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Unsolvable Woofer Pumping (Phono only)

I'm at a loss for trying to find the source of my "woofer pumping."  It's most noteworthy when playing something that is mostly/all treble, and the woofers of my Focal Aria 906s are going nuts (inaudibly, of course).  Turntable is a Debut Carbon with Ortofon 2M Blue.

Initially I was told it's an isolation problem, so I better isolated my TT, even put it right on the concrete floor to test!  Next I thought maybe a problem with the TT itself, so tried a couple others, no change.  So I figured it must be acoustic feedback, as with the TT stopped and stylus on a record, I could produce woofer pumping by tapping on certain parts of my stand...but it is also not this! I turned off my amplifier and recorded from the pre-out to a Tascam digital recorder and played that back afterwards and the pumping STILL happened! So I tried an Schiit Mani phono stage, no change in woofer pumping...I was sure it had to be my pre-amp...

So a local audiophile came over with a couple of pre-amps and we tried those.  The only time the problem went away was when the subsonic filter that one had was engaged.  So, I've ordered some Harrison Labs "FMODs" (20Hz high pass) to see if they will help.  If they do, I may order a KAB RF1 one day...but don't want to spend that much if I don't have to.
Any other ideas on what could cause this?!

tl;dr: Woofer pumping not caused by isolation, acoustic feedback, phono/preamp or a compliance issue...what's happening?!

branden_8091
mike_in_nc
1,147 posts
 
Record warp is another possibility.
branden_8091 OP
60 posts
 
@mike_in_nc Thanks, I tried several different records. Do you think a record clamp would help? I've been meaning to get one.
three_easy_payments's avatar
three_easy_payments
2,097 posts
 
Record warp was my first reaction as well.  Is the woofer pumping worst when the tone arm is near the outer edge of the record and best as it moves close to the dead wax?
millercarbon's avatar
millercarbon
12,531 posts
 
Well you've certainly taken all the right steps to narrow it down, so good on you. With a lot of the most likely culprits eliminated we are down to only a very few good options left. One being, its perfectly normal flutter, you're just not used to seeing it. Most people with entry level tables have been raised on CD and don't know some amount of this is normal. Not being there we can't know if this is it or not.

Moving on.... your table has a felt mat and let me guess, you're not using a record clamp. So records lay kinda sorta flat. But not really. Problem being, anything other than absolutely flat gets amplified by the time it gets to your speakers by about a hundred thousand times. 

The solution is a record clamp. Not weight, clamp. Specifically, you want one that's cupped on the bottom, or has something like an O-ring going around the outside. Anything to create a space between the record label and the center of the clamp. Then you also want a thin washer, just barely thicker than your mat, that fits over the spindle. The idea being the clamp will press the record down forcing the edges into the platter making the record lay absolutely flat.

A good clamp used like this will almost always be a nice improvement, which since it will also work on future better tables makes it a fine investment no matter what. 

Since you say its noteworthy mostly when just treble this implies to me you don't really notice it with bass-heavy music, which in turn implies to me that whatever flutter you're seeing is no more than normal music level or less. Which to me means perfectly normal, you're just not used to seeing it. But whatever. Try a clamp and see.


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