U67 de-emphasis network

[GeorgeToledo]

I know this would be a form of distortion of course, but I mention it becuase of reading various comments / threads / info discussing the nasty hard clip various mics have internally, like the 87AI... and this got me thinking about THIS as a way around that.

Much of the objectionable sound from U87AI doesn’t come from clipping the FET, it comes from clipping the output transformer. It is worth engaging the pad whenever there is even a chance of it being needed.

 

[kingkorg]

King Korg said
"It is exactly the same as using eq in post"
(sorry the qoute feature was messing up)

Or is it?
Consider that the negative feedback signal is a selected range of the audio signal that's captured by the capsule, phase reversed and being fed directly into the very same transducer that captured it. The most obvious effect was stated in the U67 marketing. Stating that the there was no need for a pop filter as pops were intercepted before the amplifier.
In other words, the low frequencies that caused the pop are eliminated by the reversed signal pushing the diaphragm in the opposite direction to avoid the diaphragm collapsing. Imagine what it's doing at high frequencies...
This is not just eq its also affecting the behaviour of the capsule.

You are suggesting some kind of dynamic action. Or maybe i got it wrong? It doesn't work that way, IMHO. Both low and high end curve are static and not dependent on the signal input. Easy to test by feeding the signal into the circuit. Also vintage k67 are easy to collapse anyways which they eventually fixed by increasing the diaphragm tension. However i see the point, and it is absolutely plausible the fed back signal does help counter the mechanical action. I'm not sure to what extent tho, and what the path is in u67.

@soliloqueen you are in the middle of testing this "issue" with vintage capsules. Maybe see if you get different result if the capsule is in actual 67 circuit?

 

[kingkorg]

@OPR one more question. What cap is responsible for low end feedback to the capsule? Is there even path for low end attenuation? @RuudNL maybe you could chime in. I'm not really familiar with u67 circuit. I just don't see it.

 

[Khron]

What cap is responsible for low end feedback to the capsule? Is there even path for low end attenuation?

I believe it's described here..?

I don’t know it is of interest but here is the patent paper explaining the low frequency feedback arrangement.
https://patentimages.storage.googleapis.com/63/fe/ce/b47b9b1fca6f3d/US3207848.pdf

 

[kingkorg]

I believe it's described here..?

So through S2 switch. Obviously i was focusing on the signal coming back from the transformer. Thanks!