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

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

DONT PUBLISH UNTIL TRIED OUT BETTER AUDIO AMP AND SPEAKER.

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.