Dumb idea for a mic circuit?

[OneRoomStudio]

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]

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

 

[radardoug]

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]

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.

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.

 

[tk@halmi]

I see so the 4.7K resistor drops almost all of the negative rail voltage.

Make sure to use at least 1W resistors.

 

[k brown]

With a slightly higher trafo ratio, could C1 be eliminated?

 

[radardoug]

There are many ways to skin the cat, but putting +/- 60 volts on those two transistors makes no sense. Also, your cap values dont make sense. If you are doing a dedicated supply, +/- 15 seems more appropriate for the amp.

 

[radardoug]

With a slightly higher trafo ratio, could C1 be eliminated?

Those two are in no way connected.

 

[k brown]

Those two are in no way connected.

Doesn't bypassing the source resistor increase gain?

 

[OneRoomStudio]

There are many ways to skin the cat, but putting +/- 60 volts on those two transistors makes no sense. Also, your cap values dont make sense. If you are doing a dedicated supply, +/- 15 seems more appropriate for the amp.

The cap values were set large enough that they wouldn’t affect the frequency response for simulation purposes. C2 could go as low as 1uF maybe, without losing too much low end, and C1 could go down to maybe 4.7uF or could be removed, as @k brown mentioned. Removing it would improve linearity but reduce gain.

Since the output transformer is a step-down, the ratio would have to go down with C1 removed to retain a similar output level. Luckily, the output impedance of this circuit is pretty low and you could probably get away with a 3:1 or maybe even 2:1 OT.

 

[abbey road d enfer]

Thoughts?

It's necessary to add a capacitor at the input for proper simulation.
Without it, the circuit is improperly biased.
With the capacitor, the simulation show that Q1's Vds is only about 0.5V. I don't think it's right.
It shows in the fact that the circuit distorts heavily at about 300mV, which is paradoxical for a circuit powerd with +/-60V rails.
I agree that it may be good enough for typical capsule levels, but for me it indicates inadequacy.
Proper cascode operation requires a different biasing of the FET.

 

[abbey road d enfer]

With a slightly higher trafo ratio, could C1 be eliminated?

Q2's collector sits at about 35V. No xfmr, whatever the ratio should operate with that much DC.

 

[k brown]

Q2's collector sits at about 35V. No xfmr, whatever the ratio should operate with that much DC.

I think you're tallking about C2.

C1 is the source bypass cap I referred to.

 

[abbey road d enfer]

I think you're tallking about C2.

C1 is the source bypass cap I referred to.

Aah, ok.
Without C1, the circuit's gain decreases from about 20dB to minus 20dB.

 

[abbey road d enfer]

Luckily, the output impedance of this circuit is pretty low and you could probably get away with a 3:1 or maybe even 2:1 OT.

The circuit's output impedance is almost entirely defined by Q2's collector resistor. It's the standard constant-current behaviour of a cascode. The output signal is largely affected by the load that's applied.