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Low power amplifier study plans
AlfredoT One Transistor One Watt Amp
AlfredoT One Transistor Five Watt Amp
1.2 Watt FM Amp
70mW FM Amp (about halfway down the page)
VHF Power Meter & Dummy Load
The AlfredoT Antenna Tuner

Alfredo said: While I am still trying to work the bugs out of the final stage of the 50W (PEP) power amplifier, the driver stage from this amplifier can be used as a stand-alone low power amp, capable of output on the order of 1.5W (PEP). Here’s the schematic: More…

The Circuit

This simple push-pull power amp will produce several hundred milliwatts into a 50 ohm load with just a few milliwatts of drive from a 50 ohm source. It can be used from about 450 kHz to over 2 MHz. The key bennies are:

• Low cost–2N3053s are low-cost, general-purpose TO-39 silicon transistors.
• Broadband design never needs tuning.
• Feedback keeps the distortion down.
• Push-pull design reduces the complexity of the output harmonic filter.
• Class AB biasing lets transistors run cooler and saves power.
• The circuit will operate satisfactorily from an unregulated 12 volt supply. Automotive and battery supplies are a-ok.

Modification for other frequency ranges should be fairly straightforward. For lower frequencies, down to about 100kHz, just wind the transformers with more turns, while maintaining the same turns ratios (hint: doubling the number of turns makes the inductances of these windings increase by a factor of four, thereby bringing the lower frequency limit down by a factor of four). For higher frequencies, different toroidal cores, as well as windings with fewer turns need to be used, as the Amidon T-94-15 cores are only recommended for use up to about 2MHz. Cores for higher frequencies can be found in the Amidon Associates catalog. At some point, however, the 2N3053s will start to run low on gain; one cannot expect to build a good VHF amplifier with this circuit. I have not tested this circuit at the high limit of its frequency response.

This amplifier can be used to boost the milliwatt-level output of the AM Stereo Exciter up to about 300mW (note that the peak envelope power of an AM signal with 125% positive modulation is 5 times the average or unmodulated carrier level. This circuit can be included in the same enclosure as the exciter and powered off the unregulated 12 volts present at the input pin of the 7809 3-terminal regulator. If this is done, check the inputs and outputs of the regulators to make sure that excessive ripple doesn’t result when the RF power control is turned up. CAUTION: It is very easy to go over the FCC part 15 limits when using this amplifier on the AM broadcast band. Keep in mind that the rules limit power to 100mW DC input to the final stage.

Construction

Although the circuit will probably be stable on perfboard, printed circuit boards are recommended because they are more rugged. When winding the transformers, wind the tapped windings as shown in the diagram below. One half of the winding is the red wire and the other half is the green wire. This way, both halves of the tapped winding cover the entire circumference of the core and couple equally to the other windings.

On the input transformer, wind the primary and then wrap the toroid with a single layer of electrical tape. This reduces inter-winding capacitance and prevents shorting. Then, wind the secondary. On the output transformer, wind the primary and wrap it. Then, wind the feedback winding and the output winding. Placing these windings on the outside allows them to be re-done to optimize the design. There is always room for small improvements.

Be sure to use heatsinks on the 2N3053s, as they will get hot when RF drive is applied. Low-cost, clip-on cooling fins should be adequate.

Modifications

This is a relatively simple circuit that can easily be adapted to other applications. However, keep in mind the maximum ratings of the transistors: Collector-emitter voltage must never exceed 40 volts, so the supply voltage should be kept below 20 volts (a 15 volt maximum is probably a good idea). The collector current must not exceed 700mA, and power dissipation must not exceed 5 Watts per transistor.

For low voltage operation (say, 6 Volts), it may be necessary to reduce the value of the emitter resistors in order to maximize the collector to collector voltage swing. This will cause the input impedance to go down, and it may cause the bias point to become less stable. It will also be necessary to add more turns to the output winding to get adequate output power.

Recommended reading if you’re going to try this: Winding Toroids and Using RF Transformers.

Also check the AM Stereo Repository for more of Alfredo’s work, and other interesting files, some of which some people might find offensive.

FM Transmitter Reviews from around the web. Most of these are old, but the firms noted are still making kits and the fundamental information is still valid. What isn’t valid is the assumption that all these kits are Part 15 compliant – they are not, so be attentive 🙂

www.irational.org/sic/radio/bc-ware-lcd-pll.html – Broadcast Warehouse 1W FM LCD PLL No Tune Exciter/Transmitter Kit Review

FM Transmitter Reviews from around the web. Most of these are old, but the firms noted are still making kits and the fundamental information is still valid. What isn’t valid is the assumption that all these kits are Part 15 compliant – they are not, so be attentive 🙂

www.irational.org/sic/radio/bc-ware-lcd-pll.html – Broadcast Warehouse 1W FM LCD PLL No Tune Exciter/Transmitter Kit Review

www.irational.org/sic/radio/panaxis-fmx.html – Panaxis FMX PLL FM Exciter Review

www.irational.org/sic/radio/veronica-1w.html – Veronica 1 watt FM Transmitter Kit Review

Tripod.com hosted reviews here – pop-ups galore!

www.members.tripod.com/~transmitters/begin.htm – FM Transmitter Kits for Beginners

UK108 – Universal FM Transmitter with Mic and Line Inputs (Assembled)

User Report

For a fast, quick way to try out Part 15 “microbroadcasting” this kit is a painless way to get started. I purchased this completly on a whim, but soon found that 1) it is really fun play with radio broadcasting and 2) what practical uses one can make of such set. It has also inspired me to go for my amateur license!

UK108 – Universal FM Transmitter with Mic and Line Inputs (Assembled)

User Report

For a fast, quick way to try out Part 15 “microbroadcasting” this kit is a painless way to get started. I purchased this completly on a whim, but soon found that 1) it is really fun play with radio broadcasting and 2) what practical uses one can make of such set. It has also inspired me to go for my amateur license!

I have little or no formal electronics background – I’m a software guy who tinkered long ago to make computers, and still does some repair and refurbishing of my own stuff. But audio and radio electronics I’ve pretty much avoided. I found this kit provides a nice entry that very well may lead to more complex projects (I’m already looking at some of the AM kits from other suppliers)

CanaKit Universal FM Transmitter with Mic and Line Inputs
  Payment via Credit card
  Order placed 7/18
  Item received 7/22
  Boxed, kit in blister pack

Includes
  Transmitter on printed circuit
  Four AA battery holder
  Attached (30 inch?) wire antenna
  One page “manual”

For an “on the whim” project that I just dashed off, this is more fun than I expected. And, unlike most solitary activities (programming, rockets, etc.) this one is actually communal and ENJOYED by my family.

I’ve listened a lot over weekends, and experimented some.

  * Range is pretty much my lot – I could get to the alley and the front sidewalk. Spotty, depending on obstacles, and not for Walkman (i.e. mobile) listening. I suspect better antenna’s, transmitter location, could improve things.

  * Playlist: Seussical (the upcoming musical at my daughter’s High School), Dixie Chicks Simspsons songs, lots of country, rock and more.

  * Radios: my mini-shortwave picks it up well – very handy for the gazebo in my yard. The radio by my upstairs computer does too; nice when we are sitting in the den. A bathroom radio on the second floor can get it too.

Observations:

I listen to radio a *lot* mostly walkman earbuds, or headphones at work. While it is very nice to have so many options the funny thing I’ve found with “my” radio station is that it is kind of liberating to be untethered from the radio – it just sits there and makes music. Like a cheap paperback, it can go anywhere!

Also nice is the “just pure music” aspect – I listen to too much news – commercial free, speech free (except the couple of “Radio Free Klingon” blurbs I recorded – only 4 out of over 400 tunes, so they don’t come up much).

I thought I’d use this to extend the streaming audio stuff – I don’t really care if I do now. I don’t want to give up control of the playlist.

The other funny thing is that the audio quality is not an issue. It isn’t perfect – I have occasional “hum” problems – but it is surprising how much you can get out of one little speaker. (maybe years of walkman abuse has taken its toll?)

From our old site There has been new FM PLL Kit showing up on ebay these days, so I grabbed one and gave it a try. The product web page is http://www.edmdesign.com/intro.htm Overall impression – great to play with, might be frustrating for a beginner. The version without a case or power supply needs both and should be considered a kit. Be prepared to do some assembly.

From our old site There has been new FM PLL Kit showing up on ebay these days, so I grabbed one and gave it a try. The product web page is http://www.edmdesign.com/intro.htm Overall impression – great to play with, might be frustrating for a beginner. The version without a case or power supply needs both and should be considered a kit. Be prepared to do some assembly.

Features:
Compact size
Microprocessor Controlled
Easy Frequency Programming
Digital Frequency Readout with 7-segment LED display
Non-Volatile Memory
Transmissions from this unit can be received on any reasonable FM radio MONO or STEREO between 87.7 ? 107.9MHz

You must be able to find a fairly quiet spot in this band to operate the unit otherwise range and stereo quality will be affected.

When assembled, this kit is a digital synthesized unit (meaning no frequency drift, tuning or fiddling). Frequency is phase-locked against a accurate and stable quartz crystal (similar to the one in watches) This unit’s frequency is programmed by means of a CMOS microprocessor without any DIP switches and complicated look-up tables. Pressing the UP or DOWN buttons makes it very easy to set the frequency. Transmit frequency is clearly indicated on a 3 digit, 7 segment LED display.

Frequency response: 20Hz – 15kHz
Level: Line-level input via RCA type sockets
Stereo Separation:
Typically about 40dB
Operating Power Requirement:
Needs a clean regulated 12v DC source with current capacity of 150mA for the standard power and 250mA for the high power option.

Operating the unit with an un-regulated wall wart type transformer will introduce mains hum into your transmissions.

I set up an “instant OTR outlet” using a completed 10 mW PLL XMTR kit, a guitar compressor for a little punch and a CD player for program input. I pushed it all out through a simple dipole.

Total cost was $100.00, $70 for the transmitter kit, $20.00 for the compressor and $10.00 for the CD player.

The primary problem in setting up this system was that the transmitter kit needs extensive post purchase work to be useful. That’s not a criticism, per se, as it is only $70.00 and it’s and otherwise nice unit – just be aware it’s a kit and not plug and play.

First, it must be mounted in a metal box, as the circuitry has no protection from stray signals.

Second, you need a real clean power supply or you will be slammed with ripple. I’ve been using Eveready lantern batteries until I could build a clean enough PS.

Range is a bit too much for comfortable Part 15 use, I’d probably add an antenna attenuator to knock the range down a bit if I was going 24/7. I was getting a car radio range of about 3000 feet north-south, and 2000 feet east-west. Portable radios couldn’t pick me out of the cross talk on the band as well, and I got about half that on the portables.

This transmitter is also available in a 100 mW kit unit for $100.00 – Probably a guaranteed bust if left running for any period of time

UPDATE: EDM Designs FM 100 Mw Transmitter Kit with case.

The New 100 Mw kit, completed and mounted in its case

I ordered this thing on Friday last week. Wednesday the following week I got the kit in the mail. Nicely packed, easy to assemble, easy to get the transmitter installed in the case, included a power supply to use after assembly.

I got the whole kaboodle to avoid blowing the thing up by plugging in the wrong kind of AC adapter by mistake (it’s happened).

I immediately sorted through the FM dial to find 3 adjacent channels that were clear – remarkably, I found them.

I plugged in the long wire they include with it, and away I went. A short time after opening the box my kit was assembled and I was listening to my radio.

The design is rugged. The box has mounting ears and 4 screw holes.

There is an RCA jack for the antenna, 2 for the stereo input, and the power supply plug.

I don’t know what else to say other than : Wow. VERY nice.

And for people who are wary of buying from overseas: this was shipped from the EDM affiliate in California.

I most definitely give this bad boy 2 thumbs up.

Kit Assembly Instructions from EDM

Basic summary of assembly steps

We are often asked about the skill level needed to get a unit completed and operational. Below is a quick summary of the steps to be performed by the buyer to get the unit functional. More detailed instructions and precautions about static sensitive devices, polarity etc. will be provided in the manual.

Remove microprocessor from protective foil and insert into socket holder on PCB

Insert digital displays into respective sockets on PCB

Connect the power wires to a suitable 12VDC source.

With the EP versions, install the supplied power connector and fasten with screws

Solder the power wires, Red and Black to power terminals on the power connector

For versions without enclosures, you may construct your own enclosure or purchase from another supplier

For versions without power units, you may construct your own or purchase from another supplier

Follow operating instructions supplied for specific model series in manual

More Images Here

Operating Manual Here

A nice review page for Part 15 Transmitters and other interesting products, from:
Low Power AM Transmitter Reviews

Great information from Jolly Old England.

Author Paul Stenning reminds us that his website, including all text and images not otherwise credited, is copyright 1997 – 2006, Paul Stenning.

A nice review page for Part 15 Transmitters and other interesting products, from:
Low Power AM Transmitter Reviews

Great information from Jolly Old England.

Author Paul Stenning reminds us that his website, including all text and images not otherwise credited, is copyright 1997 – 2006, Paul Stenning.
All details are believed to be accurate, but no liability can be accepted for any errors.

I have been using a Rangemaster AM1000 transmitter for almost five years now, and not a week goes by where someone doesn’t call or send an e-mail asking how I enjoy using it, or if the unit is worth the money that I paid for it. I hope to answer all these questions and more here in this document…

Read the rest of Todd’s Review here: www.toddjenkins.com/radio17

I have been using a Rangemaster AM1000 transmitter for almost five years now, and not a week goes by where someone doesn’t call or send an e-mail asking how I enjoy using it, or if the unit is worth the money that I paid for it. I hope to answer all these questions and more here in this document…

Read the rest of Todd’s Review here: www.toddjenkins.com/radio17

Be sure to note Todd’s photos at the bottom of the page – very nice antenna/transmitter mounting information.

Visit Rangemaster here: www.am1000rangemaster.com