negative Feedback in a Schoeps circuit?

[Ishi]

Please refer to the above schematic Mr. maye4 were posted.
"Coupling caps"  mean  C3 and C2 （４７ｎ）?

My simulations show quite a lot of difference in relative Z though the absolute difference is small.  Differences become more important at LF cos the coupling caps see different impedances.

 

[micaddict]

The negative feedback in a Neumann circuit is used to reduce the high frequency lift that is inherent to the K-67 capsule design and the high frequency hiss that was inherent in the head amp (two birds with one stone). It is static in its operation. What is dynamic about it?

-James-

There are much wiser guys (electronically speaking at least) than me here, so I'm not the one who should be answering this. But if we take the U67 (first mic to get the K67 capsule) from practical experience I can say that the brighter and harsher a source gets, the more the mic gets into smoothing mode. It sure feels like dynamic EQ.
As for the negative feedback in this case, the rather special thing is the connection of it to one of the transformer windings. And the U67 has a tube too, of course. Interestingly, the U67's sister M269(c) has a lower output tube (AC701 vs. EF86) "feeding" the tranny (please excuse my non-scientific attempt to get this across), and this mic has less of the high frequency taming going on, be it dynamic or not.

 

[RuudNL]

It might feel that there is a dynamic process going on, but it isn't.
The compensation network in a U67 (or whatever microphone using a K67 capsule) is fixed, meaning there is no level dependant processing.
In the Schoeps circuit, you could simulate the U67 approach by adding an capacitive (frequency dependant) feedback between the drain of the FET and the polaristion voltage of the capsule.
Some people increase the value of the capacitors between the base and the collector of the PNP transistors at the output.
This has the same effect as adding a capacitor between the drain and the source of the FET.
Although this will cause a HF rolloff, the 8 KHz boost that most cheaper K67 capsules have, remains.

This is the frequency response of an uncompensated K67 capsule:

and this is the frequency compensation used in a U67:

 

[micaddict]

Thanks, Ruud. 

In any case the U67 has a transformer (linked to the NFB) and a tube. Bot will saturate or more and more approach saturation at a certain level. How will this interact with NFB circuit?


Henk

 

[RuudNL]

Core saturation of the transformer might play a role, but I think the output of a U67 should be in the order of line levels, before you get an audible effect! (Neumann always used good quality transformers!)
Also core saturation is frequency dependant. Distortion as a result of core saturation will occur especially at low frequencies.
The level to get a transformer core distorting at high frequencies, is substantially higher.
And referring to the tube: as long as a tube is correctly biased, the point where you get a compressed output is in general far above the standard operating level. Circuits that are not properly designed might suffer from this effect, but I think the people at Neumann knew their stuff!

 

[micaddict]

Yes, one would expect both to be reasonably linear within their operating ranges.

 

[MagnetoSound]

To help illustrate the situation, bear in mind that the feedback ratio across the spectrum is always fixed.

There will of course be more dBs of feedback with louder sources, but the ratio is the same.

Otherwise, the freq response of the mic would be grossly SPL dependent, which would render even a simple response plot meaningless!  ;D

 

[maye4]

Here is my final touch.

Add inductors on the out puts of the mic to futher smooth out high end.

here is I singing pink moon of nick drake.

full of air but without sibling.

and without any eq or compression

change .jpg to .mp3

 

[maye4]

And just for fun,how much will you pay for it?

after that I'll reveal the price of the stock mic.

 

[RuudNL]

Do you have an updated schematic diagram?
(Or: what inductors did you use? Isn't the effect of the inductors dependant on the impedance the microphone is terminated with?
In any case it is a L-R filter when the inductors are in series with the output.)

 

[maye4]

Do you have an updated schematic diagram?
(Or: what inductors did you use? Isn't the effect of the inductors dependant on the impedance the microphone is terminated with?
In any case it is a L-R filter when the inductors are in series with the output.)

and LC too.And Inductor mess around with time domain too.It has a suttle saturation effect.

updated schematic is not necessary.Just two inductors before xlr.47uh to 200uh,suite your taste.Big the inductance,smoother the high end,but more distortion

 

[maye4]

The stock mic is ugly!

So I did a little finishing touch:

My own logo!

Just for fun and a good mood!

 

[maye4]

It's good to have a little voodoo magic on your recordings(I assume this Hieroglyphic of my culture looks like a voodoo sign to you)

 

[maye4]

Left is my vienna mod(mxl v67 circuit).
I made a series of my moded mics.

 

[HellfireStudios]

Doesn't the Neumann U-87 have 47uH inductors on its output? They are bypassed in the standard U-87, but are used in the U-87p. The only difference between these two mic circuits is their output levels. the inductors lower the output. Just google U87p and check out M. Schneider's answer to the differences between the U-87 and the U-87p.
What could be happening is you have lowered the output of the mic. This would require more gain from your preamp, and that could be the source of the coloration.

-James-

 

[RuudNL]

the inductors lower the output.

That's a completely new point of view for me! 

 

[maye4]

Doesn't the Neumann U-87 have 47uH inductors on its output? They are bypassed in the standard U-87, but are used in the U-87p. The only difference between these two mic circuits is their output levels. the inductors lower the output. Just google U87p and check out M. Schneider's answer to the differences between the U-87 and the U-87p.
What could be happening is you have lowered the output of the mic. This would require more gain from your preamp, and that could be the source of the coloration.

-James-

He said resistor，not inductor。。。
http://www.neumann.com/forums/view.php?site=neumann&bn=neumann_archive&key=1000224154

 

[HellfireStudios]

The KM-84 use 47ohm resistors in his way, but the schematic shows 47uH inductors for the vintage U-87. Dan's DIY87 project even includes them in the BOM (though, technically they aren't needed for standard operation).

-James-

 

[RuudNL]

One can doubt if the 47 ohm series resistors have a significant effect on the output.
Say the input impedance of the amplifier is 2000 ohms
In that case 94 ohms in series (2 x 47 ohm) will give a voltage drop of only 2000/2094 or 4%.
That is less than 0.5 dB!
What will suffer is the signal to noise ratio, because the noise generated by the resistors is in series with the output of the microphone.

 

[MagnetoSound]

A couple of points ...

1) U87p is the early low impedance version, transformer strapped for 50Ω and has 47R build out resistors for optional signal pad (bypassed in 'standard' U87)

2) The U87/U87i also has a balanced RF filter on a little PCB in the connector housing, comprising two series inductors (47uH) and two shunt capacitors (330pF), which is standard regardless of whether 7-pin or XLR version

These are two separate features, best not confused.

HTH.