Audio

JAMMA boards provide a signal-level audio signal which has enough oomph to run an 8Ω speaker directly. Indeed, you could mount such a speaker in your SuperGun and have it play sound that way. However, if you want to hook up to an external amplifier or the TV via SCART, the signal-level audio needs to be dropped down to line-level. Not doing so will likely result in at best a very loud set-up and at worst a fried amplifier and/or TV set.

Some SuperGun builders use a simple voltage divider circuit to do the level conversion. This however usually means that one side of the audio cicuit gets grounded, which can potentially damage some JAMMA boards. To be on the safe side, I decided to use an isolating attenuation circuit to keep the signal level and line level sides separate. Such a circuit isn't particularly complex and therefore not hard to build yourself, but it's also possible to use an off-the-shelf converter to do the job. These are readily available from suppliers of car audio equipment.

[IMG: level converter] [IMG: level converter guts]

If you're only interested in running JAMMA boards, which only do mono sound, you can simply connect the JAMMA speaker+ and speaker- pins to the inputs of such a converter. If you have a stereo converter, the left and right inputs can be spliced together to provide sound to both channels.

I however wanted to support stereo MVS boards too, which use a different system. Instead of speaker+ and speaker-, you get a left+ and right+ and the ground signal takes place of the "minus" for both channels. In this scenario, both the "minus" inputs of the converter are therefore connected to ground and the "plus" inputs go to their respective "plus" pins on the MVS board.

In order to switch between the two modes, I used one DPDT (double pole, double throw) switch. This is essentially two separate switches in one housing, which both change togther when the switch is flipped. It's wired up like this:

[IMG: audio switch]

When the switch is in the upper position, both the left+ and right+ of the converter are connected to speaker+ whilst left- and right- are both connected to speaker-. This provides the same signal to both inputs so we get mono JAMMA sound on both outputs. When in the lower position, both left- and right- inputs are grounded and the separate left+ and right+ are connected to separate inputs, giving stereo MVS sound.

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