Resister Pad

Discussion in 'General Electronics Chat' started by userdnl, Jun 21, 2011.

  1. userdnl

    Thread Starter New Member

    Jun 21, 2011
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    I'm trying to go from a consumer line level output (-10dBv) (1/4 inch headphone from an audio mixer) to a consumer mic level input (~-60dBv based on my sources) (Sony HandyCam). By what I am reading, an L pad is the simplest device which can meet my needs. The most basic example I am looking at drops 40dB using a 10k and 100 ohm resistor. However, I am reading that consumer mics are actually 600ohms which, based on this source, would make a 600 ohm resistor more appropriate for R2. Also, it seems, based on my sources, that I need to drop 50 dB not 40. Since this is just a voltage dividor, converting both dBV ratings to V and dividing gives a ratio of ~300. So a ~190k resistor would be appropriate for R1. What do you guys think? Am I right in my reasoning? Or should I just go with the 10K/100 model that seems to work for everyone else? Thanks ahead of time!
     
  2. userdnl

    Thread Starter New Member

    Jun 21, 2011
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    I'm afraid that I am getting very confused here. I would appreciate if someone could confirm whether I'm on the right track. I read somewhere else that the input impedance must be greater than the output impedance. Does this mean that I would have to switch the configuration of my lpad and have the 600ohm camera by R1? Or would I need a T-pad? Or is it simply not possible? This explains why that first example used 100ohm shunt. No idea why they would put line in on the r1 side though, since impedance 1 must be greater than impedance 2. Line level is 100ohm while mic level is 600ohm.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    You do not need to match impedances. A low impedance source from the attenuator will actually reduce noise when it feeds the 600 ohm input which is good.

    The amount of attenuation should be less than you think because a volume control will adjust it to be exact.

    Line level is not 100 ohms. Frequently it typically has a 10k ohm load.

    The attenuation when the divider is 10k to 100 ohms (in parallel with the 1.2k input impedance of the mic preamp) is 108 times. Then a loud sound at the mic will produce 1V RMS at the output of the preamp.
     
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  4. userdnl

    Thread Starter New Member

    Jun 21, 2011
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    Thank you for sharing your expertise Audioguru. I could just slap a few resistors on a piece of perfboard per the instructions in that first link and call it a day, but I'd much rather learn something from the whole process.

    From what you are saying, I take it that impedance really doesn't need to be considered. I can use any value resistors that would produce the desired attenuation, in this case 40dB.

    You say that lower source impedance is actually good. I would assume this is because the voltage peaks are lower and less peaking will occur? If the impedances were off by two much, wouldn't the audio be too quiet?

    I'm actually a bit confused about the difference between input and output levels and, on second thought, I'm thinking that perhaps the input impedances are purposely higher than the output impedances for the reason you just said. I'm reading on wikipedia that line level output is 100ohms while input is, as you said, 10k. Seems like a very large difference to me. Is there somewhere I can read up on why things are done like this?

    To calculate the input and output impedance of an lpad attenuator, I assume output is simply the shunt resistor and input is the series combination of the two?

    When you refer to the mic preamp, you are referring to a device contained within the camera? Just wondering were you got 1.2k.

    Once again, thanks for taking the time to help me out. Just trying to better understand things.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Those figures may be close enough for resistances, but line output is expressed as an AC voltage level, close to 1.25 VAC - http://en.wikipedia.org/wiki/Line_level
     
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  6. Audioguru

    New Member

    Dec 20, 2007
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    A signal source resistance of 100 ohms feeding an input that is 10k ohms is not "loaded down". If the 100 ohm signal source feeds a 100 ohm input then there will probably be high distortion and the level will be cut in half.

    To make a 40dB attenuator (1/100th) Use a 10k resistor in series with a 100 ohm resistor. The other end of the 100 ohm resistor connects to ground.
    The signal across the 100 ohm resistor is 1/99th the source voltage that feeds the 10k resistor.
     
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  7. userdnl

    Thread Starter New Member

    Jun 21, 2011
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    Thanks guys. Looked up impedance matching, input impedance and output impedance on wikipedia and that's helped to clarify things a bit. It seems audio work is not always an exact science like most other circuitry. Just something that's going to take some getting used to.
     
  8. userdnl

    Thread Starter New Member

    Jun 21, 2011
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    I built the 40dB attenuater as described by Audioguru and shown in my first link. While it does seem to bring the line level down to a proper mic level, I did notice some unexpected hum when the audio feed was muted. Even if the attenuater is unplugged from the mixer there is a slight hum when the camera is plugged in and slight static when it is on battery power. In either case, touching the ring of the attenuater decreases the buzz (odd that this helps when the camera is on battery). I could see why having the extra line resistance could cause I ground loop, but I don't understand why the static is there when on battery power or even when the attenuater is just hanging. Is it just an issue with the camera's design? Is there anything I can do?

    On a side note, we purchased a new mixer recently and it will give acceptable operation if the gain knob is turned far down (still peaks and cuts out at times and has a bit of static but its by all means acceptable.) Was just wondering, is there any technical difference between this external attenuater and the boards internal one?
     
  9. userdnl

    Thread Starter New Member

    Jun 21, 2011
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    Still haven't figured this out. Is it possible that adding a decoupling capacitor after the attenuator would help? Or would this just degrade the signal further?
     
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