Conclusion

Because this project was realised while I was trying to sell my house, it was built without the full resources of the workshop and the hi-fi. But despite this, I'm pleased with the design and construction, and the sound quality has exceed expectations.

As a finished product, this amplifier is seriously cute, and has elicited lots of positive reaction from friends and colleagues. The design details, including the hiding of the connectors and the use of birch ply, make for an interesting look.

The LM4780 runs slightly warmer than the LM3875 because it has a higher bias in the output stage. The quiescent current is around 50mA per channel - compared to around 17mA for the LM3875. With 30V supply rails, this translates to 6 watts of heat with no signal - the case runs at around 15°C above ambient under no-signal conditions. I quite like this heat - it reminds me of my old Musical Fidelity A1... Fortunately, because music has a high PMR (peak to mean ratio), moderate use doesn't raise the temperature much - the large amount of aluminum present in the chassis helps here because it has a decent amount of thermal inertia. The overhanging top panel also helps, despite only being 1mm thick - what matters for heatsinking is surface area, and although it isn't optimally aligned for good convection, you can feel a healthy current of warm air rising from under the rear of the panel. Of course, if the amplifier is overdriven, the thermal sensor will mute the amplifier well before the chip has reached unsafe limits, ensuring reliable long-term operation...

One lesson learnt regards the woodwork; while I took great care to ensure that the rebates cut into the side panels were perfectly accurate, the wood shrunk slightly over the first few months of operation. This means that there is a noticeable step between the wood and the top/bottom panels - especially on the amplifier where the rebate was deepest for the top chassis. It's not a huge problem, but I'll know to account for this next time.