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amplifier thud/crack


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hi all, I'm after a bit of technician know how here.


I have a bass combo (100 watt) that makes a really nasty crack/ thud sound when its turned off, it happens even when the volume is turned right down.


this spike also gets picked up by the mixer and then goes through the pa. Up until now I have dealt with this problem by making sure the bass amp is the last thing to be turned off, but I feel that I'm treating the problem and not the cause.


so I wandered if any one has any ideas about a, why the amp does this. and b, what I can do to stop it happening.


cheers in advance to you all.

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There's usually a suppressor wired across the mains switch, it stops the 'crack' as you operate it. Check if there is one..might just be a small capacitor on certain examples, replace like for like. Dont forget the suppressor cant suppress if you just switch off at the socket or just wiggle the mains plug out! The suppressors (RC networks) can fail open circuit with age.


Of course it might be a bias-off resistor thats gone but this is rare, it allows the split power rail to die lop-sidedly as the capacitors discharge, thus making a softer bump through the speaker. Check the suppressor first though!

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thanks kevinE,


my limited knowledge of electronics made me think it could be a supressor ( thinking of motors ect interfering with televisions ).

but I wasn't too sure, I'll try the cap and see if it was just that, hope it was, I can just about get my head around bias for valve amps but would not know where to start with solid state stuff.


cheers for the advice.

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A couple of bits of information please:

Is the instrument amplifier connected to the mixer (DI out, or similar), or is the switch off spike radiated via the mains connection?

What make of amplifier is it - some (poor?) designs have large switch on and off thumps and cracks, others with delayed muting are totally silent. I have heard amplifiers that momentarily loose stability and oscillate and whistle when powering down, although this is rare!


John G

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Hi, thanks everyone.


the amp is a torque tb100b bass combo (100 watt).


the amp is not DI'd or connected to any other equipment


the spike must be going through the mains.



further to this :


I just finnished changing the .01 uf capacitor across the mains input to the transformer, it did the trick, just can't believe that maplin had one in stock, the proper X2 type as well. so amp is now back to normall for a cost of 16p


thanks for your support kevinE


thanks mixermend for showing intrest.



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