I have been restoring some old '50s and '60s tube amplified radios, fitting them with Bluetooth while maintaining the original electronics and functionalities.
I achieved this by connecting the Bluetooth audio to the record player input of the radio. This worked relatively okay.
Now I am thinking of making a radio into a smart speaker, using a Raspberry Pi Zero W and the open source Google assistant software and connect this to the radio.
This is where my problem lies. As the Raspberry Pi Zero does not have any audio output except for over HDMI, I will need some sort of circuit. From the research I have done it seems the way audio works on the Pi is using the PWM pins.
There are some boards online, like the PAM8403 as well as pHAT DACs. However, it is unclear to me if this will work as I intend, as they are all meant to be the final amplifier and directly drive the speakers, not be amplified again through, for example my tube amplifier system in the old radios.
The points of confusion originate from the following:
- PWM can be used for the audio, I am confused if this is then used as I2C, i.e. digital which has to be converted to analog, or is this PWM used to create an analog signal? If so, then I might be able to directly pass it into the radio?
- The PAM8403 board is a class D (digital) amplifier. I have read that connecting together the L- and R- could cause problems, but to connect it to the radio I have a L, R and common ground.
- The PAM8403 is 3 W output, the radio however I think is used to a very low powered input signal from the record player, would this not blow up the entire radio? Could I just fix this by using a simple voltage divider?
I guess what it comes down to is that I need a low-power analog signal from my Raspberry Pi. I have found things like HifiBerry, but I am looking for a more budget-friendly solution as I believe those are a bit of an overkill. I would prefer a ready-made solution that I can buy but am also open to making my own if it is not too complicated.