Path: utzoo!mnetor!tmsoft!torsqnt!jarvis.csri.toronto.edu!rutgers!ucsd!swrinde!emory!stiatl!rsiatl!jgd
From: jgd@rsiatl.UUCP (John G. De Armond)
Newsgroups: sci.electronics
Subject: Re: Help to eliminate pre-amp power-up click
Message-ID: <589@rsiatl.UUCP>
Date: 16 Nov 89 10:04:01 GMT
References: <184@vlsi.waterloo.edu>
Reply-To: jgd@rsiatl.UUCP (John G. De Armond)
Organization: Radiation Systems, Inc. (a thinktank, motorcycle, car and gun works facility)
Lines: 52

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
>(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.  

How 'bout a real KISS solution.  Actually, two.  One is less KISS than
the other.

The KISS solution:

Simply bridge to ground the OUTPUT of the preamp with a DPDT mercury-wetted
reed relay.  Connect the NC terminals to the output of the preamp such that
when the relay is deenergized, the output is shunted to ground.  Connnect
the NO contacts to the output jack such that when the relay is energized,
the signals are passed straight through.  Energize the relay from the
rail through a time delay network that consists of a capacitor across the
coil and a resistor in series.  Choose appropriate values to give
a couple second delay.  I'd also suggest that you bridge the resistor
with a reverse biased diode to provide a quick discharge path for fast
recovery during short power transients.  Be sure and put a back-biased
diode across the three term regulator to protect it from the discharge 
current.  You probably know that :-).

The slightly less KISS solution is basically the same as above except 
that  you fire the relay with a comparator set up to trigger
when there is no DC component on the output of the preamp.  This scheme
ensures that only enough time to stabilize the preamp is used 
during startup.  Handy when you just GOTTA have that quick music
fix :-)

This solution has many benefits.  The MW reed relay will last forever,
will not introduce any noise, thermal emf or any of the other effects
imagined by the golden ears, is very fast and totally silent.  It will
not be destroyed by accidental transients fed into the output.


PS:  If you need some relays, let me know...

John

-- 
John De Armond, WD4OQC                     | Manual? ... What manual ?!? 
Radiation Systems, Inc.     Atlanta, GA    | This is Unix, My son, You 
emory!rsiatl!jgd          **I am the NRA** | just GOTTA Know!!!