Next up : Vacuum Tube Stage
The audio output from the DAC is AC-coupled to the grid of the vacuum tube. The tube operates in a classic "cathode follower" configuration, where the output is simply taken from the cathode of the tube, and AC-coupled to the next stage. The anode (plate) is connected to 100V, supplied by a little dedicated DC/DC converter inside the headunit. This converter need not supply lots of power ... even though the potential across the tube is ~100V, the current demand is low, probably only a few milliamps. I can't quite tell the bias current of the tube ... I still can't find the part number used by Panasonic and a classic load-line analysis is tough without a datasheet The heater for the tube is powered by 6.3 volts.
The "cathode follower" does not provide any voltage gain. It's simply a buffer stage, which can provide current gain if needed ... but the net effect in this headunit (besides the great looks) is to simply add some "tube coloration" to the signal path.
Sonic "accuracy"? Probably not. The cathode follower no doubt increases the measured distortion of the signal path. But you know, there's just something about tubes ... maybe it's just the nostalgia factor for those of us old enough to remember audio before transistors. But the love affair with tubes just won't die, or even pass quietly into obscurity. Check the diy audio sites ... even in 2005, some half-crazed audio nuts are still looking for ways to follow modern DAC stages with tube output stages ... just like what we find here
