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From: lauck@bergil.DEC
Newsgroups: net.audio
Subject: Re: power on/off sequences
Message-ID: <362@decwrl.UUCP>
Date: Mon, 28-Jan-85 19:47:23 EST
Article-I.D.: decwrl.362
Posted: Mon Jan 28 19:47:23 1985
Date-Received: Wed, 30-Jan-85 07:53:30 EST
Sender: daemon@decwrl.UUCP
Organization: DEC Engineering Network
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> Example:  my
> power down sequence is the following: 1) preamp gain to zero,  2) power
> off the amp,  3) wait 30 seconds,  4) power off the preamp.  Without the
> delay or the zero gain I blow speaker fuses.
> 
>                    Tony Lauck
>                           decvax!decwrl!rhea!bergil!lauck

>>This is interesting; does this sequence also apply when there is a
>>signal continuing through the system? For example, if you have a
>>tuner playing and it is NOT turned off, so you get signal  through
>>the power amp as it discharges its capacitors? 
If I turn off the amplifier while the rest of the system is playing, I
get good sound for a while, then distortion, then silence.  I haven't timed 
it, but I would get there is 10 seconds of quiet music or so.  It takes
over 3 minutes before the power led on the amp goes out with no signal.

>>What I'm wondering is whether the fact that your power amp capacitors
>>are still charged when you power off your preamp causes any turn-off
>>thump of the preamp to be amplified and fed to the speakers? 

My old preamp has a capacitive coupled singled ended output circuit.  It 
generates about 10 volts DC on turn on or turn off.  My power amp is DC 
coupled, with 1 volt for about 100 watts into 8 ohms.  It can output over 12 
amps.  My woofer fuse is 3 amps.  Without the delay, loud sound, blown 
fuses (woofer at least, sometimes midrange as well).  The "thump" scares my 
wife who fears I am fooling with the system and promply leaves the room...

The 30 second delay avoids the problem.  Perhaps I should get a better 
preamp which has crowbar circuitry or better power supply rejection.