DIY: Bluetooth-to-AM-adapter

In Norway, AM broadcasting is long gone, and the signals from foreign stations have a tendency to drown in noise from modern electronic devices, unless you live in a rural area. To make my old AM radios more usable, I made an AM transmitter hidden in a picture frame, which receives its audio signal via bluetooth. This allows me to stream music from a cell phone to vintage broadcast receivers.

As basis for the AM transmitter, I used the schematic from an article in Nuts&Volts magazine. This circuit was laid out on a rather large PCB of about 147x171mm.

I customized the transmitter circuit for receiving audio signals from an on-board bluetooth adapter, changed all components to SMD, and made a compact PCB layout of 25*94mm.

Transmitter schematic. Click for high resolution.

The oscillator is based on a CMOS inverter (SN74HCU04N) and a crystal. There are few available crystals with frequencies in the medium wave range. This is solved by using a crystal of higher frequency, and using two cascaded flip-flips to divide the oscillator frequency by 4. I chose a crystal of 3.579545 MHz, which gives a modulation frequency of 895kHz.

The audio source is a MH-M18 bluetooth receiver module. The stereo signal from this module is mixed down to mono by to resistors (R3, R4) and amplified by an OP AMP (LF351D) before entering the mixer/modulator stage.

The mixer stage is made of discrete transistors, and basically lets the audio signal control the amplitude of the 895kHz signal. Some may argue that the flip-flops deliver square waves, which one would expect to spread the signal over a wide spectrum, but both the modulator and the antenna are tuned to the selected frequency, and in practice the harmonics aren’t causing much trouble. The output stage is a BUF634 high speed buffer.

The modulator is tuned by a tank circuit consisting of L1 and C11 in parallell with C13. The tuning would be easier if one of the capacitors was adjustable, but I couldn’t fit a trimmer cap on the small PCB. The solution was to calculate a proposed capacitance value from the formula
C = 1 / (4π² f² L), solder in one capacitor with a smaller value than that, and temporarily solder in a tuning capacitor which is adjusted until the modulator delivers max amplitude. Then I desoldered the tuning cap, measured its capacitance, and soldered in a fixed capacitor with the closest standard value.

Power supply

Since the power supply would be too thick to fit inside a picture frame, i made a separate DC PSU. The circuit is pretty basic, with a mains transformer which delivers 2x12Vrms, 210mA. This output is rectified and filtered, and voltage regulators 7812 and 7912 stabilizes the voltage at +12V and -12V for the transmitter circuit. The positive voltage is further stepped down by 78L05, to deliver +5V for the oscillator.

PSU Schematic. Click for high resolution.

The bluetooth adapter needs an isolated 5V supply to avoid ground loop problems which would cause noise. This is handled by a B0505S isolated DC converter.

I designed the PSU PCB to fit inside the enclosure of an old “wall wart”.

The originally black “wall wart” was painted white, to be less visible against a bright wall. As DC power cable, I used a flat ethernet patch cable.

The antenna

A 6mm copper pipe, bent to fit inside a picture frame of 80×80 cm serves as an antenna.

This loop got an inductance of approx. 3µH. Using a 10nF capacitor in parallell, the antenna was tuned pretty close to the transmitter frequency of 895kHz.

The finished bluetooth-to-AM-converter, with transmitter and antenna inside the picture frame, and separate power supply.

This is a very low power transmitter, but the range is sufficient for indoor use, and it sounds pretty good 🙂

In this short demo video, a Tandberg Sølvsuper 3 is receiving signals from the AM transmitter:

Retronics