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Building a single channel variable U-Pad

Discussion in 'Electronic Repair Modifications DIY' started by Guitarfreak, Oct 13, 2011.

  1. Guitarfreak

    Guitarfreak Well-Known Member

    I read up on pads of various kinds, and I want to design a single channel variable pad for use between a microphone and the preamp when recording drums. The preamp in question is the channel preamp of the RME Fireface 800, which has an input impedance of 2k. Here is what I want to do. I will build an external enclosure with XLR in and out mounted to it. From there I will wire both the + and - contacts to the CW inputs of a 10k dual gang linear pot and using 20k as a shunt resistor. These values are chosen because 20k is 10x 2k and that should not affect the sound of the preamps and work solely as a signal reduction, which is desired. The pot value is chosen because if this were a simple unbalanced circuit, a voltage divider made from 20k and 20k would result in an even 50% reduction in signal level, which is fine for the application I think, but seeing as how the circuit is balanced, the R value turns into 1/2R for both conductors which carry signal to recreate the same amount of reduction. The output will be taken from the junction between the series and shunt resistors, which turns out to be the wipers of both pots and the shunt resistor will bridge both pot wipers.

    Am I on the right track here? Feel free to correct me or add anything you feel is relevant.
  2. RemyRAD

    RemyRAD Well-Known Member

    Rather than wasting your time to create a variable pad, Shure already makes an XLR barrel Pad. In fact they make numerous ones that have switchable characteristics. These are balanced pads. You don't need continuously variable, trust me. 10, 20, 30, DB switching should be more than adequate. That's why you have gain trim. Hey, this is the way all professional consoles used to be built. So what's the point for this? What is the real application intended for?

    That guitarfreak I know has something up his... something?
    Mx. Remy Ann David
  3. Boswell

    Boswell Moderator Distinguished Member

    I didn't quite follow the pot and shunt resistor arrangement you were suggesting. It's necessary to be a bit careful when constructing a balanced pad for a mic input that you use a configuration that works with phantom power. You have to pot across the balanced inputs and not have any reference to ground.

    I've never had the need to pad an FF800 microphone input, even with lively mics on loud sources. A pad switch on a microphone body itself is a different matter, as this can adjust the dynamic range of the mic for the sound source it's exposed to. If you really need a pre-amp input pad, I would go for a fixed attenuation one, as Remy suggested.
  4. Guitarfreak

    Guitarfreak Well-Known Member

    I don't actually need the pad myself, I am trying to help out a friend who has need for one before his FF800. I saw those in line pads, and I think that they are cool and all, but I think it would be great to have some sort of control on the level of pad in an instantaneous manner. Not to mention designing one which does not alter the sound of whatever passes through it for brighter or darker by using the correctly calculated shunt impedance for the given application. You are familiar with the quest I am sure :D Here is a picture which displays the idea from the original post. I know these sort of things are tough to explain verbally. Words are shallow and audio is a deep science.

    (Expired Link Removed)
  5. djmukilteo

    djmukilteo Well-Known Member

    What sort of source input are you using?
    The 2k input impedance you listed indicates the mic preamp inputs (7-10)?
    If this is for a guitar or synth, then I would think using an outboard DI and then running that balanced lowZ into the FF preamps would be all that's needed. The input trim controls should act as your pad unless your trying to drive a really hot line level into the preamps...but then that's why the external balanced DI would be useful.
    Also the built in instrument DI (input 1) on the FF has those switchable instrument options (limiter, speaker cab sim etc.) if using a guitar that can be selected.
    Then there are the line level inputs either from the front (7-10) or the ones on the rear that could also be used.
    Any of those inputs can be trimmed with TotalMix set to adjust your levels going in.
    If this is all experimental (i.e. building breadboard pad configurations), then I guess you just need to do your calculations and build what you are thinking and then try it out...without some idea of what your trying to accomplish it's difficult for anyone to give you advice.
    Those inline pad connectors are all fixed value pads and good for balancing varying multiple input sources in order to match levels to the input of a mixer, but that's about all they do.
    Resistor based pads don't impart any sort of "tone" on anything either if that's what you were thinking...
    Hope that helps
  6. Boswell

    Boswell Moderator Distinguished Member

  7. Guitarfreak

    Guitarfreak Well-Known Member

    I did neglect the idea of the pad impedance summing with the output impedance of the mic itself. It really seems that without creating a balanced amplifier stage in and of itself, the entire concept of an inline mic pad is flawed in and of itself... Seems sort of like a pick your poison situation. You either do like I did and put too much series resistance between the mic and pre, while keeping the load network as close to unloaded as possible. Or on the other hand, you use a more appropriate series resistance and in order to get a good enough reduction ratio, your shunt resistor is going to end up pulling the impedance of the mic pre low. This solution seems almost like we are treating the mic pre as a current operated device... Hmm.

    A valid point. Just trying to help out a friend. He's not using an external preamp, just the Fireface, and he says that when recording drum set, in order to get the close mics to a reasonable level he has to have the mic channel gains very low, almost off. I told him it might be better to pad the mics a bit and then run the channel gains a bit higher. He asked how much pad he needed and here we are.
  8. Boswell

    Boswell Moderator Distinguished Member

    No, not really. A simplified way of looking at connecting up this type of equipment (including in-line attenuators) is that the input impedance is relatively high and the output impedance is realtively low of each piece in the chain. The relative defininition of "low" and "high" here could be a factor of 10 difference between them, which would be enough not to make much of a dB change when loading an output with the next input.

    So a mic has an output impedance of a couple of hundred Ohms, and would normally plug into the FF800 2KOhm input. This fits with what I said in the previous paragraph. If you make the output impedance of your attenuator also around 200 Ohms by using a 220 Ohm shunt resistor, the FF800 input thinks it's seeing the mic directly and is happy in noise terms. It's then a matter of increasing the series resistors in the attenuator to give you the required attenuation value, given that anything above about 1K Ohm is not going to trouble the mic that is driving it. I'm assuming that the microphones in use are not the type that change their tonal qualities with load resistance.

    By the way, the max input level of the FF800 mic inputs is +11dBu. I somehow don't think that even close-miking a drum kit with high-output mics will cause pre-amp overload, although it is exactly the situation where I would be looking for pad switches on the mics themselves to give the mic's internal circuitry a bit more headroom to work with.

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