U67 de-emphasis network

[OPR]

I did the original test with pink noise and a drum beat. Makes sense what you wrote
Also this is with u87. The NFB circuit is way different.

You're up a bit late! 3:30 am in Norway..

Now when you say the audio Nulls do you mean that the residual gives you absolute silence?
nothing at all?

Can you post the wavs?

What distance from the speaker are you working at?
I have a bunch of 87's here and can repeat your test with the 67 also.

 

[kingkorg]

You're up a bit late! 3:30 am in Norway..

Now when you say the audio Nulls do you mean that the residual gives you absolute silence?
nothing at all?

Can you post the wavs?

What distance from the speaker are you working at?
I have a bunch of 87's here and can repeat your test with the 67 also.

Not absolute silence, it was close to the noise floor. I'll see if i can dig up the files. It's at 40cm, coaxial speaker.

 

[ccaudle]

devising a test

A test which would not require an acoustic test environment would be injecting a frequency sweep signal into the feedback path, and measure the differential voltage across the capsule. I think everyone would agree that any effect on capsule behavior would have to be proportional to the voltage applied to the capsule.

 

[GeorgeToledo]

Under what mechanism would varying backplate voltage not affect movement of a diaphragm?

 

[ccaudle]

Under what mechanism would varying backplate voltage not affect movement of a diaphragm?

If the entire capsule "floated" on that backplate voltage, and the diaphragm voltage increased or decreased directly with the backplate voltage then there would be no affect on the movement of the diaphragm, since it is the difference between the backplate voltage and diaphragm voltage which determines the attractive force.
From a quick look at the circuit it appears that the driving impedance of the feedback network and the impedance of the capsule at 100Hz are both much lower than the input impedance of the buffer stage, so I don't see why there should be much voltage across the capsule due to the feedback.