M49 Alternate schematic for cardioid only

[elskardio]

*I finished assembling the mic. Pics here*

Hi Guys,
I'm planning to build Oliver Archut's alternate M49 schematic.
I want to build the mic for cardioid only. Can I simply remove the back diaphragm polarization?
Thanks

 

[rock soderstrom]

Can I simply remove the back diaphragm polarization?

Yes, you can. This will work.

 

[k brown]

Hi Guys,
I'm planning to build Oliver Archut's alternate M49 schematic.
I want to build the mic for cardioid only. Can I simply remove the back diaphragm polarization?
Thanks

This may be of some help:

https://repforums.prosoundweb.com/index.php?topic=33882.15

 

[micaddict]

A pure cardioid M49. Sounds nice. Yes, it will sound different from the cardioid setting in a multipattern mic. You may actually prefer it. S/N ratio can be a little better, too.

BTW, you can have both in one mic. Horch RM2J is one example; Wunder Audio CM7 G is another.

Come to think of it, wasn't there a switch (S2) even on the M49. Man, I'm really rusty.

Anyway, although I cannot help you with minute technical details, I will be following your efforts with interest. Good luck and keep us posted!

 

[TLRT]

You can put a switch to disconnect the polarization circuit (disconnect the 1000pf cap from ac701 grid) you can see it is S2 on Neumann schematics

 

[elskardio]

This may be of some help:

https://repforums.prosoundweb.com/index.php?topic=33882.15

Interesting reading!

“The theory of none voltage difference at cardiode setting sounds great but the reality is that 60V (backplate) vs. 60V (back-membrane) will still push against and repell, displacing the membrane and changing overall the sound pick up.”

Sounds less than ideal…

To my understanding the polar pattern will be more hypercardioid with the back membrane disconnected. I always liked mic that are more directional. I will try to compare both configuration

 

[Khron]

“The theory of none voltage difference at cardiode setting sounds great but the reality is that 60V (backplate) vs. 60V (back-membrane) will still push against and repell, displacing the membrane and changing overall the sound pick up.”

How and why, i wonder? And push/repel against what? The rear diaphragm and the backplate?

 

[elskardio]

How and why, i wonder? And push/repel against what? The rear diaphragm and the backplate?

This is a quote from Oliver Archut by the way.

 

[elskardio]

Hi guys,
I've updated the drawing in the first post.
I'm working on a pcb for the microphone and currently planning to make 2 floating points for the capsules hi-z connections.
Any other particular details I should be aware?
cheers

 

[Khron]

currently planning to make 2 floating points for the capsules hi-z connections.

Why two?

 

[elskardio]

Why two?

Sorry but I'm not following...

 

[Khron]

The tube grid is indeed a high-impedance node, but how and why is the other side of the capsule ALSO a high-impedance node?

Remember how capacitors work? They block what, and allow what to (virtually) pass through?

 

[rock soderstrom]

Sorry but I'm not following...

The tube grid is indeed a high-impedance node, but how and why is the other side of the capsule ALSO a high-impedance node?

Remember how capacitors work? They block what, and allow what to (virtually) pass through?

I think Khron is telling you that you only need one high impedance floating point (isolated turret) because we are talking about impedances and that means alternating current. For AC, the capsule side of the C2 capacitor is more or less at ground potential because the capacitor is a frequency dependent resistor (or conductor). Ergo, you only have a high impedance point. Right or wrong?

 

[rock soderstrom]

Right or wrong?

The more I think about my (and Khron's) explanation, the more uncertain I become, because if you look at it that way, then the grid of the tube would AC wise also be on ground potential, since the capsule is actually also a capacitor.

Brainfart?

 

[Khron]

the grid of the tube would AC wise also be on ground potential

It technically sort of pretty much already is, via the grid resistor But not through the capsule itself, because the capsule CREATES the signal, rather than shunting it away from elsewhere.

..... Right..?

 

[rock soderstrom]

It technically sort of pretty much already is, via the grid resistor But not through the capsule itself, because the capsule CREATES the signal, rather than shunting it away from elsewhere.

..... Right..?

I'll have to think about that now, I think I'll take a hot bath, that helps! Ahoy!

 

[OneRoomStudio]

I think it's worth treating it as a high impedance point since the capacitor creates a frequency-dependent "pass-through" for AC. If you lower the impedance to the backplate due to some circuitboard impedance or contamination, you would also effect the frequency of the RC filtration of C2. There's a reason R5 is 100M and not just 1M.

 

[Khron]

I think it's worth treating it as a high impedance point since the capacitor creates a frequency-dependent "pass-through" for AC. If you lower the impedance to the backplate due to some circuitboard impedance or contamination, you would also effect the frequency of the RC filtration of C2. There's a reason R5 is 100M and not just 1M.

Make it 10uF then, and then you can reduce the resistor to 100k, to get the same 0.16Hz turnover frequency. Same thing, it's a bog-standard RC filter, no voodoo-magic going on there.

R5's 100Meg in order to achieve good filtering with "just" the 10nF of C5 (physically smaller and more long-lived than an electrolytic, and to not have the grid resistor as the only 100Meg on the bill-of-materials).

 

[OneRoomStudio]

Make it 10uF then, and then you can reduce the resistor to 100k, to get the same 0.16Hz turnover frequency. Same thing, it's a bog-standard RC filter, no voodoo-magic going on there.

R5's 100Meg in order to achieve good filtering with "just" the 10nF of C5 (physically smaller and more long-lived than an electrolytic, and to not have the grid resistor as the only 100Meg on the bill-of-materials).

No disagreement there. Just saying that with the parts selection as-is, there would be no issue with making the R5/C2/Backplate junction hi-z and calling it a day.

 

[elskardio]

Most U47 & M49 microphones I see have the backplate connected to a floating point. I figured there must be a benefit...