Guitar tube amp with JFets as audio switches

Discussion in 'The Projects Forum' started by muadib25, May 6, 2012.

  1. muadib25

    Thread Starter New Member

    May 6, 2012
    Hello folks! :)

    I am building a home project of a Mesa Boogie Dual Rectifier Solo using Jfets instead of LDRs for switching between the two channels.

    A network of 3 transistors will be providing 2 control voltages, Vc1 and Vc2 respectively, at -15 Vdc. They will be controlled by a footswitch that will be changing their states for the JFets, Vc1=ON while Vc2=OFF and vice versa. This I 've found in Kevin O'Connor's TUT.

    The JFets will be dividing the audio signal between the first and the second preamp stage, and between the 2 EQ sections. Here is a pic of the preamp stage, JFets in yellow boxes:

    1. How do I place the JFets, series or shunt? Also in what orientation, drain or source towards the audio signal input?
    2. Do they require any extra/special addition of components to work? (e.g. are the 1MΩ resistors I have already placed required?)
    3. Any other consideration I should take into account?

    Thank you very much! :)

    Best Regards,
  2. Nichesound

    New Member

    May 19, 2009
    I see this is something I am looking at, also have the TUT series, but wanted to know if you implemented this? I had worked on a Blues Jr. that used something like this to send a signal at -15volts to activate the FAT switch. Seems like a lot of work where a simple switch could have done this, but it did get me thinking about electronic switches. Did you ever work this out?
    Let me know if you have any schematic that you may have drawn up. I use autocad to draw up all my layouts/schematics.
    Johnne in Seattle
  3. muadib25

    Thread Starter New Member

    May 6, 2012
    Hey Johnne!

    Well, I had mailed Kevin O' Connor himself and he had helped me nail this out. Here is the schematic I drew that worked for me (see attachment below). I have 2 rails and the example from TUT that controls the ON-OFF action using a latching SPST Footswitch.

    Note that you dont need a resistor from Drain to Gate or Source to Gate.

    Let me know if you have any questions!


  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    some years ago, I built up an audio/video switcher for a video music system, I used quad bilateral switches, dont quite remember the number but in the 4000 series cmos chips. they will switch low level audio and video easily up to 35 mhz. made a good switch too, no thump when switching. seems like 4016 or 4069 chip.