At first I wanted to name this plinth design the "Test Mule". But a suggestion from a friend sounds better. From here on I name this plinth style "the mule". I like that this name echos an Azimov character from the Foundation series of novels. The Mule.
The Mule Plinth
Having already taken dimensional measurements on the bearing spindle shaft, the bushing bore and a visual exam of the thrust, we know that bearing wear is minimal, except that the thrust pad is in need of renewal. For this part of the project, the deck has been re-assembled. The bearing, spindle and bushing has been gently cleaned and then reassembled with fresh 20 wt turbine oil for lube.
The next step is to make a test mule to study and measure the performance of this machine. Specifically, the goal is to get points of reference for rumble and wow/flutter performance. This data will help to determine the electrical/mechanical condition of the motor unit. If there is significant wow/flutter, it will point toward problems with the controller electronics. If significant rumble is reported, it will point to excessive wear at the platter bearing. But I've already taken early listening sessions with this deck and know that the unit reproduces music quite well and with no audible evidence of low frequency rumble.
Like all my plinth designs, it starts out as a sketch, then, once I have a rough outline of the shape, I refine the shape in my 3D cad software of choice, Rhino3D.
This "test mule" plinth is intentionally light. The idea is to produce a method of supporting the motor unit and tonearm together in a single structure. But this structure needs to be one that does not depend on high mass to dampen whatever mechanical vibrations the motor unit may produce. Rather, I want to be able to readily measure for rumble and wow/flutter without having to consider that the plinth may be masking some of the mechanical noise. However the design does provide a hollow chamber in each of the 4 vertical cylindrical supports to hold a volume of #9 lead shot as a means to dampen the plinth at its support point. This idea is to help "plant" each cone foot solidly onto the support surface.
The plinth needs to be a rigid structure that can allow this player the ability to play music and sound good. The better the condition of the motor unit, the better it will sound.
3d cad quick shade. In this photo self tapping machine screw inserts will tap into the wood of the plinth, then accept the 1/4-20 threads of the allen bolt, which screws into and holds the Stillpoint cones. The advantage to this arrangement is that there is a slight range of vertical adjustment at each cone which can be used to level and stabilize the deck.
Earlier this year I purchased a used set of Stillpoints cone feet for this project. These cones are well regarded by many audiophiles in the know. However the manufacturer has replaced this model with another that claims to be an improvement. The advantage for me is that I can now purchase this old technology at 2nd hand rates, while more affluent audiophiles upgrade their equipment with the newer Stillpoint cones that are significantly more expensive. Progress is good.
Next step................... build the actual thing.
The Stillpoint cones connect via 1/4-20 threaded studs to the thread inserts that have been installed within the Test Mule plinth. This method allows height adjustment at each of the 4 cone footers for leveling.
Setup and ready to spin records
It is rough cut and unfinished. Some sanding. It has some dark brown stain. But rough.
After finish pics:
Finish coating: highly textured black enamel. Prior to paint, three successive coats of sanding sealer. Sanded between coats. Then 3 coats of the textured black with sanding between those coats.
But next I changed the pile-up method on top of the Minus-K. The TD124 slate plinth slabs were removed and replaced with an 18 x 18 x 3 inch granite inspection plate. On top of the granite is a Neuance platform. The Neuance platform will interface with the ceramic cone feet under the Mule plinth and should serve to dampen vibes coming off the sp10.
As noted in my review of the Minus-K isolation platform, there is a minimum(90 lbs) and a maximum (155 lbs) weight load requirement. That is why when I removed the TD124 slate plinths I had to replace those with something of similar weight. In this case the Granite is actually heavier than the Thorens' slate. The granite by itself weighs 108 lbs. The Mule with SP10mkII weighs 31 lbs. combined weight is then 139 lbs standing on the Minus-K. And the Minus-K happily floats the sp10 in its Mule.
Some brief, subjective listening observations on how this setup sounds:
Fast. Very musical. Dynamic. Great bass. A very good sense of energetic drive. Better this way than when I had it in the earlier plinth. Detailed but not really analytical sounding. Nice.