Cabinet resonance problem with Marantz MM9340 amplifier

The MM9340 amp is a four channel Class D amplifier producing 275W into 8 Ohms and 400W into 4 Ohms. I am using it to drive four subwoofers, which it can drive with ease. The problem is, the cabinet resonates seriously and appears to be in beat with the music. It does not appear to be airborne related nor is it originating from the stand. The resonance appears to generate from within the amp itself. Has anyone experienced a similar problem such as this? Any suggested solutions?


Lots of good information here, thanks!

@4krowme I'll get to this tomorrow and check out the magnetic field.   I have a small compass I can use to check the field.  If the resonance is internally generated, removing the steel top plate might squish it.

@mrdecibel The Dynamat is designed for sound absorption damping resonance while Peel and Stick is roofing material and the datasheet states nothing about sound absorption.   Have you found that Peel & Stick is as effective as Dynamat?  Dynamat is certainly more expensive, that is for sure!

Yes, I have used Peel and Seal with excellent results. It bends around corners, is more pliable and easier to apply in many situations vs. Dynamat. But, I like Dynamat very much. There are other things available as well.

Yes, Dyna-mat has good things going for unless you need to remove it. Wow. And somehow little bits of it end around the shop. Vinyl damping sheets are lighter and less messy. It doesn't get stuck to the blades of your shears.

Yes, I do have some news.  I took the cover off the top, discovered it was 12 gauge steel, seriously heavy stuff.   A total of 12 non symmetrical screws held it in place, with a dab of some sort of Locktite like substance.   There was one screw in the middle of the cover, which is something you rarely see in high end equipment.  I suspect the Marantz engineers knew there was a resonance problem of some sort.

It is not magnetic.   A large 1+KVA torridal transformer is the power unit, which traditionally has well behaved external fields.  I took a large powerful magnet and ran it over the sides and top of the until while playing bass heavy music that got the resonances rolling.  But I could feel nothing from the magnet, which leads me to believe the resonance is mechanical and not magnetic.   Even over the transformer I felt nothing.

As for the chassis resonance and sound quality, it is marginally better with the cover off than on, but not to any real significance.   The resonance is worst on the sides, top, and back than on the front, probably because of the 1/8 inch thick faceplate.

I'll have to completely disassemble the unit to apply Peel and Stick material, but I also suspect this is treating the symptom, not the real problem.  I couple of layers under the torridal transformer is a good idea, to be sure.   But, I still remain clueless on the source of the problem.