Monday, April 11, 2011

Stereo FM Transmitter with BA1404

High quality FM Stereo transmitter using BA1404 for MP3, MP4, iPod

BA1404 HI-FI Stereo FM Transmitter:

Whether you want to create your own radio station, transmit the music around the house, or simply create a wireless link between your iPod and a receiver in your car, this transmitter will let you do these things easily. With BA1404 HI-FI Stereo transmitter you will be able to transmit MP3 music from your iPod, computer, discman, walkman, TV / SAT receiver, and many other audio sources.

The above FM transmitter design is a result of many hours of testing and tweaking. The goal was simple; to test many existing BA1404 transmitter designs, compare their performance, identify weaknesses and come up with a new BA1404 transmitter design that improves sound quality, has very good frequency stability, maximizes transmitter's range, and is fairly simple for everyone to build. We are happy to announce that this goal and expectations have been met and even exceeded.

The transmitter can work from a single 1.5V cell battery and provide excellent crystal clear stereo sound. It can also be supplied from two 1.5V battery cells to provide the maximum range.

Suggested PCB layout for BA1404 Stereo FM Transmitter:

Secrets Behind Sound Quality and Frequency Stability:


One of the qualities of BA1404 FM transmitter is excellent frequency stability. This is mainly due to a use of high quality 3.5 turn variable coil. Tunable RF coils are ideal for precise frequency tuning because their magnet wire is halfway embedded within the plastic, which minimizes frequency drifts. Regular air coils are not preferred for professional broadcasting because the coil expands and contracts with temperature changes. That's the very reason why variable coil was chosen as a substitution for an air coil and a variable capacitor.

Another quality of the presented BA1404 transmitter is a crystal clear stereo sound and improved sound separation. There are several factors that account for improved sound quality and a separation. First reason is the use of 38 KHz crystal which provides rock solid frequency for stereo encoder. Another reason is the use of two 1nF decoupling capacitors one for BA1404 chip and another for 3.5 variable coil. These capacitors have to be as close as possible to a BA1404 chip and a variable coil because this will GREATLY improve the sound quality, sound separation and even frequency stability as well. What they do is filter out the noise in the incoming DC voltage. If the noise enters BA1404 chip stereo generator will include it in a transmitted sound affecting both the sound and multiplex signal that is responsible for generation of the clear stereo signal. If that noise enters it will also be included in a generation of subcarrier frequency affecting the frequency stability. Most people are not aware of how important this is and might place them in a wrong location, away from the target components which provides no use, or worse decide not to use these capacitors at all.

Another factor that is extremely important and which improves overall quality of the whole BA1404 transmitter including frequency stability, sound quality and sound separation is the use of the ground plane on the transmitter’s PCB. It is recommended that ground plane should always be used in circuits that deal with higher frequencies.

For optimum transmission range the length of the antenna should be 1/4 or 1/2 of the wave length of the transmitted frequency. Use this simplified formula to determine the antenna length for 1/4 wave length antenna type:

(30000 / Transmitted Frequency) / 4 = Antenna Length (in cm)

(30000 / 88MHz) / 4 = 85cm Antenna
(30000 / 108MHz) / 4 = 70cm Antenna

The RF coil is usually the most critical part of the transmitter. The ideal coil for this circuit is 3.5 turns with ferrite core usually found on FM radio. See the coil below:

Here is an alternative RF coils:

2.5 turns 48 - 59nH (Red) ------> AM radio oscillator coil
3.5 turns 65 - 79nH (Orange) -----> FM radio oscillator coil
4.5 turns 90 - 109nH (Yellow) ----> AM radio IF coil
5.5 turns 109 - 132nH (Green)
----> FM radio IF coil


  1. proof na po ba ito?
    at san nyo po nabili yung 38khz na crystal?

  2. Here without the CRYSTAL..

  3. The crystal is only needed (i though) for stereo sound, if you would like to have mono, then you don't need a cystal.

    On my project at school, we diddn't use the crystal. And it still workes :D

  4. I was trying to fix the Tx freq. in the fm band without using variable tuning ,i.e., i wanted only one single freq. in the 88-108 Mhz band. Can you suggest th e suitable L-C combination at pin no.10 of BA1404?, Another thing is that, due to the unavailibity of 38 Khz crystal, can I use, 32.706Khz , instead , for pilot tone?

  5. ayuda. puedo usar un cristal de 32.7 khz? puedo hacer la bobina con cable esmaltado enrollado en nuclo de ferrita.????

    help me.. I can use a crystal of 32.7 and I make a coil with a simple cable smalt and center of ferrita??

  6. i didn't get the ic ba1404...... what can i do for same.....