Capsule polarization question

[vmanj]

Hello everyone.
I saw this polarization scheme for the capsule.
What are the disadvantages or advantages of such a scheme?
Will there be 60 volts (at the anode) and at the capsule when switched on this way?

 

[RuudNL]

Usually you want the microphone capsule biased from a stable voltage.
This is not the case if you take the polarisation voltage from the anode.
In fact, the capsule capacity will now work as a feedback capacitor.
Never seen such a configuration before.

 

[LevinGuitar]

Maybe it's a mistake and the polarization voltage comes from directly from B+? Could it work that way?


Is this the microphone?

 

[Voyager10]

Assuming the pentode has high gain, the AC signal at the anode will be nearly equal to the capsule output signal, as the tube will be trying to keep the grid voltage constant. If I'm understanding correctly it's also relying on negative feedback to keep the output impedance at the anode low, presumably to drive a relatively low ratio transformer.

It has one fewer wire needed than a normal cathode follower, is that the big deal?

 

[vmanj]

Maybe it's a mistake and the polarization voltage comes from directly from B+? Could it work that way?


Is this the microphone?

Well, yes, only 19a9 uses a triode, but there are also similar circuits with a pentode (in triode mode).

 

[vmanj]

Assuming the pentode has high gain, the AC signal at the anode will be nearly equal to the capsule output signal, as the tube will be trying to keep the grid voltage constant. If I'm understanding correctly it's also relying on negative feedback to keep the output impedance at the anode low, presumably to drive a relatively low ratio transformer.

It has one fewer wire needed than a normal cathode follower, is that the big deal?

Those. will it "look" like a cathode follower?

 

[rock soderstrom]

Maybe it's a mistake and the polarization voltage comes from directly from B+? Could it work that way?

Yes, that works. The SELA T12, for example, takes B+ directly as the polarization voltage. As it is only 60V, no voltage divider is required.

In the circuit shown in the first post, the polarization voltage would vary with the conductivity of the tube, which makes no sense to me.

 

[rock soderstrom]

Well, yes, only 19a9 uses a triode, but there are also similar circuits with a pentode (in triode mode).

6Ж1П is a pentode (here in quasi triode mode) and is very similar or equal to the 6AK5/5654/EF95 you will find in the SELA T12. Nice tube.

Edit: This type of tube also works very well with low voltages.

 

[vmanj]

6Ж1П is a pentode (here in quasi triode mode) and is very similar or equal to the 6AK5/5654/EF95 you will find in the SELA T12. Nice tube.

Edit: This type of tube also works very well with low voltages.

I have a 6Ж4П (6AU6) tube, of course I’ll try these two options for capsule polarization later, but first I wanted to ask a question, maybe someone has already done this.

 

[terry setter]

Hello everyone.
I saw this polarization scheme for the capsule.
What are the disadvantages or advantages of such a scheme?
Will there be 60 volts (at the anode) and at the capsule when switched on this way?

A simple improvement would be to take the capsule voltage from above the plate resistor, use a voltage divider to get the 60V you're after, then add a small capacitor to help smooth any remaining "noise" from the polarizing voltage.

PS: You'd probably want more than 60V at the top of your plate resistor.

 

[vmanj]

A simple improvement would be to take the capsule voltage from above the plate resistor, use a voltage divider to get the 60V you're after, then add a small capacitor to help smooth any remaining "noise" from the polarizing voltage.

If this is what you meant, then this is the standard scheme that is used in most cases.
But my question was in order to understand why the engineers made such a non-standard type of polarization.

 

[terry setter]

Hi, sorry I missed that.

Ah, the "WHY" of engineers...!

 

[vmanj]

Hi, sorry I missed that.

Ah, the "WHY" of engineers...!

I meant the engineers who developed the Lomo 19-a9 microphone.
If, of course, the diagram is without error..

 

[Biula]

Those. will it "look" like a cathode follower?

No, the output signal is taken in the wrong spot: a cathode follower takes the output signal... well... from the cathode! The Lomo 19-a9 is NOT a cathode follower topology, capsule polarization is provided by the same voltage of the plate (fed by the plate load resistor), which floats with the capsule variable voltage; meanwhile, the grid and the capsule backplate tries to stay voltage constant via the bleeding resistor (that is the only very high-value resistor, no "1 Gig" resistors here: a cost-effective measure for sure!). The signal is then tapped from the plate and sent to the balanced output transformer, as in standard plate-loaded mics. It is a very unusual and creative design, using fewer components and a lot of feedback for linearization... very soviet stuff!

 

[vmanj]

No, the output signal is taken in the wrong spot: a cathode follower takes the output signal... well... from the cathode! The Lomo 19-a9 is NOT a cathode follower topology, capsule polarization is provided by the same voltage of the plate (fed by the plate load resistor), which floats with the capsule variable voltage; meanwhile, the grid and the capsule backplate tries to stay voltage constant via the bleeding resistor (that is the only very high-value resistor, no "1 Gig" resistors here: a cost-effective measure for sure!). The signal is then tapped from the plate and sent to the balanced output transformer, as in standard plate-loaded mics. It is a very unusual and creative design, using fewer components and a lot of feedback for linearization... very soviet stuff!

Well, of course, I compared this circuit with a “cathode follower” conditionally.

The original documentation says that this is an anode follower; I have never seen this in tube microphones before.

If there is a lot of feedback, then the circuit is more linear and will probably have less distortion, i.e. more “sterile” sound... like a cathode follower circuit.
But maybe I'm wrong.

Nevertheless, I wonder if perhaps such a non-standard circuit adds something special to the sound of the microphone?

 

[Biula]

Well, keep in mind this was the microphone Steve Albini used on the "In Utero" Cobain vocal tracks... a friend of mine, who used the Lomo a lot, describes the sound as "a Sony C37 with an M7 in it"... so your reasoning seems right!

 

[vmanj]

Well, keep in mind this was the microphone Steve Albini used on the "In Utero" Cobain vocal tracks... a friend of mine, who used the Lomo a lot, descscribe the sound as "a Sony C37 with an M7 in it"... so your reasoning seems right!

Those. its sound will be more sterile, and closer to the cathode follower?

 

[Biula]

Yes, maybe close to a cathode follower: but I never consider a cathode follower microphone to have a "STERILE" sound! My words to describe it would be instead "neutral", "responsive" and "with infinite headroom"... the right choice for certain applications (e.g. drums overhead, loud sources in general).
In addition, the weird LOMO configuration probably has more amplification gain (hence, hotter output level) when the cathode follower only has 0.8-0.9 in its non-inverting tube stage.

 

[vmanj]

Okay, I'll try both options, I think it will definitely sound different.

 

[vmanj]

I tried to make the polarization close to that of the Lomo 19-a9 microphone, but the voltage on the capsule was too low.
Maybe I'm measuring incorrectly?