A spectrum analyser you could actually afford! ... There was nothing like that back in the day. Now, however, there are some very nice, very useful,... and very small pieces of test equipment that can be used for all sorts of projects. In this case, The aim was to play around with some audio inputs to the LM386 amplifier on the Retro Shield, and see what the output looked and sounded like.
The LM386 is a very nice, simple amplifier, and replaces the LM380 that I wanted to fit on the Retro Shield; but didn't have enough room. To simplify projects, I added the discrete components needed for the amplifier to the Retro Shield, making it an easy plug-and-play, like on many of the old school many-in-one experimental kits. The circuit is shown.
To mix old school with new school, the signal input is the Retro Waveform Generator project, the output goes to a pair of earphones via the 3.5mm jack, and a breakout connection to the new school TinySA; a 10dB atten. is added for protection