Turntable Resonance Fix

Then the noise is extrinsic to the turntable and a correctly isolated turntable will get rid of it completely.

Which reminds me, in 1978 I lived in an apartment building in down town Miami on the top floor. The Mechanicals were on the roof and I could hear all of it on a Micro Seiki turntable I had for a short period. I realized it was not the turntable because the noise would change and sometimes disappear. Finally the light turned on. I pulled the carpet away at its edge and listened to the floor with a stethoscope. Sure enough the noise was coming from the building and the special feet on the bottom of the turntable were doing not much of anything. I had an LP 12 several months later which solved that problem. 

@mijostyn, I’m not sure the problem is extrinsic. I conducted these measurements with either no speaker output or extremely low speaker output. I considered noise from mechanicals as a potential source of vibration early on in this process, so I made sure the air conditioning system wasn’t running nor were either of the refrigerator compressors in the nearby bar nor the water heater in the adjacent utility room. This doesn’t exclude sub-sonic vibrations (with associated harmonics that could creep into the audible range) occurring from outside the house, so I confirmed the integrity of the system’s isolated state by keeping the motor drive off, placing the needle on the non-turning record and observing the results on the analyzer. For an extra degree of thoroughness, I took readings both with the drive belt taken off the platter as well as installed normally. No measurable noise occurred in either of these configurations with the drive motor off. To eliminate the possibility that the drive motor itself was the culprit, I kept the motor and belt off while hand-turning the platter to a slightly faster rate than normal and allowed its inertia to slowly dissipate. The original resonance problem was observed using this method. 

If I understand you correctly, your experiments suggest the resonance comes from the bearing but the successful addition of the springs leads you to propose that to achieve its original magnitude the resonant energy must be traveling into the shelf then back up into the turntable/tonearm. Yes?

Usually you don’t want to spring load something that is already spring loaded. You probably don’t need the second set of springs under the shelf.

@lewm This is correct. However, it still seems implausible that an amplification of that magnitude is possible through the bearing/plinth/feet/shelf and back up to the tonearm. I'm trying to account for other possibilities, but I'm just not sure where to look. Is it possible that the tonearm itself is contributing to the resonance? And furthermore, that the tonearm's resonance is somehow being amplified as it travels down to the shelf and back? 

I'm considering reversing the configuration, measuring again, and then reconfiguring with the Nobsound springs and re-measuring just to be sure. 

Good advice re: spring-loading the shelf that holds an already spring-loaded turntable. Those will be coming out.