|soapfoot wrote on Sun, 16 January 2011 08:47|
The voltage divider created by R7 and R8, given an input voltage of 105v, will output a voltage of 63v.
However, between that point and the capsule backplate, there's a series resistance of 100M. I was trying my best to do the math (not my strong point) and I'm wondering how there can be +63V on the capsule backplate? Wouldn't there be some (rather large?) voltage drop across that 100m series resistance?
No, the load is purely capacitive, this means no DC current does flow through the capsule or C1, except some parasitic due to imperfect isolation.
Therefore the voltage on both sides of R1 is the same.
R1 + C1 do form a LPF to remove hum and noise from the capsule bias voltage.
It is tuned to 0.17Hz , suppressing hum about 50dB.
C1 2nd function is to create a low impedance AC ground coupling for the capsule backplate.
R6 + C3 do the same for the tube's plate voltage, the filter x-over is about 6Hz, filtering hum by about 20dB.
The filter even reduces DC fluctuations, caused by Line AC change, reaching the signal output.
R5+R6 form the plate resistor, where only R5 is active for signal's AC.
Together they set the DC working point for the tube in the plate branch.
R2 is the negative biasing resistor, R3 the positive biasing resistor (by placing the tube into positive voltage related to ground).
Together they are responsible for the grid/anode working point of the tube.
R3, in theory, acts as a negative feedback resistor too.
In fact this is neglectible, to my measurements there is no gain difference if it is bypassed with a cap.
The resistance of 29 Ohms is simply to small to build up a significant feedback voltage.
It's inductance (it's wire-wound) can, of course, build a RF filter the reduces the possibility to catch up radio interferences.
The capsule capacity together with R2 form a HPF, tuned to 21Hz.
Increasing R2's value would widen the LF response but make the mic more sensitive to sub-sonics like wind and plosives.
The same applies to C2, which does form a 12dB/Okt. HPF together with the output x-former's inductance.
I can't tell the x-over frequency of this one.
I guess (only guess) it's around 30Hz.
R2 does not have any direct influence on the HF response of the mic. Even the parasitic capacity of R2 only dampens the signal over the whole audio range very slightly (less then 0.1dB).
But changing the low end can make us believe the high end does sound different.
Increasing R2 would bring the tube circuit out of the tubes spec's. In fact it's already far of (x200), the spec says no more then 0.5MOhms (for Pentode use, in this quasi-triode circuit it doesn't seem to be such a problem)!
So changing it will change the working point and stability of the circuit.
100MOhms allow some more negative charging of the grid (by parasitic electrons), which adds to the bias caused from R3.
This would mean less current is flowing through the tube, the working point is shifted, distortion can increase.
I've never measured the amount of extra biasing, although it's possible using a high voltage probe with an Ri of e.g. 1GOhm.
One could even do a relative measurement by shorting R2 with a low value resistor and measure the dc change on the plate.
DC on the plate should go down if the high R2 value did change the biasing.
BTW: The U47 circuit is very nice for understanding how a tube condenser mic works, as it's the most basic one possible.
It uses the least number of parts for a working mic.
In theory only the hum filters could be left away or build different.
E.g. you could leave away R1 and increase C1 to 1uF to achieve the same result.
If you have a perfectly regulated PSU you could even leave away R6 and C3, and replace R5 by 130kOhms, but that's all.