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Old 11th August 2013, 01:25 AM   #1
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Default which resistors for U47 clone?

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.

I just wanted any opinions as to which 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.

Click the image to open in full size.

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.

Click the image to open in full size.

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.

Click the image to open in full size.

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
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Old 11th August 2013, 03:15 AM   #2
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Hi Mike
It would be helpful to know something about the circuit these parts are to be used in , Is a schematic available ? Some info on the U47 original here: U 47 - Wikipedia, the free encyclopedia

Cheers / Chris
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Old 11th August 2013, 03:34 AM   #3
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Hi Chris, this is a variant of the U47 circuit. I've circled in red the corresponding functioning resistors in the U47 original schematic.

Click the image to open in full size.

Thank-you,

Mike
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Old 11th August 2013, 11:03 AM   #4
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Hi Mike
Thanks that makes things a bit clearer. The extremely high values of resistance necessary as a result of the use just one power supply voltage that gets resistive divided into various branches. Its fortunate you have found a supplier of those parts. I agree the epoxy cased ones are a better choice.

Cheers / Chris
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Old 11th August 2013, 11:12 AM   #5
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Thank-you Chris for your advice. I was leaning towards the epoxy-encased ones too, since I think they will be easier to work with and better protected from the elements.

My only concern about them is they are not matched in terms of voltage rating - the 100M is 4kV, and the 200M is 10kV. The 10kV is also physically twice as large. In your opinion would this mean the 200M is way too overkill for the circuit - in other words would it introduce any extra noise or eat some of the current in any way? I am just thinking of when I was a kid and my dad told me you can't use a gigantic wire for a small current (he was an EE and brought home samples of 6" thick cables)...

Also would the fact that they are not matched voltage-wise cause any other mismatch in this circuit, if any at all?

Thanks again,

Mike
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Old 12th August 2013, 12:08 AM   #6
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Hi Mike
Yes the voltage requirement on resistors is overkill. We can answer if the original schematic is representative of the replica that the highest voltage in the circuit is 105v. A rule of thumb is to plan for twice that with components.

I did a simple simulation check on this circuit, the voltage values at the microphone elements are correct. The resistance of 700M and 60M looks ridiculous at first. Until you consider the circuit involves those values in parallel with R8, (and of course R8 dominates being the lower value) . when the element is switched on. Also the circuit is cleverly routing the correct current for biasing the element.

It sounds like you are trying to fit these components in a small space. An alternative could be suitable number of series 1206 or 0805 surface mount components, observing the wattage and voltage rating to not be exceeded.

Cheers / Chris
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Old 12th August 2013, 01:31 AM   #7
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Thank-you Chris for the info. Much appreciated!
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Old 12th August 2013, 01:33 AM   #8
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PS would it cause any kind of problem to use the 10kV part anyway...add extra noise or anything? It does fit. It's the only other epoxy-coated resistor I have at 200M; the other option is the "bare" version and I'm hesitant to use it in case any oils/fingerprints/dirt could get more easily on it and cause a change in the resistance or something...

Thanks man!

Mike
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Old 12th August 2013, 02:50 AM   #9
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Quote:
Originally Posted by PhrazeMaster View Post
PS would it cause any kind of problem to use the 10kV part anyway...add extra noise or anything? It does fit. It's the only other epoxy-coated resistor I have at 200M; the other option is the "bare" version and I'm hesitant to use it in case any oils/fingerprints/dirt could get more easily on it and cause a change in the resistance or something...

Thanks man!

Mike
Hi Mike
It will be fine, the higher rated voltage won't add extra noise. There would only be worry if the component was lower in rated voltage, which in this case it is the exact opposite.
Cheers / Chris
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Old 12th August 2013, 06:44 AM   #10
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Hi Chris, thanks!! The part will fit, and actually the fact that it's larger just makes it easier to see the difference. My only last concern - inductance/capacitance; I notice these resistors have parallel lines of tracks. The Ohmite resistors specifically say no inductance; these do not. I've sent an email to the rep asking this.

Thanks again!

Mike
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