Resistor Question

[Platefire]

On the attached 200M resister spec, it's hard for me to imagine a resistor with this much resistance. To me it seems it would be the same effect as installing a SPST switch and permanently leaving it in the off position. So if so what would be the purpose?

[sluckey]

On the attached 200M resister spec, it's hard for me to imagine a resistor with this much resistance. To me it seems it would be the same effect as installing a SPST switch and perminatly leaving it in the off position. So if so what would be the purpose?

Well, first off, it ain't so. An open switch contact has an infinite resistance. So the open switch contact resistance is infinitely greater than a 200MΩ resistor. Or, you could say a 200MΩ resistor is infinitely smaller than an open circuit.

[Platefire]

OK, I never even thought about a 200M resistor but a friend building microphones is asking me where can he get them. I did a search for that value just like the spec description at the Mouser Site and nothing. Any idea where that might be obtained? I guess next, e-bay:>)

[sluckey]

Why did you post this in the "Technician Listings" forum? Nobody ever comes here. Would you like me to move this thread to a more popular board so others will see it too?

[Platefire]

Well it was an accident and I didn't see anyway to move it.
BTW, I did find a 200M 1/2 watt Metal Oxide R at Mouser but if you want a 1/4 watt its a six week order lead time. According totheir spec data the 1/4 and 1/2 watt are the same physical dimensions and the 1/2 watt is in stock. Thanks for answering myquestion

[PRR]

A capacitor microphone is 30pFd which is 100Meg at 50Hz. The resistor must be larger to avoid bass-loss, AND because the inherent hiss (rumble) of the resistor can be shorted-off by the capsule capacitance.

1Gig is common for FET mikes. 200Meg is an approximate upper goal for tube mikes because grid current.

It WILL work with 50Meg (or 22Meg+22Meg). Few studios can show the small noise improvement at 200Meg. The bass-loss is small enough to EQ-out later.

I don't think you want to be thinking 1/4 Watt. In fact "2W" has been done. Not for power, but so the body size keeps the two ends apart and reduces leakage.

groupdiy.com used to be a place to talk about this.

[Platefire]

PRR I don't really know a thing about building high end mics. The guy that ask me about this owns/runs a recording studio and hasbuilt his own Mic pre amps and now mics. I'm guessing that a schematic he was going by call for that resistor and he didn't have a source. Mouser sells a 1/4 Watt 200M but they don't stock them and are special order. When you do order them it's a long wait time until they come in. The 1/2 Watts they have in stock

[HotBluePlates]

... Mouser sells a 1/4 Watt 200M but they don't stock them and are special order. ... The 1/2 Watts they have in stock

I don't think you want to be thinking 1/4 Watt. In fact "2W" has been done. Not for power, but so the body size keeps the two ends apart and reduces leakage.

Re-re-read and absorb the bolded parts of what PRR said.

The condenser microphone's capsule (that picks up the sound) is the "30pF cap" PRR mentioned.  There has to be an electronic circuit built right-at the capsule to get the electrical signal from that capacitor/condenser/capsule.  And like every other electric circuit, "how low" is set by R x C, and "C" is a puny 30pF so "R" has to be huge to get any bass response from the mic.

Your oily fingerprint will probably be something like 20-40MΩ.  So when you pick up a 200MΩ resistor with your bare fingers, you will probably create a leakage-path across the body that's like 20MΩ in parallel with the desired 200MΩ ---> you just turned it into an 18MΩ resistor.  Now the mic rolls off bass at about 300Hz instead of 50Hz, and sounds gutless & tinny.


So avoid leakage by buying the bigger-body resistor.  And you'll probably want to buy some of those nitrile gloves at the auto parts store so you don't transfer any finger-oil to the resistor body.

[Platefire]

I re-read it and your explanation on top of what PRR said and it's starting to get me in the ball park, I think
 
I was thinking in the confines of limited space of a microphone, small physically would be good. So your saying the 1/4 watt would not be able to maintain a consistent 200M resistance because the electrons would find another route even through a oily finger print, and weaken the bass response?

So should I be telling my friend, he should be installing the largest watt rating resistor he can fit in the Mic circuit?

[PRR]

Gloves are good, but a condenser mike usually has to be washed in strong cleaner after assembly, and even after a few years use.

Also you don't build it on Fender-board, or even Doug's good G10 board. The early British experimenters jumped on the then-new methacrylate (Plexiglas) used in WWII airplane canopies because it is so inert and easy to clean.

[HotBluePlates]

... I never even thought about a 200M resistor but a friend building microphones is asking me where can he get them. ...

I was thinking in the confines of limited space of a microphone, small physically would be good.

It seems there are modern kits for building tube condenser mics that use 1/4w or 1/2w resistors.  But we can see the board fits in a fairly small space within the mic body.

But the thing the kit copies used larger-bodied resistors in a different spot right below the capsule & grill.  So depending on the mic and the board/layout used, there may be a fairly large amount of room.