Path: utzoo!attcan!utgpu!watmath!att!rutgers!mit-eddie!mit-amt!mit-caf!ankleand
From: ankleand@mit-caf.MIT.EDU (Andrew Karanicolas)
Newsgroups: sci.electronics
Subject: Re: Help to eliminate pre-amp power-up click
Message-ID: <3467@mit-caf.MIT.EDU>
Date: 16 Nov 89 02:09:07 GMT
References: <184@vlsi.waterloo.edu>
Reply-To: ankleand@mit-caf.UUCP (Andrew Karanicolas)
Organization: Microsystems Technology Laboratories, MIT
Lines: 36

In article <184@vlsi.waterloo.edu> rlbrenn@vlsi.waterloo.edu (Robert Brennan) writes:
>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

I take it that you have exonerated the power-amp by turning on the
power-amp without the pre-amp connected to it to see if there is a glitch.
One of the simpler ways to solve your problem may be to use a circuit that 
delays switching on AC power to the power amp relative to the pre-amp.  
This would allow the pre-amp output to settle before it can cause trouble.  
Of course, you can also switch on the pre-amp and the power-amp manually to 
avoid loud glitches. A friend of mine had trouble with power up glitches
on his Denon receiver, the solution there was to use the speaker switches to
connect the speakers after the receiver was powered up and to disconnct them
before it was powered down.  Many receivers have a delay built in to the
speaker protector circuit at the output of the power amp (the 'click' you
hear with many receivers and amplifiers shortly after AC power is applied is
the relay in this protection circuit).  
If your power amp has such a speaker protector, then you could modify it to
include this delay.  If it does not include one, the problem you are 
experiencing is one of the many good reasons to include one. There are audio
purists whose beliefs are incongruent with such circuits though and rely
on fuses.  In such a case, delaying AC power application to power-amp would be
an easy way out. I guess you could also delay connecting the pre-amp output 
to the power-amp input but I've never seen the problem dealt with in that 
manner.  

This was a little longer than needed perhaps, good luck and have fun!

Andrew N. Karanicolas
Microsystems Technology Laboratory
ankleand@caf.mit.edu