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Simple open-source condenser studio microphone with a modern twist

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Better make those 47 Ohm resistors very high precision then. In audio range the output impedance of the opamp is a bee's dick.

PS. AFAICS with something modern like an OPAx828, a 0.1% difference between the 47 Ohm resistors causes more impedance mismatch across the audio range than dropping the bottom opamp.


--- Quote from: Spirit532 on July 19, 2023, 05:13:50 am ---

--- Quote from: RFDx on July 19, 2023, 05:08:31 am ---Why do you need a 1GOhm bias resistor in front of the noninverting op-amp?

--- End quote ---
Avoids DC charging of the input. Given the extremely high(many GOhm, and not particularly well characterized) input impedance of the opamp, it can cause a serious charge to build up on the input and just rail it. I've tried it without the resistor, and found that this happens.
--- End quote ---

What I was wondering was why do you need 10 x 100 MOhm in series? You would get the same result with 200 MOhm for R13 and the opamp bias resistor.

--- Quote from: RFDx on July 19, 2023, 05:08:31 am ---https://en.wikipedia.org/wiki/Virtual_ground
--- End quote ---

That's right. A node that has the same voltage potential as GNDA but is not connected to it would be a virtual ground. The inverting input of U1B is indeed a virtual ground. The non-inverting input of U1B, which is connected to the (real) GNDA from the rail splitter, isn't.

Have you looked at the JLI electronics mic kits?
How does it compare?

Also, have you considered using two 6V zener diodes in series to create the virtual ground and 12V supply? That would result in a lower power-on time and reduce component count a bit.

This is seriously cool, thanks for sharing!  I can't wait to make one.


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