Audio Circuit (Sega Mega-Tech)
The audio circuit on the Sega Mega-Tech is identical to the Mega Drive, except for the final stage.
Check out the Sega Mega Drive Audio Circuit wiki page for much more detail.
In short, there is:
- PSG audio generated from the 315-5313
- FM Synthesis generated by the YM2612
- No Edge Connector audio
These are mixed together and sent to the LA700N audio amp.
The output for left and right speaker are the connector CN3. Pins 1 and 2 are left output to a speaker, and pins 3 and 4 are the right speaker.
The speakers' volumes are controlled by the two potentiometers above the YM2612. Left to lower, right to increase.
You must have the 12V power applied for LA7400 amp to get its power and work.
At idle with or without a speaker connected, and no audio, it should pull 110mA approximately.
When driving both speakers, it can pull up to 2A at full volume.
If you want to ignore the amplifier output and 12V rail, you can just connect a powered speaker (that accepts mic level input) to the YM2612 audio chip, with ground on the Analog Ground pin 19, and audio out on pins 20 (right) and 21 (left).
NOTE: On Master System games like After Burning for example, the audio out of the YM2612 will be very quiet. To test properly apply the 12V power and tap the audio from the actual output CN3 header.
