The Pixie2 is a simple QRP CW transmitter that dozens of ham radio operators have successfully built. (QRP is ham jargon for low-power operations, and CW is the simplest method of sending Morse code merely by turning a carrier-wave on and off.) The Pixie2 is usually built for the 40 meter band but it will work on frequencies from 1000 kHz up to at least 15 MHz. It is said to output a couple hundred milliwatts of RF.
The circuit can be amplitude modulated quite easily. A small audio amplifier feeds audio current into the 8-ohm side of a transformer. The 1k ohm side of the transformer is inserted in the V+ supply going to the Pixie's output transistor.
This modified Pixie2 is called the Talking Pixie. It has 18 components (not counting circuit board, jacks, power supply and external audio amp). Building it on a prototyping board only takes a few minutes if all the parts are available.
The level of the audio fed to the transformer is adjusted until the best sound quality is achieved. The Talking Pixie will not sound as loud as commercial stations but the user must avoid the temptation to over-modulate; nobody will listen to an over-modulated signal.
C1: 100 pF
C2: 220 pF
C3: 82 pF
L1: 150 uH
L2: 22 uH
Q1: 2N2222 or 2N3904
Q2: 2N2222A (metal can type) or 2N3866
R4: 10 or 15 ohms (experiment!)
T1: 1000 ohm to 8 ohm audio transformer
The frequency is crystal-controlled. A crystal for the frequency you're interested in will have to be ordered if you don't have one handy.
The transformer must be rated to handle at least half a watt of audio; a very tiny transformer will not sound good and will have too much resistance on the 1K winding.
L3, C6 and C7 form a low-pass filter to attenuate the harmonics generated by the circuit. Specific values for various frequencies can be found on the Medium Wave Alliance's filters page.
L1 and L2 are factory-made axial molded chokes.
The impedance and bandwidth of the antenna will affect the sound quality of AM transmitters like the Talking Pixie. What sounds good on the test bench with a 50-ohm dummy load attached to the output might not sound as good with real-world antennas like short end-fed wires. Some kind of antenna tuner might be helpful. (By the way, two 100-ohm resistors in parallel make an adequate dummy load for this rig.) Needless to say the size and efficiency of the antenna will have a major impact on the range.
If you build the circuit on a prototyping board (as shown above), you can experiment with many variations on the circuit design.
Here are some modifications that have been suggested...
Martin Spencer suggested using a FET (such as 2N7000) instead of an NPN transistor for Q2. This could give more linear modulation. Replace L1 with a 5K variable resistor; remove the crystal for a moment and adjust the resistance for about 2 mA drain current.
Mark Weiss wrote: "You can put another transistor in series with the PA and use it as a series voltage source. By varying this voltage control element with the audio signal, highly linear modulation is achieved. Transformers tend to present variable impedances, causing the PA to be less stable under varying load conditions. A direct-coupled modulator can offer the potential for great tolerance of loads that aren't precisely +50 j0."
Other QRP CW transmitters can also be modified for amplitude modulation. You will find schematics for such transmitters in ham radio books and magazines, and on websites operated by QRP clubs.