Problems with the All In One boards

A project log for OPL Studio - OPL2 / OPL3 MIDI Interface

a MIDI interface for the OPL2 Audio Board and OPL3 Duo!

maarten-janssenMaarten Janssen 02/27/2023 at 20:130 Comments

Since a couple of days I have the first prototypes of the OPL Studio all in one boards where I combine the OPL Studio and OPL3 Duo onto a single PCB. I'm quite happy with how boards look and how easy it is to assemble them, however...

There is a big problem with the all in one design and it is noise. Noise from the LCD, the pointer, the SD card and the internal buzzer. It's like when you have headphones connected to your laptop and you can hear the computer do its work or buzzing when you move the mouse, but much louder! If there was one thing that I wanted to get completely rid of with the new design it was noise, but I made it much, much worse than just a little buzzing on the speaker output. Now also the line output has unacceptable levels of noise.

So far I've tried adding series resistors to the SPI lines and adding a bunch of capacitors on the 5v rail for the analog portion of the circuit, though when I got to 4700 uF there was still too much noise. Probably next I'll start messing with the 5v line. Maybe a voltage regulator will clean up the signal? Maybe I should get rid of the separate supply all together and just use a single supply for both the digital and analog parts of the circuit? I don't know...

With my very limited knowledge on the subject I doubt whether I'll be able to fix all the noise issues with these boards. If anyone reading this has any tips for me that I can try then I'd like to hear! For now I think that these all in one boards are not going to happen any time soon unfortunately.