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Unwanted distortion on my bass amp.

Started by grue, March 04, 2010, 04:56:17 PM

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Hi.  I'm hoping somebody here can help me diagnose this.

I have a Mesa Walkabout bass amp (hybrid) which has been giving me a faint buzzy distortion at all volumes, especially on the low notes or when I dig in.  The sound doesn't seem to be affected by turning the volume up or down.  It sounds a bit like the rattle from a loose part on the speaker cab or something, but I have tried it on 3 different cabs and they all had the same fuzziness.  The amp has an effects loop, so I tried running the bass directly into the return and got the same result, so it looks like it's not a preamp problem.

Some history:  A couple years ago I had to replace the power transformer after it melted (because of a faulty speaker cable jack).  Maybe some other component was damaged when this happened?



SCope?  Scope the power amp power rails to see if a filter cap has gone soft.   otherwise, measure them, and hte plus and minus sides about the same, and set the meter to AC and see if the ripple is about the same.

Whatever op amps might be in hte power amp stages, check THEIR power rails as well, the same way.


Could also be
the output stage bias current is a bit low due a component problem.
This buzz usually disappear at higher volumes and returns when the sound is about to die.
the speaker is rubbing on the cone.
You can try with another speaker if possible



It sounds very similar to severe underbias in the output stage. Being a valve driven MOSFET output stage I'd guess there is very little negative feedback to elliminate crossover distortion. Most amps bias generator networks are wired so if the bias pot goes open circuit (usually the case from contamination of the tracks) the output stage will loose all bias. MOSFETS needing several volts of Vgs will suffer from severe crossover distortion as there will be a considerable amount of signal around the 0v crossing area on the output that will not be amplified. There are alot of distortion mechanisms that come into play in such an instance but let's not worry about them hey ;)

You should be able to check across the source resistors with no signal applied and no speaker connected for a couple of MV of voltage. If you measure 0v then this is the case. If you can identify the gate connection then measure the voltage difference between the gates of two FETs from opposing power rails, if you measure 0v again it is definetly the bias generator that needs attention/ re-adjustment.


Thanks for the replies!

Enzo:  Unfortunately, I don't have a scope.  I'm also a bit of a noob, so I would rather not mess with the caps on a 300 Watt power amp.  Death would be very inconvenient right now.  If the bias isn't the problem and the caps look OK, I guess I will probably take the amp to a tech to check this out.

mandu:  I ruled out the speaker.  The buzz is present within the note, but isn't there when there is no signal.

ll81:  Thanks, that sounds like a good avenue to pursue.  It would definitely be nice if this was all just some gunk in the bias pot.

J M Fahey

Agree on the bias theory.
When I switched from bipolars to MOS I was initially turned off (pardon the pun) precisely for that, the horrible sounding crossover distortion, *much* worse than that on bipolars.
Besides, negative feedback works by lowering open loop gain as much as needed as to match the "weak" part of the cycle; but with "0" gain, no feedback in the world will do, and that's what happens when you use cheap switching Mosfets instead of proper audio ones (expensive).
I had to learn the hard way.


'I ruled out the speaker.  The buzz is present within the note, but isn't there when there is no signal'

Did you try with another speaker? All speakers are quite when there is no drive.