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Thread: Passive attenuators(volume control) & HP anomily-for monitors??

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    Passive attenuators(volume control) & HP anomily-for monitors??

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    I read that a passive attenuator (volume controller) can create a HP filter due to high capaciitor rating in monitor(powered speaker) cables impedance rating.
    An other words a decent passive attenuator can cause loss of HF due to mis -match of impedance of the controller and monintors**especially in long cable runs.

    Is there any truth to this?

    What is considered long cable lengths anyway? 10ft? 20 ft?

    To boil it down, I want a monitor passive volume controller between my DAW interface to a set of power monitors- to set the volume.
    I'm looking at box with a passive attenuator vs SPL volume 2-which is actually a active box

    Any one have a clue? mshilarious?

    T
    Last edited by 1tonio; 10-12-2008 at 18:00. Reason: .

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    Quote Originally Posted by 1tonio View Post
    I read that a passive attenuator (volume controller) can create a HP filter due to high capaciitor rating in monitor(powered speaker) cables impedance rating.
    An other words a decent passive attenuator can cause loss of HF due to mis -match of impedance of the controller and monintors**especially in long cable runs.

    Is there any truth to this?

    What is considered long cable lengths anyway? 10ft? 20 ft?

    To boil it down, I want a monitor passive volume controller between my DAW interface to a set of power monitors- to set the volume.
    I'm looking at box with a passive attenuator vs SPL volume 2-which is actually a active box

    Any one have a clue? mshilarious?

    T
    Cable capacitance is nominally 30pF/ft. Can be higher or lower, but that's a reasonable estimate for a typical line cable.

    A long cable length would be something that caused audible band (20kHz) attenuation--to avoid any attenuation, keep the corner frequency (-3dB) point at 100kHz or so.

    Given this formula:

    Z = 1 / ( 2 * pi * 100kHz * 30pF * ft)

    then:

    Z * ft = 53000


    A passive attenuator would cause HF rolloff only if its output impedance was high enough to cause a problem given the cable length. So if you need say a 100 ft cable length, then you need an attenuator with output impedance of less than 530 ohms. You can roughly take 1/2 the value of a pot (assuming you are using a pot as an attenuator) as its worst-case output impedance, so that would indicate a 1K pot. If you only need 10ft, 10K would be fine, but I don't see a reason to use more than 5K. I don't know the design of these attenuators, but it's possible if not likely they are using a fixed impedance design, so they might simply state output impedance in their specs.

    Of course you can use longer cable lengths with lower value pots, but the tradeoff is you will be loading down your output device, and at some point you will degrade its performance.

    If you are used a fixed inline attenuator, you can simply locate it on the monitor end of the cable and avoid any such problems. I know you aren't doing that, but just for anybody that was curious . . .

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    woah- great info MS !! thanks alot

    Actually its a stepped passive attenuator here http://www.goldpt.com/sa1x.html

    It deffaults to 10K but can confiure as 5K, 10K, 20K, 25K, 50K, or 100K per request as you can see on thier site.

    So basically I should be OK using 10' cables w/ 10K configuration , yes?

    At $440, it might seem overkill, but its about the same as SPL volume 2 price wise. However the Volume 2 is an active.

    Was considering a Central station. Has nice bells & whistles, but read about issues with my main concern of a good volume knob. I don't need the bells & whistles plus the ticket of $600 woppers.

    Any other decent attenuator out there? I don't have the knowlegde nor the time for making one DIY.

    thanks

    T
    Last edited by 1tonio; 10-13-2008 at 12:26. Reason: .

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    Quote Originally Posted by 1tonio View Post
    So basically I should be OK using 10' cables w/ 10K configuration , yes?
    Yeah 10K is fine there. I dunno about anything else, I would just make one myself. That 4 gang stepped attenuator is gonna be an expensive part. It could be made a lot cheaper if you had separate left and right controls, and dirt cheap if you were happy with two pots. The pot approach would get the parts cost to about $20, but it would be tricky to match left/right level.

    Me, I just use the software volume control . . .

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