Hi Nigel,
Good question!!
The power produced by an amp into 8R is given by the voltage peak to peak figure squared, then divided by 64.
If you assume that the dead zone, 2.45V, is the only period when both upper and lower output devices are actively passing current, and then add a further 0.2 volt to both ends of this zone for each output device to switch off, then this corresonds to 127mW of output power into 8R.
That is, for a quiescent current of around 65mA, the Class A output power of the Lifeforce is just 127mW - and no more.
If you increase the quiescent current to say 500mA (at which point the output stage of the 25W amp would be dissipating a cool 9W per device, which is pretty warm, but not hot), then the Vbe bias voltage would increase to 3V, and the switch off zone would increase around 0.8 volt each side, giving us 330mW of Class A operation. As you can see, these figures are not very favourable to Class A operation.
Full on Class A would require no less than a quiescent of 2.4A, including reactive reserves. This would be a total amp dissipation of 43.2W per device, and give a power output of close to 25W continuous into 8R. These sorts of dissipations would require huge heatsinking, around four times more than normally supplied with the LF 55W, and yet I rather doubt that it would sound that much better than a LF25. On top of that, the outputs would likely need replacement every couple of years, and you'd get a bit sick of this thing pumping 86W of wasted heat into your listening environment.
In my view, Class A is not the answer. This is akin to achieving more performance with a tripling of engine size; it is not particularly efficient, there are huge heat problems, and in fact the sound would not be as dynamic.
Cheers,
Hugh
