Electronics > Open Source Hardware

Simple open-source condenser studio microphone with a modern twist

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Spirit532:
It's not doing nothing, it does provide a differential signal(that I've measured), but there is an imbalance due to the way they're linked. Solving this would require adding a buffer, which would likely blow the power budget. It's already stretched pretty thin with the high impedance takeoff.

Marco:
Voltage wise it's creating a differential signal on the XLR connector, but it's creating it from an imbalanced connection to the capsule. A voltage differential signal an sich has no real value at mic level, it's not like the mic level inputs will saturate, it's cargo cult differential. If you made the top opamp a simple unity gain buffer, removed the bottom opamp and just connected the bottom coupling capacitor directly to GNDA you would have superior noise.

If GNDA was connected in the middle of the string of bias resistors instead of the bottom, the connection to the capsule would be balanced and the differential amplification would have some justification.

Spirit532:
It's a differential signal going down the cable for common mode immunity, there's absolutely no point in doing differential pickup from the capsule.
All audio interfaces expect a differential input by default. That's what the second opamp is for. Yes, you can ignore that and take it single ended from ground.

Marco:
It's not the differential signal an sich which gives you noise immmunity, it's the matched impedance and a differential input on the receiving end. You can have matched impedance on a single ended output just fine.

That's why this type of differential amplification is cargo cult, it looks at completely differential signal chains and confuses an unimportant artifact (the signal being differential to a reference voltage) and confuses it for being essential. It ain't, all you are achieving by recreating the artifact is increasing amplifier noise.

Spirit532:

--- Quote from: Marco on July 19, 2023, 11:56:37 am ---It's not the differential signal an sich which gives you noise immmunity, it's the matched impedance and a differential input on the receiving end. You can have matched impedance on a single ended output just fine.

That's why this type of differential amplification is cargo cult, it looks at completely differential signal chains and confuses an unimportant artifact (the signal being differential to a reference voltage) and confuses it for being essential. It ain't, all you are achieving by recreating the artifact is increasing amplifier noise.

--- End quote ---

It's partially true, but having it be even slightly mismatched differential will give an edge over completely single-ended. It matters less when the whole thing is shielded, and it is, but as this is an opensource project I'm expecting other, less experienced people to run into blunders like running cheaper unshielded cable, plugging it into questionable gear and such, in which case it does actually help.

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