Stupid or clever? You decide - U87 meets Schoeps

[Khron]

Right, so a couple days ago i had this brainwave (or brainfart? ;D ) - if that PNP phase-splitter could be used in the stock BM800 circuitry, with the Schoeps-like "output stage", that pretty much left the JFET input stage free to be / do whatever you wanted. So why not implement / add on the U87-like negative-feedback-EQ components?

So far i've only added the appropriate components into my modded-Schoeps schematic, and spent "a while" struggling to shoehorn in the extra components onto the already quite-densely-populated board, but i managed to succeed. Well, minus the connection to the pad switch, but i'll leave figuring that out for a later date.

I've got a couple of 32mm sE Electronics K67-like capsules, and a 34mm K67 out of an AKG Perception 200, that i can use for testing. Not exactly sure when, but i'm gonna breadboard this thing, and see what happens ;D

 

[Wordsushi]

Oooh, I think that would be really cool.

 

[RuudNL]

Yes, this should work. (I don't see any reason why it shouldn't! It is the standard U87 input circuit with an electronically balanced output.) 

I did this in a different way in the past: I added an extra NPN transistor stage with the base coupled to the source of a standard Schoeps circuit. The ratio between collector and emitter resistor of this transistor determines the gain of this stage. The collector of the added NPN transistor now has an amplified signal with reverse polarity that can be used for frequency dependent feedback. (De-emphasis)
Have a look at the attached circuit diagram to get an idea. (Here the NPN output is used to act as a 'low cut' The 1 G.ohm at the switch isn't correct, the 150 pF is the 'pad'. )

 

[ln76d]

I did few deemphasis circuits including in schoeps topology - with this one, if memory serves me well, there was a problem to find proper values for filtering (different than Neumann values).
Anyway, still have problem with reading your schematics - my eyes hurts

 

[Khron]

Looks fine at 100% zoom on my fullHD (1920x1080) screen. Might not, on lower-res screens at under-100% zoom.

Anyway, still have problem with reading your schematics - my eyes hurts

 

[ln76d]

Looks fine at 100% zoom on my fullHD (1920x1080) screen. Might not, on lower-res screens at under-100% zoom.

The way you drawing, with all these "pads" not resolution

 

[Khron]

Those connect to the switch-"carrier" pcb, sorry about that Pad 1/8 and 2/9, when unconnected via the switch, are the high-pass; when connected, it's "flat". I still have to see how i can wire up the switch for the attenuation, though...

 

[RuudNL]

To be honest, I prefer a more complete circuit diagram too!
Now it looks more like a puzzle...

 

[Khron]

The switches are on their own board :-/ The way things go together can be seen on my BM800 thread, for what it's worth.

 

[RuudNL]

Today I did some measurements on the circuit I described before.
Core of the circuit is the generic Schoeps circuit, as seen in many commercial microphones.
It proves that it is possible to create exactly the U87 de-emphasis: - 3 dB @ 40 Hz and -4 dB @ 16 KHz.
Of course you can increase the value of the 47 nF capacitor, if you don't need the LF rolloff.
In fact, it should be possible to create a small add-on PCB, to add de-emphasis to an existing (Chinese?) microphone.
In the circuit diagram the PNP output stage is omitted, but this is not important to understand the principle.