I beefed up the amplifier I built couple months ago for this article. I was not satisfied with the tiny sound so I bough a PNP/NPN pair BD679/BD680, high-voltage op-amp LF412CN/NOPB, and a nice jack from Sparkfan. Results you can see... and hear!

The extra op-amp is for mixing left and right channels.

I had to bypass one high-current track with a wire instead of using the matrix (not visible, not the black wire), but the rest worked. Also, by-pass capacitors are absolute MUST when so much inductance is present in the tracks.

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Engine sounds from THOR