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  • Active Passive Input

    I need to add a switch or change one of the input jacks for using an active bass on the amp that has been fixed in this thread, http://music-electronics-forum.com/t45926/. Would a switch be the correct way or is there a way to wire one of the inputs for active and one for passive? The inputs are currently wired the way they are on a Fender amp. I suppose it would be better if one input was active and one was passive but they don’t have to be.

    One way I found is to use a DPDT switch and resistors but not sure of the value of resistors that should be used, see attached image. Probably need to drop the input 15 to 20dB, just a guess.

    If the switch would work, would anyone have an idea of what value resistors to start with?

    Click image for larger version

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    It's all about the bass. Lock in the groove and stay out of everyone else's way.

  • #2
    I would run a resistor to ground through a SPST switch to lower the level in active mode. Value will depend on taste, input impedance, and what's already there. You can experiment with resistor value. Maybe even use a pot first to test. Dial it in, then measure it to add a fixed resistor. Here's a picture of a Trace Elliot input circuit for example. You could even use a pot and make it variable.

    Click image for larger version

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    If you just want 2 different inputs, one for active and one for passive, just add a resistor to ground on the active input to lower the level.
    Last edited by The Dude; 04-21-2018, 12:55 AM.
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • #3
      Thanks. I'll give the switch that way a try.
      It's all about the bass. Lock in the groove and stay out of everyone else's way.

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      • #4
        If you have two input jacks and they are wired like a zillion Fenders, then you ALREADY have active and passive jacks. They are 6db different. The Fenders used 68k to make a 2/1 voltage divider, that Trace schematic above uses 100k to make a 2/1 divider. Same same. Back when they didn't call them active and passive, but that is all it means, The active is the one padded down for the hotter signal.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Your correct about the input wiring, I mentioned that in the post, I probably should have mentioned that it's not enough of a drop with the active bass I have. Figuring probably -15 to 20 dB might do it.
          It's all about the bass. Lock in the groove and stay out of everyone else's way.

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          • #6
            The two resistors in the Fender arrangement are either in parallel for high gain or in series, forming a voltage divider for low gain. When both are the same, like 68k, then the voltage division is half. 2/1. if you want an even larger pad, make the resistors different. 47k and 100k would make 3/1 say. But the two jacks and their switching functions work the same, you just change the numbers to suit.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              -20dB seems like overkill; are you sure that input stage overload is the issue?
              My band:- http://www.youtube.com/user/RedwingBand

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              • #8
                Thanks Enzo
                It's all about the bass. Lock in the groove and stay out of everyone else's way.

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                • #9
                  Originally posted by pdf64 View Post
                  -20dB seems like overkill; are you sure that input stage overload is the issue?
                  Agree with the -20dB being a bit much for myself but just thinking about it due to occasionally I have to be part of the house band at a couple of Blue's jam's, OH BOY, and there are a few knucklehead's that use an active bass that they like to turn it up toooo loud. Don't particularly care for doing the jam thing but sometimes I get roped into it.
                  I usually bring in 2 amps and only let them use the one they can't turn up loud before it clips hard, but bringing two amps in at some places is a bit of a pain.

                  Maybe I'll put in 2, one on one of the input jack's for myself and an additional on a switch to knock it down more and label it KH.
                  Last edited by J Luth; 04-21-2018, 06:53 PM.
                  It's all about the bass. Lock in the groove and stay out of everyone else's way.

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