Tuesday, April 15, 2025 Accessible FM Radio Case Study - Part 3 - Audio Tests

[Part 1] [Part 2] [Code and Design Files]

After some promising results with the PCB loop antenna, in this post, I'm starting to look at the audio output.

The intention for the final system is to design a single PCB incorporating the power supply, radio receiver, power amplifier and antenna. But before launching in to this, I want to do as much testing and developing as possible using the modules that I have to hand.

In Part 2 I was getting reasonable signal strengths from the PCB antenna, but I hadn't really listened to the reception, except through headphones.

The TEA5767 module I used has an audio amplifier built-in, but its the now obsolete TDA1308 which is a stereo amplifier offering 80mW at 32Ω per channel. No good for driving a speaker directly.

To try out what I have so far, I had a rummage around and found an old version of the MonkMakes Amplified Speaker and connected it to the output of the radio module and to the 5V supply of the Arduino.

I was not expecting results. The Amplified Speaker, is a very low power quite tinny thing and the quality was pretty awful. Key point were:

• Terrible sound quality
• Significant hiss
• Clicks every time the Serial Monitor reported the signal strength.

Touching the antenna to increase the signal strength got rid of the hiss. I am not too worried about this, as the FM reception at our house is pretty bad. Window-sills being required even for really good FM receivers.

The USB interference is not be a problem for the final device, as it will be battery powered, but it does serve as a warning that interference could well be a problem for the future.

I wanted to make sure that the TEA5767 output wasn't inherently low quality, so I connected the audio out of the TEA5767 module to the line-in of HiFi. The results were very acceptable. The USB click was still there, but overall reception was better (slightly different position).

Looking at datasheets and browsing component catalogues, I quite like the look of the PAM8320. It's a class D (think PWM) amplifier that can provide up to 20W. I don't need anything like that much power, but having some oomph in reserve is usually a good thing.

I think its getting close to the time, where I start to design the system as a whole.