[Don Pearce[_3_]]

On 13 Sep 2010 16:36:26 -0400, (Scott Dorsey) wrote:

wrote:

The 33 Meg resistor is being bootstrapped to at least 330 Meg. The
bootstrap is direct coupled. The output Z of the capsules I'm using
is ~50 Meg at 40 Hz.

How did you get that number? What is the capacitance of the capsule when
idle?

I'm getting plenty of low end, and no obvious
distortion. The problem with a higher value grid resistor is the bias
becomes drifty. The U47 used a 60 Meg grid leak, which I think is too
low, but there was no bootstrapping in that case.

As long as the leak resistance is low enough to permit all the spurious
charge appearing on the grid to be grounded out, you're fine. The amount
of spurious charge that appears there is dependant on the kind of tube you
use and it's one of the more important things to select when you pick a
tube for microphone applications.

Are 1 or 2 Gigohm thick film resistors quiet enough in this
application? I can't find metal film above about 50 Meg.

Read the previous message. The capsule is bypassing them, it doesn't
matter how noisy they are.

Oh it does. The same RC pair that is lowpass to the bias and its
associated noise is highpass to the wanted signal. In the region of
crossover (ok cutoff for both) any resistor noise will be coming
through loud and clear.

d