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[mike_mccue]

I've been looking at the circuit boards of Chinese microphones in articles about mods and thinking of maybe trying something soon.

What is the purpose of the small white standoff posts that the capsule leads are soldered to?

If you want to experiment with a DIY turret board mod will you need those special posts? Where can you buy a few?


Thanks,

best regards,
mike

[klaus]

Would be great if you could add an illuminating picture.

[mike_mccue]

[mike_mccue]

I searched for several days before posting here but wasn't finding much info. This morning I found a reference to the phrase Teflon Insulated Terminals and a simplified explanation about why the high impedance at those connections in the circuit needs a very high level of insulation to keep the signal from being diverted.

Searching for that phrase quickly lead me to a Keystone parts page and several vendors.

Sorry if I wasted anyone's time.

I do have a question about attaching them to something like a piece of garolite; can I use something like super glue? What sticks to teflon?

Thank you.


best regards,
mike

[klaus]

The principal idea of the stand-offs is to insulate as much as possible the FET's gate from the rest of the circuitry.
With other words: on a circuit like Neumann's U87 through 2002, they simply bent up the FET gate leg, made a direct wire connection to the head of the mic, thus avoiding circuit board contamination and possible lowering of impedance from the 10gig range it is supposed to operate in.

If you keep this principal purpose in mind, you can proceed in a similar fashion.

Now, what is curious about the picture you attached: the stand-offs can only do their job (actually, only the FET stand-off is of interest and usefulness here, as the source and drain egs operate in the mid-to-low impedance range and don't ever need special attention) if there is no circuit board connection underneath. If there is, possible impedance lowering is not really addressed with adding a white plastic collar. And if there is no circuit board connection, you don't really need the stand-off in the first place.

[mike_mccue]

Thank you for the reply Klaus,

That photo is not indicative of the circuits I am curious about but it happened to show off the plastic item clearly. I am primarily interested in tube circuits and thinking about trying a mod or two based upon hard wiring components to a hand made turret board.

I took a photo of a mic I had here where the white parts were easy to see and didn't consider the fact that it was FET circuit.

Many of tube mic mods I've been reading about reuse the PCB boards. I like making turret boards so I thought I'd try that instead and so have wondered about these plastic items.

When you say

"if there is no circuit board connection, you don't really need the stand-off in the first place."

How does that apply to a turret board or an eyelet board where they are riveted to the board material (garolite?) but the circuit is not running along the actual board surface?

I don't think I've ever seen the plastic insulators on mics built with that sort of construction.


best regards,
mike

[klaus]

Considering the fact that almost all tube mics nowadays use plastic tube sockets, I would not lose sleep over this issue.

[mike_mccue]

I'm imagining you are referring to direct point to point straight to the tube socket type construction.

I've been thinking about using a sub mini tube and so hadn't thought about that.

I am going to try using a small turret board instead of a pcb and hand wiring the connections.

This is a photo I found with google images and cropped to size so that it's easy to see the 4 plastic mounts just below the top plate.

They seem to be associated with the high impedance resistors surrounding the capsule.

best regards,
mike

[radardoug]

I know you can get small teflon standoffs from Radiospares, I don't have the number but they are used in rf work as well. The pin pushes in and expands the teflon slightly to lock it in the hole.

[radardoug]

Turns out the RS ones (433882) are now unobtainium, but have a look at these guys.
http://www.mac8usa.com/tht-teflon-terminal-fx.php

[klaus]

Great resource you found. Thank you!

[mike_mccue]

Hi everyone,

Thanks for the links to the stand offs.

I had found similar available at Keystone Electronics but thought posting a link might violate the terms of use for this forum.


Here is the Keystone info, you can find these parts at many vendors:

http://www.keyelco.com/pdfs/npb2007-teflon-products.pdf


I am still anxious to understand if these are necessary when assembling a circuit using a small hand made turret board and hard wiring or if they are only necessary when working with PCB traces.

I found that the comment about plastic tube sockets just confused me further.

Can anyone put this in perspective for me?


Thank you for sharing.


best regards,
mike

[Jim Williams]

IIRC Digi Key carries the Keystone and Millimax stand-offs. They are used to isolate sensitive high impedance nodes from anything that may allow current leakage. As Klaus said, in the past they used what is known in the electronics biz as "dead bug" construction.

The sensitive leads were pointed up in the air and soldered there instead of on a pcb. Old designs with DIP packages looked like a dead bug when wired upside down.

Air is a very good insulator compared to average FR-4 circuit board material. Some older mics like AKG 451's soldered the fet gate to the pcb, bad idea. Best to pull that lead up into the air and connect the blocking cap to that point.

Modern tube mics are now using the stand-offs too. With 200M input impedances, not too critical. I use 1 gig ohm "grid leak" resistors in my tube mics, that makes that node more sensitive. It is very important to clean those high impedance nodes with a good solvent. Any flux or hand oils will create a leakage path. The resistor body, wires, pcb and the standoff must be clean. If you see solder flux, clean it off!
Afterwards I use a polystyrene dope conformal coating to insulate those areas from any contamination or moisture. That is what will hold the stand-off in place.

Plastic tube sockets have more leakage than their ceramic counterparts. I use Kimber pure silver/teflon wire to connect the isolation stand-off to the tube socket.
Then the socket is cleaned off of any dirt or flux. Then coat that pin and surrounding plastic with the conformal coating. That will do a good job controlling any leakage.

[klaus]

I had found similar available at Keystone Electronics but thought posting a link might violate the terms of use for this forum.

There is never an issue sharing resources, as long as the sharer is not affiliated with the resource (exceptions in rare cases).

I am still anxious to understand if these are necessary when assembling a circuit using a small hand made turret board and hard wiring or if they are only necessary when working with PCB traces.
I found that the comment about plastic tube sockets just confused me further.

(Jim just posted a detailed response, while I was typing too):
The ONLY critical issue of the connection between a tube or FET and a circuit board is conductivity: maintaining the super-high (10gigΩ +) impedance of the tube's grid or FET's gate is paramount for keeping the noise of the circuit low. When the socket's plastic material is contaminated or of low quality, or when the stand-offs between FET gate and circuit board are of a kind that does not have super-high resistance (sometimes measured as Ω per mm), the lowering of the circuit's impedance affects the performance of the mic; increasing noise, and alteration of the frequency response result.

[mike_mccue]

Thank you both very much.

The explanations have been helpful.


best regards,
mike