Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!uakari.primate.wisc.edu!aplcen!unmvax!ogccse!blake!whit
From: whit@blake.acs.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: Help to eliminate pre-amp power-up click
Message-ID: <4596@blake.acs.washington.edu>
Date: 22 Nov 89 08:17:45 GMT
References: <184@vlsi.waterloo.edu>
Reply-To: whit@blake.acs.washington.edu (John Whitmore III)
Organization: University of Washington, Seattle
Lines: 53

In article <184@vlsi.waterloo.edu> rlbrenn@vlsi.waterloo.edu (Robert Brennan) writes:
>Here is a circuit for a split 12 volt power supply that I have used
>[in] low current applications (at end of article).
>
>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. 
>
>      ___  ___________
     ~   BB      _____|____                    _______
         BB      |bridge   | Pos.              |     |
     AC  BB-+  +-|rectifier|---+----+--    ----|7812 |-----+------ +12 V
         BB |  | |_________|   |    R1 \___/   |_____|     |
     ~___BB_|_ | _____|      __|__  |____|_       |      __|__
            |  |             _____  |   -+- C1    |      _____
            |  |               |    R2  ---       |        |10uF
            +------------------+----+----+--------+--------+-------+ Ground
               |             __|__       |      __|__       
               |             _____ 1000uF|      _____        
               |               |      ___|__      |10uF
               |               |      |     |     |
               +---------------+------|7912 |-----+------------ -12 V
                             Neg.     |_____|

	If your only trouble is the abrupt turnon of the power
supplies, ramping the input to the regulators will help; I show an 
example above, in a modification of the positive regulator.  The
R1/C1 pair form a turnon delay; about 1 second for R1= 1k ohm, C1= 1000 uF.
R2 should be much larger than R1 (30 k ohm) and just discharges C1 
when the preamp is powered down.  The symbol above C1 is intended to be a
NPN transistor (or Darlington, in which case R1= 10k ohm, C1= 100 uF)
which is connected as a "capacitor multiplier" to C1.  The emitter is
connected to the 7812; for the negative side, use PNP transistor.
	It is important that the input to the 7812 be above 15Vdc, so
the transistor (which "steals" two volts) will only be biased correctly
for a bridge rectifier output of circa 17Vdc.  I once made a version
of this circuit with a power MOSFET for the transistor (N-type for
positive), but that requires the pulled-up voltage on the gate to be
at least 5V higher than the required output; unless there are other
power supplies available, that will not work.
	A more elegant scheme would be to use a single regulated power
supply to control both + and - voltages (i.e. a tracking regulator),
with a slow-start connection to the voltage reference that drives the
regulators.  I have tested the ramp-the-input scheme shown above,
and the output does not ramp as smoothly (the input goes from six to 
eight volts while the output goes from zero to eight volts) as the
input.  If the same scheme is used with a LM317 regulator, it should
be more effective than with a 7812 (because the LM317 operates at
lower voltages, it will turn on before six volts is applied).

... I am known for my brilliance,             John Whitmore
by those who do not know me well.