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[Khron]

R5 should be connected between R4 and R3, the voltage divider. Otherwise you’ll see close to 120v on the backplate.

That's a really great point, can't believe i missed that!

R5 and C3 filter the supply to the backplate. Very important that this is spotless noise wise. Maybe not necessary if the B+ is already spotless, but why would it be a bad idea??

For the exact same reason i objected (and still do, to this day) to the same arrangement in the FET847 circuit. 1gig is completely unnecessary there. Yes, C3 should indeed go from backplate to ground, but why would you need or want 1gig(!!!) as a series resistor there? Especially connected as drawn (by both KK and PTC)...

"Even just" the 500k from that divider, with the 10nF, gives a -3dB frequency of ~32Hz. Bump those resistors up to 1-10meg if that's not enough, but 1gig has precisely zero justification there.

 

[MagnetoSound]

Ah, I see. I thought you were objecting to the resistor, not the value. Personally, I don’t see -3dB @32Hz adequate if there is any 100Hz ripple there at all. Of course, ~100M is usually considered ‘safe’ in that position. 1G is indeed kinda overkill.

 

[Khron]

10meg & 100nF (instead of 10nF) - no reason to need to go all that "exotic"...

 

[telewulf]

Like this for instance:


with kind regards from Bremen

 

[Khron]

Like this for instance:

What's the point of that 1G going to the backplate, though?

Or if you consider / claim it's somehow "necessary", would you care to explain how come the M49 happens to work just fine without one?

Schematic:
https://groupdiy.com/threads/m49-microphone-from-6s6b-v-to-ac701k.81028/#post-1043190

 

[OneRoomStudio]

What's the point of that 1G going to the backplate, though?

Or if you consider / claim it's somehow "necessary", would you care to explain how come the M49 happens to work just fine without one?

Schematic:
https://groupdiy.com/threads/m49-microphone-from-6s6b-v-to-ac701k.81028/#post-1043190

Wouldn't that 1G be equivalent to R2 (100M) in the schematic you linked to?

 

[Khron]

Wouldn't that 1G be equivalent to R2 (100M) in the schematic you linked to?

More or less, but the exact value isn't that crucial. Anyway there's no requirement for it to be ridiculously high.

 

[OneRoomStudio]

More or less, but the exact value isn't that crucial. Anyway there's no requirement for it to be ridiculously high.

I think the idea is that by using a higher resistor, you can use a smaller capacitor (in value and in size). It's true that 1G is probably overkill, but it's sometimes easier to find 1G resistors than 100M resistors these days. I don't really see any downside to using a 1G if that's what you have around or what's available.

 

[MagnetoSound]

Like this for instance:

View attachment 123529
with kind regards from Bremen

Not quite. The 10n goes at the backplate, not the divider.

 

[telewulf]

@MagnetoSound: Aah, you mean like with the U47, right?

@Khron: I wouldn´t consider 1Gohm necessary, that was just a quick drawing. But for example in the U87 A it really is the value.

Correct me if I´m wrong, but if my maths do not let me down, the source impedance of a capsule of 100pF would be about 30Mohm at 50Hz. (If I remember correctly that is exactly the resistance with wich the capsule was charged in the CMV3).
But for the alternating voltage of the signal, the charging resistor acts as a shunt. So if you don´t want your signal generator to be overly stressed, you ought to choose the value 10 times as high, 5 times at least (wich is how you end up with the 150Mohms in the M49) unless you expressly want to achieve a bass roll off.

To compare values and schematics for myself I drew these a few years ago:


To be honest, I could never really figure out why with the U47 there is this 10nF from the backplate to ground Maybe you could enlighten me there?


The last one is NOT the original coupling schematic of the CMV 563 but my own take on that one:



Over the years some old M7 Lollies ended up in my posession and I wanted to be able to use them.

best regards from Bremen,
Wulf

 

[Khron]

@Khron: I wouldn´t consider 1Gohm necessary, that was just a quick drawing. But for example in the U87 A it really is the value.

Well, the U87 (even the -A) have a completely different capsule bias wiring than what was being discussed here.

And your M49(c) is missing the equivalent 5nF (C2 and C3 in series, both 10nF) capacitor between backplate and ground. The 200 ohm resistor for the test input can be ignored for these purposes.

And the C414EB input capacitor is 470p, not 100n.

To be honest, I could never really figure out why with the U47 there is this 10nF from the backplate to ground Maybe you could enlighten me there?

Because that's there filtering the DC applied to the backplate. You'll note the audio signal is taken from the front diaphragm, and the capsule itself is a capacitor (ie. NOT passing DC).

Other than that, a really cool summary!

 

[telewulf]

Ooops, right you are!
I tried to simpify everything as much as I could but obviously one mustn´t oversimplify.

 

[Papa Tango Charly]

Hello guy thank you guys for you input and reflexions. i 'm trying to to summarize this point to point schematic form Kingkorg with all your comments. What would you suggest. I hope this is getting better.... Thank you !!!!! peace out!

 

[Khron]

R5 = 100meg, plus a 10nF capacitor from backplate to ground.

 

[Papa Tango Charly]

Ok! there it is! thank you guys! far away from Kingkorg previous schematic...

 

[Khron]

Ok! there it is! thank you guys! far away from Kingkorg previous schematic...

Still no. 10nF from 100meg+backplate node, to ground.

https://en.wikipedia.org/wiki/RC_circuit#Series_circuit

R5 = 100meg, plus a 10nF capacitor from backplate to ground.

 

[Papa Tango Charly]

CK12 is also extremely picky in how the multipattern voltage is applied. It is extremely easy to think you are in say cardioid while being in who knows what pattern. It is VERY important that there is discharge path so that no residual voltage remains after changing pattern.

Topologies where front is at 0V and back diaphragm and backplates are at 60v coming from two different branches is my least favorite.

I like CK12 best at 40v, not for some metaphysical reason, simply because there's less attraction going on. If i could i'd polarize them with 10v.

I have put a C12 capsule in Jules opa circuit without the Hex inverter DCDC and the microphone was sounding ok ))

 

[kingkorg]

I have put a C12 capsule in Jules opa circuit without the Hex inverter DCDC and the microphone was sounding ok ))

Where does the capsule get the polarization from? What's the voltage applied to it?

 

[SparkleBear]

I think Khron means this PTC:



Is C3 (10,000 pf) there just to filter noise off of the rail? Different than the role of c6 (600pf) in the m49 to shunt HF from plate out (to reduce hf response) yes?

 

[Khron]

Is C3 (10,000 pf) there just to filter noise off of the rail? Different than the role of c6 (600pf) in the m49 to shunt HF from plate out (to reduce hf response) yes?

Yes. You can also look more carefully where each is connected.