others, you get so much more for your money that it is actually better value too.
If you want an AM transmitter to bring your vintage radio collection to life, and you can manage straightforward electronic kit construction, buy an AMT3000!
The kit is manufactured by SSTRAN in the USA. The price is $89.95 for the standard version (AMT3000-9K), or $92.95 for the version with the SMT IC already fitted to the PCB (AMT3000-9KSM). Be sure to order one of the "9K" versions as these have the 9kHz channel spacing used in the UK. Shipping to the UK is $15.70 (charged at cost).
The kit is now available without the power adaptor, giving a saving of $4 on the kit price and $2.40 on the shipping. All options are on the ordering page.
Although orders by post are accepted you would
need to pay in US Dollars, so it is much easier and quick to order online paying
Some photos on this page are Copyright © SSTRAN and are included with their kind permission.