lauck@bergil.DEC (01/29/85)
> 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.
dmmartindale@watcgl.UUCP (Dave Martindale) (01/30/85)
In article <362@decwrl.UUCP> lauck@bergil.DEC writes: > >> 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. >> >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. It seems that your preamp is built with the assumption that your power amp will have some sort of circuitry to delay connecting the speakers to the amplifier (many amplifiers have a relay for this) until the amplifier is stable, and that this delay will absorb the preamp's turnon/turnoff thump. Conversely, it seems that your amplifier is stable enough not to generate nasty transients on its own, and it assumes that the preamp won't either. Unfortunately, both assumptions are false in your case - this is just an incompatibility between the amp and preamp. It is probably best to live with it, as you've learned to, or replace the preamp (since that is the component you refer to as "old"). (All of the preamps and amps I've owned (two each) used direct-coupled outputs and bipolar power supplies, and none generated turnon transients large enough to hurt speakers.)