Dumb idea for a mic circuit?

OneRoomStudio · 2024-12-01T16:47:39-0500

I was thinking about the limitations of phantom power the other day, and I wondered what kinds of solid state mics would be designed if power weren't a consideration. One idea that occurred to me was to build a cascode circuit using bipolar 60V. The nice part about using +/- 60V is that it would make polarizing the capsule a piece of cake. The backplate could be grounded, +60 could be applied to the front capsule, and the rear capsule could be switched between +60V, 0V, and -60V for omni, cardiod, and figure-8 (respectively). You could even add some voltage dividers for additional patterns. Anyway...here's the circuit I was thinking of. I simulated it, and it seems to be solid. Thoughts?

tk@halmi · 2024-12-01T18:03:51-0500

Seems like a lot of voltage across those two transistors. What current do you get through Q1?

radardoug · 2024-12-01T18:26:51-0500

OneRoomStudio said:
I was thinking about the limitations of phantom power the other day, and I wondered what kinds of solid state mics would be designed if power weren't a consideration. One idea that occurred to me was to build a cascode circuit using bipolar 60V. The nice part about using +/- 60V is that it would make polarizing the capsule a piece of cake. The backplate could be grounded, +60 could be applied to the front capsule, and the rear capsule could be switched between +60V, 0V, and -60V for omni, cardiod, and figure-8 (respectively). You could even add some voltage dividers for additional patterns. Anyway...here's the circuit I was thinking of. I simulated it, and it seems to be solid. Thoughts?

View attachment 140825

Power is not a consideration. The phantom supply gives plenty of current and voltage to build mikes. You are not powering a power hungry device.
And if you dont like phantom, there are plenty of ways once you build a dedicated supply.

OneRoomStudio · 2024-12-01T19:03:59-0500

radardoug said:
Power is not a consideration. The phantom supply gives plenty of current and voltage to build mikes. You are not powering a power hungry device.
And if you dont like phantom, there are plenty of ways once you build a dedicated supply.

Right, the point is that with a dedicated supply, +/- 60V would be convenient for capsule polarization…so the design was based on that.

tk@halmi said:
Seems like a lot of voltage across those two transistors. What current do you get through Q1?

It pulls around 12.5mA, which is on the high side, but within the spec of the LSK170A (20mA max). There ends up being around 32.5V at the BJT collector, which again, it high-ish, but within spec.