Cabinet resonance problem with Marantz MM9340 amplifier

@fiesta75 - Well, I can call them, but it is out of warranty, so it would be at my expense.  At 80 lbs, freight & insurance won't be cheap.  The thing is, I am sure they will say "It meets specs, nothing wrong with it".  There is nothing in the specifications that the cabinet is resonance free.  And, it does work perfectly, other than vibrating.

@erik_squires - I can definitely feel it, the cabinet vibrates like crazy.  It is a steel chassis, so internal magnetics can be the cause.   That was my thinking about using a thick plexiglass cover.   

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 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.

@mrdecibel That hum is caused by a DC offset in the power line.   It happens and only a volt or three will cause it.   There are several devices made to eliminate this, as the problem is worse with toroidal transformers.  If you Google "Blocking Mains DC Offset From Transformers", you should find lots of stuff to fix that.   There is a good article here: https://sound-au.com/articles/xfmr-dc.htm

If you scroll down a bit on that article, you will find a schematic so you can home brew it.   I did cost it out and found you can buy ready made DC Blockers for about the home brew price, assuming you bought parts from DigiKey or Newark Electronics. 

Which, BTW, reminds me I should try a DC Blocker to fix my problem!

There are a lot of options out there.  iFi makes one, although a bit tiny.  I bought this one from these guys - https://www.atlhifi.com/product-category/fully-assembled-devices/

If you do buy from them, I suggest you don't use US Mail, as it takes forever to arrive.   It is well made, rugged, and clearly will do the job.

There are other options from other manufacturers, too.   You are right, it won't hurt that is for sure.  

A final follow up on this resonance problem.  I covered the inside of the unit with DynaMat Extreme.  Turns out, it helped significantly!  I put two layers under the transformer and that seemed to help as well.   It reduced the resonance a bit but more importantly, it cleaned up the entire bass region.  The bass has much more "impact", you can feel it "hit" which before was just more of a muddled mess. I told my wife the bass now had a "visceral impact", to which she agreed  I also noticed that switching from an 18g generic power cord to a home brew 10g shielded cord made a very slight improvement in the muddiness as well.   

Thinking I am on a roll here with isolating the transformer, I added four more layers of DynaMat Extreme under the transformer, which required changing to a 5/16", 5 inch long bolt to secure it.  That is one inch longer than the factory bolt.   It should not matter with a toroidal transformer but I picked up a non magnetic stainless steel bolt and hardware.   I also added a bit more DynaMat Extreme in a few places that still had bare metal.  I could not add any to the bottom of the chassis since there wasn't room for it and the PCBAs.  I thought about putting on the outside bottom of the chassis but I didn't want to mess around with the DynaMat Extreme on the outside of the unit.   The improvement wasn't as dramatic this time as before, but the improvement is now to the point where I can happily live with the result.   The fact is, probably 90% of what I play has bass but not in the extreme bottom end that aggravates the resonance.  

It was then I remembered I had a Panega AC9 power cord I bought just for this amplifier, but removed it when I rearranged equipment in the rack.   So I installed that and it made a slight improvement over the generic 18g cable.  To be fair, the improvement was only in the deep, subterrain bass region.   The bass and upper bass wasn't affected as much as I could determine.   

Thanks to all who contributed ideas to my problem here!