hi jakob (and others as well, of course),
i've been comparing your design and gus' reverse-engineered schem of the brauner to my library of classic schematics... one striking difference is the unusually high grid resistor (and actually the polarizing resistor, too). in the classic designs, you find a few hundered megohms at maximum. according to textbooks, this is supposed to reduce the problem of grid leakage current shifting the bias too much. also, a superimposed random current (shot noise) from the grid wouldn't create a noise voltage as high as with 1G...?
on the other hand, i hear that noise level gets down with increase of grid resistor. could anyone elaborate? Low pass behaviour with the tube input capacitance effectively in parallel, or something?
at last, as to variation of polarizing R's... is there a disadvantage to make it lower than 1G, other than possibly affecting frequency response? is resistor noise an issue here? is 1G possible with the now available metal film R's, while classic mic's had to use noisier carbon (-composite) ones?
cheers,
volker
i've been comparing your design and gus' reverse-engineered schem of the brauner to my library of classic schematics... one striking difference is the unusually high grid resistor (and actually the polarizing resistor, too). in the classic designs, you find a few hundered megohms at maximum. according to textbooks, this is supposed to reduce the problem of grid leakage current shifting the bias too much. also, a superimposed random current (shot noise) from the grid wouldn't create a noise voltage as high as with 1G...?
on the other hand, i hear that noise level gets down with increase of grid resistor. could anyone elaborate? Low pass behaviour with the tube input capacitance effectively in parallel, or something?
at last, as to variation of polarizing R's... is there a disadvantage to make it lower than 1G, other than possibly affecting frequency response? is resistor noise an issue here? is 1G possible with the now available metal film R's, while classic mic's had to use noisier carbon (-composite) ones?
cheers,
volker