Not designed by myself by but since someone asked about it and couldn't find this elsewhere on the internet, I decided to publish it here.
The Atari ST has a sound chip with three channels which are normally tied together to produce a mono signal. If you look at the pin out of the sound chip you see that each of the three channels has it's own pin. We can thus separate those channels and feed them to a stereo amplifier. The problem is to divide the signal of the third channel over both the stereo channels. This circuit was designed by Rob van der Haar and was published in the Dutch S.A.G. Atari Magazine, December 1988.
Schematics + build description
Stereo upgrade schematics
to cut pins 3, 4, and 38 of the sound chip loose from the motherboard and fed
them into this circuit. The sound chip is the 40 pin chip with "YM2149" or
"AY-3-8910" on it. The "monitor audio" goes back to the motherboard at one
of the pins you cut loose. The left and right audio signals goes to a standard
audio connector (RCA or stereo jack) and can be fed to an amplifier.
You can use the potentiometers to adjust how the signal is divided over the two channels.
The 'double pole' switch is used to switch between mono- and stereo-mode. The switch is necessary to switch back to mono sound while playing digital samples. This because samples sound not as intended in stereo (a lot of noise). Samples are played on an ST by adding the volume levels of the three sound channels. If you separate the channels, the volumes will not be added and the sound will be distorted.