Which resistors for U47 clone build?

[Phrazemaster]

Hi, I obtained some "high end" resistor samples from a company called Ohmcraft, and I had already got some ohmite resistors, for a U47 mic build I'm doing.

Here is a pic of the original U47 schematic, and although one of my resistor values is not the same, the circled resistors are where mine go in the clone circuit I'm building (very similar topology).

I just wanted any opinions as to which resistors to use. These are 100M and 200M resistors.

The first up is the ohmite, which has respectable specs. 1%, to 5kV, TCR unknown but I think low. They are called "Thick film planar" but the series is Slim-mox, which sounds like metal oxide.

http://www.ohmite.com/cat/res_slimmox.pdf

Next up is the Ohmcraft "bare" resistors. They are 4kV, .5%, TCR100. The rep said they are encased in a glass.

Finally I have the Ohmcraft epoxy-encased resistors. They are 4kV and10kV respecitively (I don't have a matched pair for voltage ratings - they are samples). They are .5%, TCR50.

My instinct would be to use the Ohmcrafts, since they are supposedly higher-end, and go with the epoxy-encased for lower TCR and easier handling, but then I don't have a voltage-rating match - does this really matter?

Or would there be any advantage to using the "bare" Ohmcraft version since they are a match voltage-rating-wise? Or go with the Ohmites?

I know we're splitting hairs on hairs, but since I have the options, what do you think would be best in a microphone?

Thanks,

Mike

 

[RuudNL]

R1 isn't critical at all. All noise is filtered out by the 0.01 uF capacitor. (C1)
R2 is more important, although also here the noise is filtered by the capsule capacity.
60 M.ohm/80 (160) pF. Personally I like the Ohmcraft resistors.

 

[Phrazemaster]

Thank-you RuundNL! I appreciate the explanation.

Of the Ohmcraft, would you see a problem with using the epoxy (blue) ones, even though they are not match voltage-rating-wise? I realize they are both in-spec, but having one good up to 4kV and the other to 10kV, tells me there is probably a lot more resistive material in the 10kV version.

Or would it be a problem to just use the "bare" (encased in glass only) versions that you see?

Thanks for your, and any other, thoughts.

Mike

 

[RuudNL]

...tells me there is probably a lot more resistive material in the 10kV version.

No, the resistance ('resistive material') is the same, but the isolation is better.
But as long as the resistor is in 'free air' (point-to-point or 'above' the PCB), isolation is no issue here, since air is one of the best isolators we can think of!
By the way: the resistor is a grid resistor, meaning there is (almost) no voltage across it!

 

[Phrazemaster]

Wonderful details RuunDL, thank-you! Makes a whole lotta sense.