Amp Switching Circuit Popping

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by 8len8, Feb 27, 2015.

  1. 8len8

    8len8 Member

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    I've get a Phaez amp that has 2 individual inputs, one for each channel. I decided instead of using an AB box that I'd install a switching circuit to route a single input to either channel.

    Originally the amp had each channel's preamp tube connected to its own input jack.

    I got a Granger optoFET channel select board:
    http://grangeramp.com/shop/index.php?main_page=popup_image&pID=493

    I powered it off a wall-wart and hooked it up like this (all connections via coax):

    1 NC - ground
    1 Com - clean channel 12AX7 cathode
    1 NO and 2 NC - input jack
    2 Com - dirty channel 12AX7 cathode
    2 NO - ground

    My only problem is that I hear a slight pop when switching from the clean to dirty channel. I've read that FET based channel select circuits should have quiet switching. Is there a pull-down resistor or something that I'm missing that could be causing the pops?
     
  2. J M Fahey

    J M Fahey Member

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    "An image is worth 1000 words" ;)

    Draw and post a schematic.
    Remember to label all parts (R1, R2, C1, C2, etc.) to avoid confusion.

    Even a picture of a hand drawn one is acceptable, as long as it's sharp and readable (use a real camera, not a cellphone)
     
  3. 8len8

    8len8 Member

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    It's so basic I wouldn't know what to show. There are no Rs or Cs. Just a Fet board, a jack, and 2 tubes. The list above says it all.
     
  4. VaughnC

    VaughnC Supporting Member

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    Without a diagram of the switch module, this would be tricky to sort out. Its unclear how the input jack is wired...or is that just the control input point and doesn't actually carry the audio? However, connecting to the tube's cathode's isn't right. The audio needs to connect to the control grids...and, you may possibly need to add around .1uf DC blocking caps in series with the control grids and about 470k to ground from the control grids if there aren't any present.
     
    Last edited: Feb 28, 2015
  5. 8len8

    8len8 Member

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    I guessed the cathode because it's the 3rd tube pin from the end. It could be the control grid like you say.

    In the original amp the input audio jacks just directly connected to the tubes (control grid), no resistors or caps in-line. I just placed the new optoFET board between one of the audio inputs and the 2 preamp control grid pins.

    So it sounds like there should normally be an RC high-pass network between the audio input and the control grid?
     
  6. VaughnC

    VaughnC Supporting Member

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    I'm not certain if the switching board has isolated NO, NC, C points so I thought DC blocking caps might help with the popping...and the 470k resistors would bleed off the DC between switching. Can't hurt and might be worth a try if you're not sure.
     

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