Cabinet resonance problem with Marantz MM9340 amplifier

Just following up. Any news for us ?

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. 

More info - I played the same music through the Home Theater system.   It isn't the same quality of components as the Big Stereo.  The deep bass was there but the muddy sound of the bass was not.   Given this, I am assuming the problem is definitely in the Marantz amp.

I believe I discovered about 90% of the problem.   There is a 1KVA, medical grade, toroidal isolation transformer that isolates my system from the grid.   Thus any line leakage to neutrals or grounds is blocked, eliminating any hum induced from leakage currents into the signal grounds.   Apparently, there is some sort of oscillation problem between the Class D Marantz and the isolation transformer.  Clearly, it is triggered by the music load put on the power supply which draws power from the isolation transformer.  It is important to note the Marantz also has a toroidal transformer in its power supply as well.  The two 200W monoblock Conrad Johnson Premier Fives, which are tubes, are not affected in any way.   Neither is the solid state Parasound 2250 V2 amplifier at 275W per channel. The Parasound has a toroidal while the Premier Fives do not.   But the Marantz, at 275W per channel, simply goes nuts!  About 90% of the resonance problem disappeared when I plugged the Marantz directly into the wall.  

I say 90% since there is still an unacceptable amount of vibration in the Marantz chassis.   I ordered 4 square feet of DynaMat Extreme sound deadening material, so a thank-you to all who suggested it.  I am pretty certain this will eliminate most of that 10%.   It is certainly thick and heavy enough!

As for the 1KVA grid isolation toroidal, the longer-term plan is to add two 7.5 KVA isolation transformers, along with the existing 1KVA unit, and run them all from 240VAC, thus generating three independent separate derived supplies for the stereo.  It appears I need to move that time frame up!.   If I still have a problem, then it might be time to look at a new amplifier.  I am sure there are amplifiers with better sound quality out there that will drive subwoofers quite well.

Although I am on a dedicated circuit, I own a particular Hafler designed amp ( Smart Theater Systems TA 242 I acquired from a going out of business movie theater ) that uses a toroidal transformer. When connected through my system, the transformer " hums " when I am using my OTR microwave. From the chassis only, not through the speakers. Yes, I damped this chassis and isolated the transformer using a few layers of Peel and Seal, and the hum is much lower, but I can still hear it. I suppose I can stop the humming by teaching it the words.......not my joke....Enjoy !