BA1404 - HI-FI Stereo FM Transmitter 88

About 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.

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.

Printed Circuit Board

This a suggested high-resolution PCB layout for BA1404 Transmitter. It is ready for printing and no further adjustments are necessary. Dimensions of the PCB should be 57 mm x 35 mm (W x H).

Download layout in Photoshop format (600 dpi)
Download layout in TIFF format (600 dpi)

Frequently Asked Questions

This is list of answers to questions that I have received through an e-mail so far. If you should have any other questions please don't hesitate to e-mail them to me.

What is the range of BA1404 Stereo FM Transmitter?
Range of the transmitter depends on many factors.
a) Length and type of the antenna (the longer the better, copper will radiate radio waves better than aluminum)
b) Supplied voltage (3V Max power output)
c) Weather conditions (clear skies & no humid, greater distance)
d) Elevation above the ground (the higher the better)
e) Line of sight (no buildings or trees, greater distance)
f) Whether external amplifier is used

I want to use the BA1404 Transmitter in my car for my iPod. What can i do to reduce an output power?
Supply the transmitter with just 1.5V voltage supply (single AA battery cell) and minimize the length of the antenna.

Can I replace a variable coil with regular air coil and variable capacitor / trimmer?
This is possible but keep in mind that frequency stability will be decreased with temperature drifts.

How can frequency of the BA1404 transmitter be controlled with a varicap diode?
Check this section for some theory and examples on how to use tuning diodes.

I would like to be able to transmit the music to greater distances. Can you provide any suggestions on how to modify the circuit?
You can connect an external amplifier based on the common 2N3866 / 2N4427 RF transistors. Please keep in mind that it is illegal to operate higher power transmitters in most of the countries without the proper license.

Where to get the Parts

If you are building the above Stereo FM Transmitter and have trouble finding some of the components like BA1404, 38KHz crystals and 3.5 turn high precision variable coils we are distributing the following components in our Electronic Store and a complete list of parts in Electronic Kits section.