Marshall MB450H popping on shut down

[cwmont]

To All

Has anyone worked on any of the Marshall MB450H bass heads?

My friend has one that pops very loudly upon shutdown. He says it sounds like a gunshot. He said he took it to a local warranty center where they were able to verify the pop noise. He says the service center called Marshall and they (Marshall) told them to tell my friend that it was normal for the amp to do that?

Doesn't sound like "normal" operation to me. I've owned amps that popped a little going off but he says the MB450 is loud when popping. My concern is possible speaker damage from the pop.

I told him I'd post up here and see if anyone has seen this before with these heads.

Any ideas?

TIA

[Enzo]

It is very normal for amps to pop at turn off. SOme do so more than others. It is not likely to damage the speakers, the pop is no louder than the peak signals the amp can normally produce, such as a plucked or slapped string.

[tguen63]

I have a Peavey solid state amp that is doing this also. I understand it is common and not hurting the amp, but does anyone know the cause? Is it the filter caps?

Tom

[Enzo]

Cause? If it were some bad part, we could just replace it and the am pwould never do it again. There are a variety of ways an amp can pop. Not least of which is that it takes a second or two for the power supply to discharge. During that moment, the amp can still amplify. One potential source of pops is the power switch. When you turn the switch off, the contacts spark a little, and this makes electrical noise. That is a pop right there. Power supplies don;t necessarily discharge evenly at the exact same rate, and below a certain voltage, the amp is not stable anyway. SO as the supplies settle towards zero, sometimes the amp can schlump on the way down due to uneven voltages. That is the ka-whump you sometimes hear, rather than a pop per se.

There is no "it is this thing right here" answer.

[tguen63]

Thanks Enzo. I was definitely referring to the "ka-whump" a couple of seconds after flipping the switch. On this amp (Peavey Combo Mark III 260C bass amp) the "ka-whump" is really loud and causes the limiter light to come on even with all controls turned to zero. I have been tinkering to see if I can eliminate the noise and noticed if I add another 1000uF parallel the 1000uF/35V negative filter cap, the noise disappears completely. If I add it to both 35V caps, or just the positive one, I still get the noise. Any problem with just leaving the extra 1000uF on the negative side?

Tom

[chipprogr]

For those wondering how to help that loud SNAP right when the power switch is turned off, generally a 0.1uf to 0.0033uf cap across the switch terminals help quench this high voltage kick that the switch causes when the power is interupted. Id look for a voltage rating of 500 or so on this part. Take a look at several amp scematics, you'll see them in the power turn on section of the diagram.