Electronics > Open Source Hardware

Simple open-source condenser studio microphone with a modern twist

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Marco:
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.

RFDx:

--- 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.

Zipdox:
Have you looked at the JLI electronics mic kits?
https://www.jlielectronics.com/diy-accessories/opa-alice-single-channel/
https://www.jlielectronics.com/diy-accessories/dc-dc-hex-inverter-pcb/
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.

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

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