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Old 29th December 2006, 02:07 PM   #1
Kinger is offline Kinger  United Kingdom
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Location: Lincolnshire, England
Question On & Off Thump

My Marshall MG15CDR makes a LOUD thump when switched on and when switched off - is this normal for this amp? - I also have a Fender amp of the same power rating, this only makes a gentle thump.

Perhaps some of you other users can enlighten me.

Thank you.
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Old 29th December 2006, 06:56 PM   #2
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Join Date: Nov 2005
Location: East Coast of South Africa
You will find there is a substantial off-set voltage on the output at switch on and takes a while to stabilise. Does this bother you or was this purely out of interest.

The second posibility is that the amp is driven from a single supply rail and uses an output capacitor connecting to the speakers.

At switch on the output swings to half rail and the capacitor charges through the speaker and could be responsible for a big thump.

One way of checking is turn the amp on and off and on. If the thump is not that servere at the second switch on then this is most likely a single supply rail.
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Old 31st December 2006, 11:46 AM   #3
Kinger is offline Kinger  United Kingdom
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Location: Lincolnshire, England
Thanks for the reply, the amp has a symmetrical +/- 20V supply without a series capacitor to the speaker, the thump appears to originate from further down the line somewhere in the audio processing before the power stage. The output swings between the supply rails for up to 200mS after switch off. It is therefore at max volume - I just wondered if this is a feature of this amp, or is there a fault? - I wondered if anyone else out there had a similar amp and could report whether their amp makes a similar noise.
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