Rumbly feedback

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).

@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?

If you have your turntable atop a tower-style rack (like many of us), freestanding, then it’s actually free to sway when energized - which can easily cause or contribute to that woofer flapping. If you think about it, your turntable is literally cantilevered up into space - a really bad spot for it to be in. The taller the tower, the worse it gets. Adding mass up top can just make the problem WORSE, actually.

To brace the rack can be as simple as backing it firmly against a wall, with a little foam or rubber pad wedged in between there to interface. You’re basically clamping it between wall & floor, and making it stop acting as a cantilever. This can help your flapping problem (which can be deadly to woofers - you seriously don’t want this), but may not address your audible rumble / hum feedbacks - for that you’d want to additionally put your turntable & arm pod atop squishy feet or springs.

If you have concrete flooring, then your flapping problem possibly has another cause. But usually life with such floors is "easy mode" for vinyl playback.

If you struggle to find any mitigating solutions, you can always get a KAB rumble filter - it’s pretty good, and VERY effective, but I found it to not be quite "100% audibly transparent", though it is advertised as such. And it's aways better to address the actual mechanical & structural problems, rather than just slapping an electronic filter on as band-aid. 

Yes, the separate arm pod is not a variable I’ve dealt with before. That itself might well be the culprit here. Hopefully so, becasue if not, good luck - the hum / rumble types of feedback were the most obnoxious of all gremlins I’ve dealt with. Many times I thought it was finally put to rest, only to come back again. The tables I had the problem with - VPIs - were large enough to not lend themselves easily to spring platforms that could perfectly break the feedback.

The rack bracing advice was aimed specifically at subsonic feeback (non-audible) which causes woofer flapping. I say non-audible, but it can cause chuffing or doppler artifacts because the amplitude is so large. But it won’t transmit through a wall at the necessary amplitudes. However, the wall could contribute to the audible feedback frequencies.