Turntable motor vibration, noise and mounting methods

[JemHayward]

My Transcriptors Skeleton uses the Papst AC motor that was used on the earlier, AC Gyrodeks, with a custom made pulley which allows me to run the motor at lower RPM, which reduces vibration quite notably, and my (Meldano) TTPSU allows me to alter the phase as weel, so I'm pretty confident I have the motor producing the minimum possible vibration. However, the Skeleton is basically a glass box, so I can hear some motor noise if I connect the motor rigidly to the glass box. If I suspend the motor using O-rings, it's completely silent, but it's a PITA to get the belt running properly as the motor refuses to sit straight, and I do wonder if the noise is not coming out, is it being transmitted to the platter via the belt.

I want to build a motor mounting system that will allow me to control the motor position more precisely, and allow the levelling of the motor, and silent running - wondering what the best method would be.

Any ideas or opinions?

 

[martin clark]

Could you post a pic or two of how things are supposed to fit together..? might give a few clues to the audience as to what else might be possible in the space/constraints to hand

 

[JemHayward]

Hopefully these will work:
https://photos.app.goo.gl/P2KsaHgXg5H62oVK7
https://photos.app.goo.gl/eWDCx7fr7aDqsZz36

Basically I have three threaded pillars to work with, and I also have some springs and a plate from the original motor mounting. Mounting the Papst motor to the original plate didn't work well, though I have since made a new pulley, so it's something I could re-consider, but my question is, should a motor be rigidly or compliantly mounted?

 

[YNWOAN]

Wow, that’s pretty ‘cobbled together’ there! I would start by remaking the bit of board the motor mounts to so that each mounting point reaches to about 10/15mm from the pillars - then use quite tight silicone O-rings to attach.

To encourage you in your endeavours here is the solution I bodged/designed for the turntable I’m building now:

 

[JemHayward]

Hmm... yes, your bodging is on a different level to mine!

However, it looks like you have gone down the compliant mounting route in some way? Would be interested in knowing more about the thinking behind this beautiful object!

 

[YNWOAN]

I’ll see if I can find a picture of a disassembled prototype. Yes, there is some compliance. Imagine it is a bit like a speaker cone suspension. There are two circular leaf springs that are spaced apart (it’s a bit more complex than that but that is the gist). These enable the motor to be partially decoupled in one plane whilst much more rigid in others. The O-rings that you can see are actually not decoupling but acting as springs and are actively tensioning the construct against the adjustment mechanism - this part is quite rigid.

 

[YNWOAN]

Here is a pic of an earlier disassembled prototype:

 

[JemHayward]

Ah, yes I can see how it works... so it's essentially vertically compliant, but rotationally fixed - that makes sense, and the leaf springs mean that any vibration from the motor is going to have to travel a long way to get to the rest of the mount - clever stuff! Is there any further isolation between the motor housing and the platter/arm?

The Skeleton is very compliantly sprung, so I'd doubt if any vibration would get from the motor to the stylus, but rigid mounting just sends noise into the glass box itself, and so is audible.

 

[YNWOAN]

Yes, there is some further isolation, some in the way the motor ‘pod’ is mounted and some in the way the plinth is constructed. In this deck the motor is mounted to the same structure as the arm and platter and so motor isolation has to be very good - whilst maintaining alignment accuracy and stability. WHat you can’t see, as such, is the mass of the motor is matched to the compliance of the leaf spring design.

The reason you are having so much problem with alignment is because the motor support is both too compliant and, even more so, the compliance of those mounts is very uneven. If you add a bit of mass to the base of the motor (some large penny washers glued to it for example) and then do as I suggested in my earlier post, you should find motor noise is still well rejected and that the mortor is much more stable in alignment.

 

[JemHayward]

Adding mass to the motor itself is difficult as the 'body' of the motor is attached to the spindle and acts as a flywheel. I did think about making a 'pod' to enclose the motor, though having realised the motor noise is not really being radiated by the motor body as such, I didn't follow it up. I think I'll make a motor mounting plate from perspex, as you suggest, and then use matched o-rings to suspend it from the pillars. I think I'll also mount the motor so I can adjust the spindle to platter distance so I can get the belt tension optimised.

Do you have a good source for silicone o-rings in assorted sizes?

 

[snowman_al]

Problem I see is that there is nothing to counter the 'pull' of the platter belt. (In fact there are no good angles from the pillars at all.) You have to have at least one suspension directly in line with the centres of the motor and platter spindle or it will always be drooping in that direction due to forces involved.

Off the top of my head.

I think it would be simple enough with simple tools to make a 'double' plate. Outer to hold the mounting points and an inner to hold the motor. You can use rubber grommets to slightly isolate the outer plate and make it height adjustable with washers or what ever was used in the original? If you use 4 'o' rings you improve the pulley centres 'pull' and stability and reduce the rotational twist generally. Nuts and bolts for the mounting points and you could experiment with having the 'o' rings on top on the outer plate and underneath on the inner etc. You could use much more compliant 'o'' rings too and add further isolation even?

Just my 4 pence worth.
Alan

 

[YNWOAN]

Do you have a good source for silicone o-rings in assorted sizes?

I get mine from eBay - it’s great for this sort of thing.

 

[Craig B]

Hopefully these will work:
https://photos.app.goo.gl/P2KsaHgXg5H62oVK7
https://photos.app.goo.gl/eWDCx7fr7aDqsZz36

Basically I have three threaded pillars to work with, and I also have some springs and a plate from the original motor mounting. Mounting the Papst motor to the original plate didn't work well, though I have since made a new pulley, so it's something I could re-consider, but my question is, should a motor be rigidly or compliantly mounted?

Looking at the Transcriptors mounting arrangement, the original motor was compliantly mounted via springs in compression and included height and azimuth adjustment via the theaded rods (and possibly belt tension via slightly oversized holes in the plate). The user manual doesn't go into this, focusing instead on the Vestigial.

As the pillars are present, I'd have another go with the old plate, possibly adding (or substituting) upper and/or lower rubber bushings to (in place of) the springs.

 

[JemHayward]

I did try this originally (and I have all the bits), but the thin alloy plate seemed to amplify the motor vibration, but I've changed a few things since then (motor speed, pulley) so I could give this another go, and maybe then make up a less resonant plate (acrylic perhaps).

(Indeed setting up the Vestigial arm is an art in itself and very different from anything else, before, or since, possibly for very good reasons.)

 

[Craig B]

Interesting, the first thing that came to mind when I came across that picture of the motor assembly was that the top cover plate looked like it would have made a better motor 'board'. Perhaps the original plate would benefit from a layer of bitumen damping sheet being applied to one side?

Re the Vestigial, David Gammon certainly had a number of interesting and unique design ideas. I rate him up there with the likes of Arnold Sugden and Peter Walker.

 

[JemHayward]

Interesting, the first thing that came to mind when I came across that picture of the motor assembly was that the top cover plate looked like it would have made a better motor 'board'. Perhaps the original plate would benefit from a layer of bitumen damping sheet being applied to one side?

Re the Vestigial, David Gammon certainly had a number of interesting and unique design ideas. I rate him up there with the likes of Arnold Sugden and Peter Walker.

Indeed the cover plate is plastic with a brushed stainless finish, so inherently well damped, though I suspect was done that way for cost reasons. The original motor, on that plate wasn't silent, but was quiet enough not to be a bother, it's my fault for wanting to use my TTPSU and Papst motor, though I must say it does sound better than with the original motor, even in its current 'Heath Robinson' configuration.

My original intention was to replace the Vestigial with my Naim ARO, but my Skeleton is the Mk2 which doesn't have the large round armboard, just a slot to allow adjustment of the Vestigial, so fitting the ARO was going to involve irreversible surgery, and I wanted to be able to return the turntable to it's original configuration, so I decided to work out how the Vestigial worked, and give it a go. The cheap Ortofon cartridge it came with was pretty dire, so I dug out my old Decca London Gold, as this was a combination I had seen and heard in the '80s and though it sounded pretty good then, despite all the theoretical reasons why it shouldn't. The Vestigial was all about minimising vertical effective mass to allow very low tracking weights, so almost the polar opposite of the Decca. Once I'd worked out all the odd magnetic stuff, and adjusted the tracking 'weight' finally by ear, I sat down to listen, and it is really rather remarkable. My previous deck was LP12/ARO and the Skeleton doesn't sound the same as that, but it reproduces the fine details of my 'test' LPs, has deep and un-exaggerated bass, and seems to have no vices like rumble, wow and flutter etc, and of course I'm using a Decca again, which is rather magical.

 

[wylton]

How much does the Transcriptors Skeleton differ from the Hydraulic Reference? I ask because on my Michell, most of the noise comes from the mains cable coupling the motor to the chassis. On mine, I found that a different clamping arrangement for the cable and careful adjustment got rid of most of the noise (which was a mechanical hum in the background).

Incidentally, I use an Aro/Troika on mine, but the Michell has quite a large armboard, thus it is easy to accommodate the Naim arm.

 

[JemHayward]

It's a long time since I've looked at a Hydraulic - the platter and bearing are the same, and I think the motor is the same, but I can't picture the motor mounting method in the Hydraulic. The 'plinth' is very different, as its a three legged affair with very compliant sprung feet and is very 'floaty' The Whole thing then lives in a glass box, and the motor mounting pillars are attached to the glass floor. The box isn't particularly resonant, but the glass can 'rattle' in the alloy corner posts, so I've added in some packing to secure the glass. The glass box sits on three non-compliant feet. The power cable for the motor goes through a hole in the glass floor with a grommet. The signal cables just exit through the gap between glass floor and back wall, which is a bit odd... I could easily experiment with routing the mains cable.