First time - Schoeps CMC5 schematics

[jp8]

If I understand you right I can remove C14 and GND, and it won't make a difference if I design a PCB from the scheme?

C14 should stay and be connected to signal ground on the PCBA. PCBA signal ground connects to the mic body by means of the ground connection on the XLR (pin 1 to the tab on the XLR, then from this tab through the screw to the mic body).

On another note. In the original scheme Dr5 is labeled "Ferrite bead", so I'm guessing it's an inductor. But I wonder at what value.

Dr is a German abbreviation of Drossel, which indeed translates as inductor. But in this particular case, it is a ferrite bead, which is an inductor with high resistive losses to reduce potential RF interference. In most Schoeps style incarnations it is left out, probably with the assumption that the capsule and mic body are properly closed Faraday cages.I think the RFI filter on the XLR is more important to reduce RF interference, as demonstrated in the KM84+ and KM84++ projects and in the Takstar CM-63

Jan

 

[Karl-0_0-]

Well, y'know, with all the fancy technology we have free access to nowadays, there's no real need to guess all that much...

https://en.wikipedia.org/wiki/Ferrite_bead

Lol, thanks.

 

[Karl-0_0-]

C14 should stay and be connected to signal ground on the PCBA. PCBA signal ground connects to the mic body by means of the ground connection on the XLR (pin 1 to the tab on the XLR, then from this tab through the screw to the mic body).

Thanks for clarifying.

Dr is a German abbreviation of Drossel, which indeed translates as inductor. But in this particular case, it is a ferrite bead, which is an inductor with high resistive losses to reduce potential RF interference. In most Schoeps style incarnations it is left out, probably with the assumption that the capsule and mic body are properly closed Faraday cages.I think the RFI filter on the XLR is more important to reduce RF interference, as demonstrated in the KM84+ and KM84++ projects and in the Takstar CM-63

In your opinion, if I ever print a PCB from the scheme I'm sketching on and use the AliExpress XLR pin you link to on PCBWay in a CM60 or -63 host... is the FB on Dr5 then necessary?

 

[jp8]

In your opinion, if I ever print a PCB from the scheme I'm sketching on and use the AliExpress XLR pin you link to on PCBWay in a CM60 or -63 host... is the FB on Dr5 then necessary?

Ooh, difficult to answer... Depends on some variables which we don't know upfront. How well is the mic body EMI-tight? How well is the PCB layout designed with handling RF signals in mind? Is there an RFI filter near the XLR output?

As said before, most Schoeps designs can do without. The CM-63 has a very good RF filter on its output, which is extremely effective. And the CM-63 does not even have a bead in the JFET Gate connection. I'd leave it out if you only have a short pogopin connection to the capsule. But if you do want one in, you could take the bead that is used in the KM84+. It has maximum RFI rejection at cell phone frequencies.

Jan

 

[cfanzalone]

Very interesting thread...I'm doing a mod on a CAD GXL1200...of course most all these Chinese Pencil mics use the CMC5 circuit. Your transistor replacements are helpful, although I was under the impression that a transistor in this path acts more like a switch?? (If it ain't broke, don't fix it)... I always replace the JFET with a genuine Toshiba 2SK170BL, but what interests me is replacing the 1GOhm resistors at the North end near the capsule connections...Any thoughts about those??...and their significance in the signal path??...Worth replacing??

Thanks to whomever to respond...

 

[Karl-0_0-]

Very interesting thread...I'm doing a mod on a CAD GXL1200...of course most all these Chinese Pencil mics use the CMC5 circuit. Your transistor replacements are helpful, although I was under the impression that a transistor in this path acts more like a switch?? (If it ain't broke, don't fix it)... I always replace the JFET with a genuine Toshiba 2SK170BL, but what interests me is replacing the 1GOhm resistors at the North end near the capsule connections...Any thoughts about those??...and their significance in the signal path??...Worth replacing??

Thanks to whomever to respond...

I'm certainly not the right person to answer your question, but jp8 writes on his website that replacing the 1G resistors can help with low-end noise on the Takstar CM60. So maybe the same is true for the CAD GXL1200.

 

[jp8]

Increasing the value to >1GOhm, it will reduce noise. I'm not sure there is an audible difference between different brands or types of GOhm resistors. Johnson (thermal) noise will not be different between resistors of the same value, but I don't know about excess noise, i.e. 1/f low frequency noise. Might be worth investigating. Or does someone know?

Jan

 

[Khron]

I was under the impression that a transistor in this path acts more like a switch?

Why would they?

(If it ain't broke, don't fix it)... I always replace the JFET with a genuine Toshiba 2SK170BL

I hope you also see the irony in these two statements, right up against each other...

 

[cfanzalone]

Wellll, the JFET is far more important than the transistors...

 

[Khron]

Wellll, the JFET is far more important than the transistors...

And that's a response to... which part?

 

[micolas]

And that's a response to... which part?

Please be more lenient.
It's a no-questions-asked answer, but it's a good answer.

 

[cfanzalone]

Exactly, let's be nice....
I always replace the all-important JFET...I was just wondering about the transistors if or if not affecting the signal path...that's all....
...And now I'm out of this nonsense...I tried to share and ask appropriate questions that I, and perhaps others, might ask or want to know...instead there's always someone who wants to start something....I'll do what I decide and blessings to all you others.

 

[micolas]

Exactly, let's be nice....
I always replace the all-important JFET...I was just wondering about the transistors if or if not affecting the signal path...that's all....
...And now I'm out of this nonsense...I tried to share and ask appropriate questions that I, and perhaps others, might ask or want to know...instead there's always someone who wants to start something....I'll do what I decide and blessings to all you others.

Everything affects the overall performance, but to a different degree.
IIRC in another older thread, @RuudNL specified that from his own experience, which I respect a lot, he noticed that transistors placed towards the output of the audio circuit in the microphone have little impact in worsening the quality, for example noise.
This is exactly my opinion, and that of many others, that's why I treat jFET with the greatest attention.