Gainclone Power Amplifier

I've built a “gainclone” power amplifier, based on the LM3886 Overture Audio Power Amplifier, using PCBs from chimamp.com. The case is from DIY enclosures (they would appear to be out of business). Since those cases have rather weak bottom panels, all components are mounted on a 3 mm thick aluminium plate, that in turn is mounted in the case using the same screws that attach the side panels to the bottom panel. This configuration makes the case quite rigid, and has the added advantage that no holes needed to be drilled in the bottom panel. Each LM3886 is mounted on a heatsink rated at 1.5 K/W inside the enclosure. The ventilated top of the case allows for sufficient circulation to keep the whole amplifier mildly warm to the touch.

Four LM3886 modules are powered from two unregulated power supplies, and from a single 300 VA toroid transformer. The filter capacitors total 48,000 µF.

A custom soft-start circuit is included to limit the inrush current at startup. When the push-button switch on the front panel is pressed, a relay is activated that switches on the mains power, but with two resistors into the current path. After about half a second, a second relay activates that connects the toroid directly to the mains. The first relay is no longer needed now, and is deactivated. The advantage of this circuit is that the resistors never remain in the current path for more than about half a second, even in case of a relay failure.

Pictures