Rumbly feedback

I have used tonearm pods and each was mounted on three spikes to minimise transmission of vibration. Though when you mention it happens with the Wand but not the Brinkmann, it makes me wonder if it isn't simply the fault of the pod/pedestal, but speaker vibrations affecting the unipivot arm, allowing for the pod/pedestal to be a little less solid than a built in arm..

I had amazing problems in my small-room vinyl rig (springy floor). Rumble-type feedback (audible bass frequencies) was specific to VPI tables with VPI tonerams. The 3D material arms were MUCH worse than the metal arms. This was the case for the gimbal design as well as unipivots. I think the plastic-y materials are likely to blame for being energized in the "rumbly" range, and a carbon fiber tube may be a similar story there. Everyone was concerned with higher frequency ringing on the metal arms, but this is a MUCH less significant issue than problems in low frequencies, as you’re probably finding out. Often it sounded similar to a ground hum during playback. And at its worst, it can even reach a "runaway" level (ugh).

Of course you’ll get responses from guys who never play loud, or have concrete floors, and will never encounter these kinds of issues.

My conclusion: the main culprit for rumble-type feedback is the tonearm’s design & materials, but VPI’s plinth designs certainly didn’t help things. Their HW-40 feet were the only type of VPI feet that were moderately effective at mitigating rumble feedback. These use a couple layers of very spongy foam. That’s what you’d need for buffering against LF feedback - something very spongy / lossy. "Hard" isolation like spikes, stillpoints etc likely won’t help. "Constrained layer damping" won’t touch this - these types of isolation are usually completely ineffective in bass frequecnies, and the makers never point out HOW LITTLE of the unwanted energy is actually converted to heat. Proper isolation is about successful management & redirection of energy, rather than its idealized conversion into heat.

Probably a better way to go is spring isolation. The Townshend solutions are excellent BUT it can be tricky to find the right fit for your table: 1. Their isolation platforms are a bit too small for turntables with larger footproints. 2. Their podiums can fit larger tables but have a HUGE footpring that requires a LOT of real estate for your top rack shelf. If you can DIY and tune properly for mass, this could be very cheap and effective. In my "problematic" room I ended up with a SOTA Cosmos (buily-in spring suspension, tuned to a very low frequency) and a cheap rack with bracing support (expensive CMS rack didn’t help worth a crap, and in fact may have been worse).

If you have subsonic type feedback (woofer flapping) that is likely a different cause, with a different solution (rack bracing).

@dogberry I think that's exactly it. As I said, when I press down onto the pedistal with just a little force the rumble goes away. It's like if I brace it to the HRS platform a little better it solves the problem. I wonder if a heavier pedestal would solve the problem. 

@mulveling I think the problem is more the last thing you mention, where the woofers are sort of flapping in a pattern and it's audible. What do you mean by rack bracing?