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POP problems when preamp is turned on

Discussion in 'Electronic Basics' started by tempus fugit, Nov 21, 2004.

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  1. tempus fugit

    tempus fugit Guest

    Hey all;

    I've built a mic preamp that has one xformer for the opamp (+/-15v), and
    another for phantom power (+48v). When I switch the power on, I get a good
    sized POP (about 1v or so according to my scope). I get another POP when I
    turn it off. When I switch the phantom power on, it pops again, and when I
    switch the phantom power off, it pops yet again, and I measure about -15v DC
    on my meter at the audio output.
    Can anyone clue me in as to how to get rid of these annoying (and maybe
    damaging) pops? Also, can anyone account for the DC voltage at the output? I
    have a DC blocking cap at the opamp's output, so I'm not sure how the DC
    voltage is getting there.

    Thanks
     
  2. Blake

    Blake Guest

    ----- Original Message -----
    Many audio amplifiers use a JFET as a power-on/power-off mute (an RC circuit
    on the gate to turn the FET on for the first second after power-on. The JFET
    pulls the input of the last stage to ground until things settle down.) to
    deal with these turn-on transients.



    Also, can anyone account for the DC voltage at the output? I
    Unless I misunderstand your problem, it's not really "DC", but a sudden
    change in voltage at turn-on that causes the problem. The trick is to have
    everything else settle down first, and then allow the amplifier to amplify.
    That's where the mute circuit helps you.
     
  3. tempus fugit

    tempus fugit Guest

    Thanks Blake. I knew that they used JFETs for quiet switching, but never
    really thought about how until now (never needed it before!).
    A couple of questions:
    Why a JFET (wouldn't a BJT work too)?
    Will any old JFET do, or is there one or another that will do the job better
    (quieter)?
    Does the audio signal go through the JFET?

    As for the DC, I actually measured a jump to about -15v DC which gradually
    worked its way to 0 when I turned the phantom power off. That issue is fixed
    now, though. I wired the on/off switch a little differently and there is
    very little noise when I turn on the phantom power.

    Thanks
     
  4. Blake

    Blake Guest

  5. tempus fugit

    tempus fugit Guest

    I see it. That's pretty slick - so the signal never goes through the JFET,
    the output is just grounded by the JFET, hence no signal gets through (is
    that right?)

    I can't see what kind of JFET they're using - will just a standard
    runofthemill JFET work, or does it need to be a power JFET, or something
    special?

    Thanks
     
  6. Blake

    Blake Guest

    Exactly. The RC time constant gives a delay on power-up, but the diode
    eliminates the delay on power-off so turn-off transients are muted too.
    It's a J111. See the data sheet at:

    http://www.fairchildsemi.com/pf/J1/J111.html
     
  7. tempus fugit

    tempus fugit Guest

    Ahhhh.
    I did see that - I thought it was like Q111 - the part # on the schem, not
    the actual part#.

    Thanks
     
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