Path: utzoo!utgpu!watmath!maytag!vlsi!rlbrenn
From: rlbrenn@vlsi.waterloo.edu (Robert Brennan)
Newsgroups: sci.electronics
Subject: Help to eliminate pre-amp power-up click
Message-ID: <184@vlsi.waterloo.edu>
Date: 14 Nov 89 16:45:46 GMT
Reply-To: rlbrenn@vlsi.waterloo.edu (Robert Brennan)
Distribution: world
Organization: U. of Waterloo, Ontario
Lines: 51

Here is a circuit for a split 12 volt power supply that I have used
(and I'm sure most people have used) successfully in typical
low current applications (at end of article).

That is until now...
I have recently built a pre-amp for my audio system as a front
end to a home-made 600W amp (Now, if I just had decent speakers to
handle the power ... but that's a different issue).
The system works great except for an extremely loud start-up 
pop out of the speakers.  The fast voltage rise on the power
supply rails seems to be the culprit.  It appears that the pre-amp
circuit is coupling power supply transients to the output.  Without
redesigning the pre-amp circuit (really, I just want to use it ... it was
a canned design), how can I go about 'softening' the abrupt voltage
rise on power-up?   I can't think of any off-hand that wouldn't
also sacrifice regulation.

By the way, I tried a slow turn on circuit using LM317 and LM337
regulators and a circuit from the National Linear Guidebook.  It
didn't solve the problem because the voltage didn't start from zero.
It ramped up starting at about 2 volts.  Even that much of an offset
created a massive power-up click.  I don't think the best answer
lies with a complicated circuit (that's why I'm going back to
the simple circuit below), I just need some advice from some of
you who might have had similar problems.  I'd like to have the
start-up sound like a nice expensive system (very muted).

Depending on interest, I'll repost replies.

      ___  ___________
     ~   BB      _____|____           _______
         BB      |bridge   | Pos.     |     |
     AC  BB-+  +-|rectifier|---+------|7812 |-----+----------- +12 V
         BB |  | |_________|   |      |_____|     |
     ~___BB_|_ | _____|      __|__       |      __|__
            |  |             _____ 1000uF|      _____
            |  |               |         |        |10uF
            +------------------+---------+--------+-----------+ Ground
               |             __|__       |      __|__       
               |             _____ 1000uF|      _____        
               |               |      ___|__      |10uF
               |               |      |     |     |
               +---------------+------|7912 |-----+------------ -12 V
                             Neg.     |_____|

Thanks for your help

-- 
	Robert Brennan  (U of Waterloo VLSI Group, Waterloo Ont.)
	rlbrenn@vlsi.waterloo.{cdn,edu,bitnet}
	rlbrenn@vlsi.UWaterloo.ca