A board · sheet 4/8 · manual page 26

The main amplifier sheet

The only part of Commando with no logic on it at all. Op-amps, resistors and a power amplifier, drawn in a completely different idiom from every other sheet.

▸This sheet is part ofHow the sound works

The subsystem followed end to end, across every sheet that builds it.

What this sheet is for

Sheet 3/8 produced sound digitally. This sheet turns it into something a speaker can move air with, and it reads like a different era of engineering because it is one.

The FM side of each YM2203 leaves as a serial stream and needs a companion DAC, so there are two YM3014s. The PSG channels leave as analogue already and are summed with plain resistors. LM324 op-amps buffer and set gain, a mixing pot at VR2 balances the sources, a volume pot at VR1 sets the level, and an HA1368 power amplifier drives the cabinet speaker directly.

FROM THE SOUND CHIPS TO THE SPEAKERYM3014FM to analogueSUMMINGPSG added inLM324buffer and gainHA1368power amp

Each YM2203 sends its FM voice out as a serial stream to a companion DAC, while its three PSG channels leave as raw analogue and are summed passively with resistors.

Two details on the drawing

The DACs are numbered against the sound chips in a crossed order: 8C-2 belongs to chip 1 at 11C, and 8C-1 to chip 2 at 9C. It is recorded in the model because it is precisely the sort of thing that costs an hour on the bench.

And the title block carries a piece of factory history: the mixing resistors were revised from 33K to 56K. Somebody built this machine, listened to it, decided the PSG channels sat too loud against the FM, and changed three resistors. That revision is on the schematic and it is on the board.

Every chip on this sheet

Click any of them to open the schematic with that chip selected in both panels.

8BLM32411AHA13688C-1YM30148C-2YM3014
← open the schematic for this sheet