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Old 16th August 2005, 11:55 AM   #1
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Default preamp soft start

Having put a chip preamp upstream of my comertial power amp and a volume pot in front of the lot. I now get a huge startup thump. I didn't get it from running the commertial amp standalone, so the problem lies in the preamp which runs from its own 12V switch mode PSU.

Is there a very low power soft start circuit which doesn't require a relay?

I also didn't get so much when the volume pot was between the 2 amps. Have I just answered my own question?
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Old 16th August 2005, 12:11 PM   #2
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Obviously if the volume pot is after the preamp and before the power amp it will attenuate any thumps made by the pre amp.

What you need is an output mute circuit for the pre amp. This can be achieved by either simply putting a signal (reed) relay in the signal path or by adding a 100 ohm resistor in the output of the pre amp and switching the other side to ground by a relay or transistor or FET.
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Old 17th August 2005, 03:35 AM   #3
anatech is offline anatech  Canada
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Hi Pbassred,
Is this a home made pre? There is normally a muting device on the output. Relays on better units and transistors on everything else, or worse - nothing (excepting old tube units of course). If your unit is commercial (ie a brand), it's possible the muting transistors are shot. Very common and they may test okay. Look for 2SC2878 transistors (TO-92 case).

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Old 17th August 2005, 08:46 AM   #4
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The preamp is just a cheap Velleman kit, so there is nothing there to blow. I need to put something there. something cheap.

It doesn't need to be HiFi, because this is a stage monitor. I don't even mind the BUMP! at start up. Its just unnerving to see the cone trying to leap out of the box.
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Old 18th August 2005, 01:36 AM   #5
anatech is offline anatech  Canada
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Then do a timed short across the output. You could use a switch if you want really easy.

-Chris
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Old 18th August 2005, 09:39 AM   #6
klitgt is offline klitgt  Denmark
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If the preamp tolerates that the output is shorted to ground, a "startup delay" circuit which shorts the output to ground will do what you ask for. Such a delay is based on a relay that breaks the short some seconds after power-on. This is the best way because the relay does not go into the signal path. See more about the issue here http://www.machmat.com/sales/kits/startupdelay.htm
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Old 19th August 2005, 01:28 AM   #7
anatech is offline anatech  Canada
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Cheapie kit, muting transistors or 4066/4966 to ground is okay.

-Chris
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Old 19th August 2005, 10:36 AM   #8
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Thanks for the help guys. I'll have to research a timed signal ground circuit (mono).
It occured to me that I could just arrange for the 12V switchmode to power up just before the main power amp supply. A 2 position switch perhaps.
No.... I should do it right.
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Old 19th August 2005, 11:34 AM   #9
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Or just add a muting relay and DC protection at the power amp output, a better solution IMO.
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