27-11-2016, 06:04 PM
Wow... Yes I'd certainly meant 'virtual earth' at the second stage grid, from a signal point of view, blatantly not true at DC! But assuming there's a high (AC) voltage gain in the second stage, it's a reasonable approximation. After all, even with a modern op-amp, the gain is still finite...
DC coupled would be nice, down side is the relative incompatibility of anode and grid voltages, unless some considerable level shifting is done (which usually involves either negative supply voltages, or rather high positive ones). We're lost without PNP valves...
Of course, for a phono preamp, the response doesn't need to extend to DC, in fact the RIAA curve has a LF turnover frequency of 20Hz if I remember correctly - hence the 0.01μF / 1MΩ right on the input.
DC coupled would be nice, down side is the relative incompatibility of anode and grid voltages, unless some considerable level shifting is done (which usually involves either negative supply voltages, or rather high positive ones). We're lost without PNP valves...
Of course, for a phono preamp, the response doesn't need to extend to DC, in fact the RIAA curve has a LF turnover frequency of 20Hz if I remember correctly - hence the 0.01μF / 1MΩ right on the input.
