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In Reply to: RE: Why do subwoofers thump when turning on power amp? posted by tketcham on April 11, 2017 at 09:17:51
the thump is caused by a DC offset voltage that occurs across the sum amplifier as it powers up. Some amplifiers have relays that engage the woofer after the amp stablizes - but the plate amps don't often do that.
The DC offset is usually caused because there is a charge held on one side of of the amplifier rails compared to the other - the + and minus rails will will not discharge equally. Other causes could be other capacitors in the circuit path holding a charge - for instance the Low pass filter at the input - stages.
As most say this is usually a benign effect - as woofer can usually hand several volts of DC offset for a short while. My M&K sub does not thump on turn on. A Sub I built with a Dayton plate amp does...
"The hardest thing of all is to find a black cat in a dark room, especially if there is no cat" - Confucius
Thanks for the reply. I'm not sure what you mean by the sum amplifier. Is that the power amp or the plate amp in the subwoofers?
The subwoofers don't "thump" when connected to my integrated amp or with other preamp/amplifier combinations I've tried. It's specific to the Prologue Premium/M22 combination.
I have contacted the manufacturers and am waiting for a response to see if they know what is causing the DC offset and if there's a fix.
I'm not that concerned about the subwoofer drivers but am wondering what the DC discharge might be doing to the built-in DSP circuitry.
To keep the powered subwoofers from "thumping", I just make sure the M22 comes out of standby mode before the preamp comes off mute mode during warm up. I hadn't tried this because I thought the preamp tubes had to be completely powered up and stable before I took the power amp out of standby. At least I think it's OK to have full output on the power amp while the preamp is still warming up. Hopefully a simple solution to a simple problem.
PS: Thanks again, kavakidd.
Dayton is one that is going ultra cheap now.
Better buy up the older models before it's too late.
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