Em7 Battery-less Footswitch Mod for Pre-Custom 2-Channel Amps

Discussion in 'Amplifiers' started by Em7, May 30, 2017.

  1. Em7

    Em7 deus ex machina

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    I designed this mod back in 2012 after people started to ask for a footswitch with LEDs for the 2-Channel "H" and the 2-Channel "C." The switches in the footswitch connect the V- terminals on the relays to V- on the relay power supply; therefore, there is no way to power LEDs in a footswitch in the standard amp configuration (this problem was rectified on the 2-Channel Custom by using a cable with more than three conductors and a 5-pin DIN plug to bring V+ and V- out to the footswitch). The challenge here was to power two LEDs and energize/de-energize the coils in the switching relays using only three conductors. I thought about the problem for a while before it dawned on me that I could insert N-Channel MOSFETs between V- on the relays and V- on the relay power supply and use V+ on the supply to turn the MOSFETs on (i.e., the MOSFET is acting like an electronic switch). The beauty of this solution is that the relay power supply provides enough voltage to support an LED forward voltage drop and still have enough voltage to turn a logic-level MOSFET completely on via the gate; therefore, I could insert LEDs between V+ and the gate on each MOSFET. The jack is rewired to run the relay power supply V+ out to pedal on the shield with the MOSFET gate returns running on the two signal wires. The old relay power supply V- connection on the jack gets wired to the source leads on the MOSFETs, and each of the old relay V- connections on the jack gets connected to the drain lead on its assigned MOSFET. The switches in the footswitch connect V+ from the shield to the anodes on the LEDs. The switch closes, which results in the LED lighting. The relay power supply voltage minus an LED forward voltage drop appears on the cathode, which runs back to the amp and turns the associated MOSFET on via its gate. Current flows through the MOSFET, which energizes the relay coil, closing its contacts. The beauty of this mod is that it will work with the non-LED footswitch that ships with the amp as well.

    The Basic Circuit

    [​IMG]

    There are actually two of these circuits in the mod. The second circuit handles the reverb relay coil. The current limiting resistor serves two purposes. Its primary function is to limit the amount of current flowing through the LED. The secondary function is to provide a discharge path to ground for the gate. I used a 470 ohm resistor in my build. That value limits the current flowing through the LED to around 10 milliamps.

    A Few Photos of the Build

    Circuit Board

    [​IMG]

    Back of Circuit Board and Backer Board

    [​IMG]

    Testing Signaling

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    Circuit Installed in Chassis

    [​IMG]

    Footswitch Guts

    [​IMG]


    Four Switching Functions

    Lead Sans Reverb

    [​IMG]

    Lead With Reverb

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    Clean Sans Reverb

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    Clean With Reverb

    [​IMG]
     
    #1 Em7, May 30, 2017
    Last edited: Jun 14, 2018 at 1:07 PM
  2. bodia

    bodia Authorities said.....best leave it.....unsolved

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    That's sweet! Nice iob!
     
  3. LSchefman

    LSchefman Hears Tones

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    You bet, super cool!
     
  4. Em7

    Em7 deus ex machina

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    Here is where I tapped the relay power supply V+ to power the LEDs and supply the gate voltage to the MOSFETs:

    [​IMG]
     
    #4 Em7, May 27, 2018
    Last edited: Jun 14, 2018 at 6:40 AM

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