Great question. The bias voltage is applied to the backplate. Then the diaphragm is connected to the virtual ground via the 1G resistor. That resistor “develops” the signal via the voltage change caused by the change in capacitance due to sound moving the diaphragm. ?That voltage also goes into the Opamp.?
The bias generator actually has a 1M output resistor as part of the output RC filter. One other thing to know is the virtual ground is about 6V thus the actual bias voltage is about 75V?
Some of the designs you mention that have 1G resistors on both sides do some switching of bias voltages to change the mic pattern etc.?
Hope this helps?
Best Regards,
Jules Ryckebusch?
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On Mar 10, 2025, at 20:03, matthewsewell via groups.io <matthew@...> wrote:
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Beginner here.
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I've been looking over the instructable for the True Condenser OPA and there are 2 things that I'm not completely understanding yet. I suspect they are related though. In other mics I've looked at (the MXL Schoeps-style, eg) the high voltage passes through a 1G resistor before being applied to the backplate and the polorizing cap and then to the FET gate past another 1G resistor while the capsule wire is connected to ground. I think maybe this is a typical setup for a pencil mic.
If I'm understanding the instructable correctly, the 80V (or so) is applied to the backplate and the capsules are coupled through the polarizing cap to the OPA gate past the 1G resistor. I think this makes sense (since what we're sending to the OPA is the difference between the 80V and the capsule) but I'm not 100% on this. I'm wondering why there is no 1G resistor between the 80V and the backplate. Is this not necessary since the 80V DC cannot pass through to the capsule side?
