Question about wire length on LDC capsules

[blue_luke]

Hello all, I want to build a stereo microphone in the like of AKG C24 or Neumann SM69.
IE: two capsules on top of each other at a 90°angle.
I can not find any suitable body so I have decided to make my own as I have all the machine tools I might need, all advanced hobbyist grade machines (Lathe, milling, welding etc...)
So I am at the "thinking/shoot the crap" stage and I can see a few design avenues but I have a question that is nagging me.
What is a reasonable maximum length of wires I can use between the capsules and the circuit board in the body?
For a single capsule, those wires are hmm... about 3 inches long or even less, but the top capsule would then be at least 2 inches longer I guess...
What is the effect of this length of wire?
I know we are dealing with very high input impedance here.
My first natural idea is that these wires should be kept as short as possible so maybe if I built a support that would have a capsule on top and the other underneath and a small FET PCB interface in the middle, the wires would be kept extremely short.
Then the rest of the circuitry can be housed in the body.
That's just my first idea.
Luc

 

[kingkorg]

Hello all, I want to build a stereo microphone in the like of AKG C24 or Neumann SM69.
IE: two capsules on top of each other at a 90°angle.
I can not find any suitable body so I have decided to make my own as I have all the machine tools I might need, all advanced hobbyist grade machines (Lathe, milling, welding etc...)
So I am at the "thinking/shoot the crap" stage and I can see a few design avenues but I have a question that is nagging me.
What is a reasonable maximum length of wires I can use between the capsules and the circuit board in the body?
For a single capsule, those wires are hmm... about 3 inches long or even less, but the top capsule would then be at least 2 inches longer I guess...
What is the effect of this length of wire?
I know we are dealing with very high input impedance here.
My first natural idea is that these wires should be kept as short as possible so maybe if I built a support that would have a capsule on top and the other underneath and a small FET PCB interface in the middle, the wires would be kept extremely short.
Then the rest of the circuitry can be housed in the body.
That's just my first idea.
Luc

Parasitic capacitance, inductance, noise and mechanical noise because the wires act as pickups them selves, very microphonic. So as short as possible is the way to go. If you take FET route, i'd go for individual hiZ area under each capsule. Or even something like this?

 

[blue_luke]

Parasitic capacitance, inductance, noise and mechanical noise because the wires act as pickups them selves, very microphonic. So as short as possible is the way to go. If you take FET route, i'd go for individual hiZ area under each capsule. Or even something like this?

I intend to use a set of matched Microphone parts K-47 capsules that I bought a few years ago.
I want to be able to select the pickup pattern.
I am also into making a circuit for each membranes, so that would get me 4 outputs.
As I said, I am really at the early conceptual stage as of now.
There was a thread some time ago about a group buy for a mic body that looked like the advantone stereo mic (AK-40?) but this seem to lead nowhere, so I am going to make my own....

 

[ccaudle]

You can see in the diagram from the C24 manual that the upper capsule has wires which run along the capsule support members. But as pointed out you have to be very careful of the distance of the wires to metal components because of the capacitance.

 

[ccaudle]

I am going to make my own....

This topic is of interest to me (I would also like to make a stereo microphone), so please show your progress here as you progress with the design.

 

[kingkorg]

@blue_luke @ccaudle idk if this could be relevant to you guys.

https://www.nbmic.com/stereo-diaphragm-condenser-microphone.html

Some years ago they quoted me about 170$ + shipping.