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Input Resistance of Preamp Controlled by Resistor-What Wattage Should I Use
Old 19th December 2019
  #1
Gear Maniac
 

Input Resistance of Preamp Controlled by Resistor-What Wattage Should I Use

Hi,
the input resistance of the preamp im working on is controlled by a resistor but i dont know what wattage resistor i should use to replace it. The schematic does not say.....how do i work out what to use.
Thx
Old 19th December 2019
  #2
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Radardoug's Avatar
 

Well, do the calculation. How much power does a microphone put out?
Old 19th December 2019
  #3
Lives for gear
 

If it's not specified, the power rating is almost certainly not critical (that is to say, the resistor is never going to dissipate anywhere near its maximum power rating). A standard 1/4W or 1/8W resistor should be just fine.

As Radardoug hints at, a microphone is generating power at levels at least a few orders of magnitude less than typical resistor ratings.
Old 19th December 2019
  #4
You are AC coupling this resistor, right?
Old 19th December 2019
  #5
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ruffrecords's Avatar
Quote:
Originally Posted by David Rick View Post
You are AC coupling this resistor, right?
Why would he want to do that?

Cheers

Ian
Old 20th December 2019
  #6
Gear Maniac
 

I think the real question here is does a microphone ever put out.....but my beautiful word play aside i may not have explained myself very well. I am refurbishing a channel strip and would like to change the input impedance.....in the circuit description i have of the strip " the nominal 600R input impedance may be increased up to 5KOhms by changing R147" The resistor R147 is just before the primary of the input transformer in parallel with it. Hope this helps and thanks to all who replied.
A
Old 20th December 2019
  #7
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brianroth's Avatar
 

That is a bizarre design.....what channel strip do you have? Regardless, a resistor in parallel with the primary of a mic xfmr will only have to handle 0.000000000000000000001 Watt. (OK, I'm only slightly kidding, but power requirement for that bizarre resistor will be tiny.)
Old 20th December 2019
  #8
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Radardoug's Avatar
 

So you already have a resistor in there? What wattage is it? Gosh, that was easy!
Old 20th December 2019
  #9
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JohnRoberts's Avatar
 

A 1/4W or even 1/8W should be sufficient.

JR
Old 20th December 2019
  #10
Quote:
Originally Posted by ruffrecords View Post
Why would he want to do that?
AC coupling will prevent any DC current from phantom power resistor mismatch or other offsets from flowing though this shunt resistor. It will see only the audio signal power, which is negligible.

Also you can use a potentiometer to produce variable impedance. The pot must not have DC on it or it will crackle.

Last edited by David Rick; 20th December 2019 at 06:09 PM.. Reason: pot option added
Old 20th December 2019
  #11
A 5k reverse log pot wired in after the phantom blocking caps will work well with a smooth transition. Set the minimum resistance with a fixed resistor in series, usually 300 ohms. Transistor base loading should be 10k ohms or so.
Old 20th December 2019
  #12
Gear Maniac
 

What Wattage?

I was pondering a pot with a resistor in series in place of the original resistor. Ill post the schematic, its not the best but maybe it will let us know if this idea will work.
Thx
Attached Files
File Type: pdf TR04 Schematic.pdf (235.0 KB, 14 views)
Old 20th December 2019
  #13
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ruffrecords's Avatar
Quote:
Originally Posted by David Rick View Post
AC coupling will prevent any DC current from phantom power resistor mismatch or other offsets from flowing though this shunt resistor. It will see only the audio signal power, which is negligible.
Any such dc current would be well within the rating of any 1/4 or 1.8 watt resistor. if there was any dc imbalance, which is unlikely except under fault conditions, I would be more worried about its effect on the mic transformer core than on the resistor
Quote:
Also you can use a potentiometer to produce variable impedance. The pot must not have DC on it or it will crackle.
Whole different ball game.

Cheers

Ian
Old 21st December 2019
  #14
Gear Maniac
 

How does the quality of this pot effect the sound of the pre. I have some pots i could use but they are of a middling quality...so i guess any suggestions of who or what i should use.
Thx
Old 22nd December 2019
  #15
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Radardoug's Avatar
 

The plastic ones have a plasticy sound, the steel ones are a bit steely. Pretty hard to tell though, although some folks here beleive everything affects the sound. I try and use the ones made by vestal virgins in the Bahamas, they are so sexy.
Old 22nd December 2019
  #16
Gear Maniac
 

Could someone let me know if Dug says anything useful ever, ive been ignoring him for a very long time
A
Old 4 weeks ago
  #17
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JohnRoberts's Avatar
 

Quote:
Originally Posted by andybot View Post
Could someone let me know if Dug says anything useful ever, ive been ignoring him for a very long time
A
I understand his attempts at humor but he is mostly trying to amuse himself. Ask smarter questions and he might offer smarter answers.

JR

PS I'm old and cranky too..I just hide it better.
Old 4 weeks ago
  #18
Gear Maniac
 

Must be a Benny Hill fan.
A
Old 4 weeks ago
  #19
Quote:
Originally Posted by andybot View Post
How does the quality of this pot effect the sound of the pre. I have some pots i could use but they are of a middling quality...so i guess any suggestions of who or what i should use.
Thx
Carbon track has a sound, but will you hear it? Conductive plastic is the prefered construction.
Old 4 weeks ago
  #20
Gear Maniac
 

Thx Jim, what i have is a Borne Trimpot i may use and will fit perfectly. Ill probably record some test tracks before i do anything, then ill have a better idea.A
Old 4 weeks ago
  #21
Gear Maniac
 

Thx to all those who help me set this up. Used a 1/4 ohm base resistor of 350 ohms and a trimpot in series and set it to the 690ohms of the original resistor. Compared the results to the baseline tracks i recorded and I cant tell the difference at all. Just a small test but encouraging.
Cheers
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