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?
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.


Paid Member
2004-06-06 8:31 pm
Georgetown, On
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).



2004-09-16 9:31 am
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