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modding console mic pad attenuation? Consoles
Old 17th September 2011
  #1
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modding console mic pad attenuation?

hey maybe someone here can help...
I am looking to increase the value of mic pads on my tascam m-3500, that is to say to increase the level of attenuation?

the switch for the mic pad routes the signal through a 68 ohm resistor. Do I need to increase that value or decrease that value in order to make the pad reduce more incoming volume?
Old 17th September 2011
  #2
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I cannot find the schematic for the M3500 at the mo, but I´m pretty sure that the pad circuit is at least three resistors. Post the schemo and you will get help.
Or download the "Radio Designers Handbook". There´s a chapter about Pad circuits. You´ll find all necessary tables and formulas over there.
Old 17th September 2011
  #3
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JohnRoberts's Avatar
 

Quote:
Originally Posted by enginefire View Post
hey maybe someone here can help...
I am looking to increase the value of mic pads on my tascam m-3500, that is to say to increase the level of attenuation?

the switch for the mic pad routes the signal through a 68 ohm resistor. Do I need to increase that value or decrease that value in order to make the pad reduce more incoming volume?
Yup, a mic pad is generally 2 resistors switched in series with the 2 mic input lines, and 1 resistor switched in shunt between those two series resistors to form a simple voltage divider. The 68 ohm resistor is probably the shunt leg, so to make more attenuation make that a smaller value. Say 51 ohms for another 3 db of cut, 33 ohm for another 6 dB of cut. Ideally you should increase the series resistors some, but for a one off I wouldn't worry too much about that.

JR
Old 18th September 2011
  #4
jrp
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why should he increase the series resistors?
Old 18th September 2011
  #5
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JohnRoberts's Avatar
 

When designing a mic pad from scratch you are dealing with more than just dB of attenuation. When you switch in a pad you want to maintain a similar input impedance as seen by the mic.. (nominally 1500-2000 ohms). Dropping the shunt leg will also drop the total input Z but probably not by enough to make a serious difference.

JR
Old 21st September 2011
  #6
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Thanks for all the replies!

I will check the schematics again, and see about the values of the other two resistors... sorry no scanner
Old 22nd September 2011
  #7
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I managed to find the schematic online... Mr Roberts, would you mind taking a look and seeing if you can identify the other resistors involved in the mic pad? More for learning sake, I dont really understand how the three resistors work together to keep the input load stable.
Attached Thumbnails
modding console mic pad attenuation?-tascaminputa.jpg  
Old 24th September 2011
  #8
Gear Maniac
 

I'm not sure why you need a greater PAD attenuation - as the one in your mixer calculates to be a 30dB loss.

Anyway - R110 (2.4K) effectively defines the microphone amp input impedance, so when the PAD is switched in the 2 x 1K resistors now become the microphone 'load', as the 68 ohm resistor is now in parallel with R110.

So - quite simply replace the 68 ohm resistor with a lower value to increase the attenuation - even putting a low value resistor in parallel with the 68 ohm one to try out.

What sort of microphones (or very loud instruments) - are you using?
Old 24th September 2011
  #9
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You need to do this on all channels? If it is only for a couple of instruments, why not just build pads in barrel adapters, or buy selectable pads for those inputs?

Are you trying to feed full scale digital signals into an analog board and find yourself over-loading everything?
Old 24th September 2011
  #10
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JohnRoberts's Avatar
 

Quote:
Originally Posted by enginefire View Post
I managed to find the schematic online... Mr Roberts, would you mind taking a look and seeing if you can identify the other resistors involved in the mic pad? More for learning sake, I dont really understand how the three resistors work together to keep the input load stable.
The pad switch when off shorts the two 1ks and opens the 68 ohm.

To restore the same input impedance when making the 68 ohm smaller, you could make the 1ks slightly larger a similar amount, but this is not a big deal, and probably not audible in this application.


JR
Old 24th September 2011
  #11
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yep that is just about it... i am using presonus firepods as digital interface, they have balanced outs. The board (tascam m-3500) only has unbalanced line inputs, so i am using the mic input.

I wanted to test using more pad to cool the signal vs. lowering the digital output from the DAW to compare.

Maybe there is a better way to do this (aside from getting a new board ?

If it works well, I would need to do 16 channels.s
Old 24th September 2011
  #12
Gear Maniac
 

There is a better way to do this - use the line inputs! Balanced outputs will happily feed unbalanced inputs - and you won't have the possible danger of accidentally applied phantom power damaging the outputs of your Presonus.
Old 25th September 2011
  #13
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Quote:
Originally Posted by enginefire View Post
yep that is just about it... i am using presonus firepods as digital interface, they have balanced outs...
as said above, don't use the mic level inputs to feed a line level signal into the board.
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