Building the AAP12, Thorens TD-150 Modification Part Three

Herein we continue the tale of two turntables as introduced in Part One. If you haven’t seen the series introduction, you may wish to do so now. In Part Two we built a TD-150 in a Maple wood plinth using mostly stock components. And now, finally, it’s time to build our LP12 clone, hereafter the AAP12, and mod it to the max.

As before, stripping the donor deck was the first port of call. This deck was in considerably better condition with the original wood plinth. I’m not a fan of the plinth. I think it’s unnecessarily bulky and I’m not keen on the dust cover design. I also don’t care for the way the top plate sits proud of the top plinth frame. We can certainly do better.

During disassembly I was interested to note the design of the bearing. Instead of a captive ball as on the majority of TD-150 MK II models, this bearing ends in a machined conical point. I mentioned this to mark of Vinyl Passion, who informed me that this superior bearing was used in the original MK I model. After a bit of digging I found a few other MK II owners with the same bearing, which makes me wonder whether Thorens briefly carried the design over, or used up some surplus component stock during the initial run of the updated models.

Further research suggests that this design was in fact adopted after the captive ball design, and was introduced for the TD-125 MK II and TD-160 to enable the bearing shaft and housing to be shared between models. Incidentally it is probably no coincidence that Linn’s single-point LP12 bearing is not dissimilar in design, albeit larger.

A new plinth was then assembled from 23 mm thick Cherry. As with the Maple plinth in part two, the corners are mitred and glued and the braces are glued in place, with a 10 mm rebate for the baseboard and the top plate inset by 3 mm. The thickness of the board necessitated some rebated battens be machined to support the top plate while making the plinth large enough to hold a standard-sized hinged dust cover. This was a difference of a Meer two millimetres, but I would try to use thicker wood in future.

As before, the outward-facing edges of the triangular braces were machined with clearance slots for the arm, motor and power switch cables using a hole saw mounted in a pillar drill, which enables a half circle to be cleanly cut without using the centre guide bit.

The baseboard is identical to that of the Maple plinth, made from a laminated plywood shower panel at a fraction over 10 mm in thickness. 24 mm Recesses allowed the feet to knock in tightly, and 24 mm access holes were drilled for the suspension bolts and chassis bolts.

The LP12 has a cross member spanning the depth of the plinth between the two top plate mounting blocks. This is used to hold the power supply and wiring, but I wanted to take the design one step further. A T-shaped structure was constructed from aluminium and bolted into place, spanning the depth of the plinth and continuing across to its right-hand side.

A common problem with the LP12 top plate is that it has a tendency to sag in the middle at the right-hand side, closest to the arm board due to the weight of the hanging sub-chassis. The Vinyl Passion top plates have a mounting hole in this position which is used to mount their ‘Apollo’ support between the top plate and base. This has the added effect of helping to transfer any vibrational energy from the top plate, into the base and out via the feet. This requires a hole be drilled in the standard TD-150 sub-chassis, as this supporting pillar must pass through the sub-chassis but still allow it to move freely to achieve the desired bounce.

VP’s solution is a high tech carbon fibre pillar (the Apollo). Mine was a little more simplified. I bolted a 70 mm M5 bolt tightly to the top plate, and bolted this securely to the T structure. A hole was then drilled in the baseboard to accommodate this bolt, and a nut and washer used beneath to pull both the top plate and T structure towards the baseboard once installed.

During setup I experienced quite a bit of the dreaded LP12 top plate rattle. This is quite a common issue where the top plate will rattle against the plinth when tapped at the corners, usually near the motor or power switch. This usually requires ‘dressing’ of the top plate, which involves bending it into shape to form an arch over the two central mounting blocks. The chassis bolts then pull the top plate down flat, and in theory stop it rattling at the corners.

More recently Linn has fitted a bolt to the underside of their top plate in the motor corner, which bolts through one of the triangular braces in the plinth and is tightened just enough to eliminate the rattle. This is an area where a corner has been cut and a quick fix applied to overcome a problem caused by poor design. That might help the motor corner, but what about rattling around the switch or on the corners close to the arm board?

Try as I might, I couldn’t ‘dress’ the top plate to a satisfactory level. I therefore decided to improve the mounting system by drilling a pair of mounting holes on the left-side, through which M5 bolts passed into the triangular braces beneath. This cured the problem immediately, and didn’t affect the aesthetics in a negative way. Most aftermarket LP12 top plates have a far better method of mounting than the original. I question why the original engineers considered that two bolts (and a pair of self-tapping screws) in the centre of a flexible metal plate was sufficient, but it is a problem that is easily solved. I also ditched those self-tapping screws in favour of another pair of bolts which pass straight through the mounting blocks and bolt tightly against the T structure, adding further strength to the top plate mounting.

The arm board was cut from a piece of Cherry and drilled for a Rega arm. It is mounted to the sub-chassis using M3 bolts rather than the usual small self-tapping screws. I favour the rigidity of this approach, though it is claimed that as the self-tapping screws are a ‘lossy’ form of mounting, they help prevent any ringing of the sub-chassis from impeding the performance of the arm. In theory the rubber suspension grommets, bearing dampening and indeed the arm board itself will go some way to dampen the sub-chassis, and bolting the arm board to the sub-chassis becomes far less of a compromise if it is a compromise at all. A TD-150 actually has five predrilled holes for arm board mounting – 4 along the rebate, one right centre – of which three are commonly used. I found that the mounts at either end of the rebate and the two holes central of the board gave the most secure fit while maintaining a consistent aesthetic.

The suspension springs are another product from Vinyl Passion. They are Linn spec springs, but with an anti-harmonic coating used in aircraft engines to help minimise vibration. This cures the 10kHz resonance peak in the springs, which should go some way to removing some of the typical LP12 tonal colour. The spring set, priced at £55, is supplied with current specification rubber grommets and the necessary M5 flanged bolts, nuts and washers.

The standard TD-150 motor was lubricated with Mobil DTE medium, which is an ISO VG68 turbine oil and is an ideal substitute for the original Thorens oil. A few drops were applied at the top and bottom of the motor shaft and allowed to work their way in. The SRM/Tech Thrust Bearing was then installed. This is a tiny machined part which is adhered to the underside of the motor using the supplied superglue. Once in place, a 4 mm ball bearing is lubricated with Moly grease (also supplied) and dropped into place and held by a grub screw.

Typically the shafts of these motors ride on the bearing sleeve, rather than on a proper bearing. Some years ago it was discovered that supporting the shaft with a thrust plate beneath would significantly lessen vibration and motor noise, resulting in a smoother-running motor. Tightening the grub screw achieves this. Once tightened such that the shaft raises slightly, the opposite end of the motor shaft rides on the ball bearing and thus is properly supported. The difference in the free movement of the motor is quite surprising, and the improvement in performance is significant. This is a very elegant and well engineered mod.

So too are SRM’s damping products. Made from a thick rubber polymer, they fit tightly around the given components. The MVA (motor vibration absorber) was perfectly sized to fit snugly around the motor casing with clearance for the cable exit. The BDR (bearing dampening rings) are supplied as a set of two. I installed one at either end of the housing, lining them up approximately with the locations of the two inner bushings. Finally the platter damping ring (PDR) fits tightly around the outer rim of the platter and dampens it to a surprising degree. This enables the use of a felt mat in place of the standard rubber mat, without fear of a ringing platter imparting any character of its own.

The motor was wired to Vinyl Passion’s Wave power supply. This is a neat PCB containing a similar circuit to that supplied as standard with a TD-150, including phase and spark suppression capacitors and a voltage dropping resistor. There is no fancy circuitry here and it could certainly be argued that at £55 the Wave is quite expensive for what it is. However it does make for a very neat installation with screw terminal connections and adhesive mounting standoffs, and it is also safety fused which a standard TD-150 circuit is not.

I am looking to implement a more sophisticated power supply, which will hopefully be coming in the near future. This upgrade will also incorporate electronic speed switching, as this is currently achieved by manually moving the belt across the pulley.

With the deck mostly assembled I mounted up the arm. The arm in question is a Rega RB202, extensively modified by Johnnie at Audio Origami. What Johnnie doesn’t know about tonearm building, if there is anything, isn’t worth knowing. The man is a fountain of knowledge which he is always happy to impart, and his work and attention to detail are top notch.

I opted for an RB202 with a Cardas 33AWG OFC internal rewire and a super OFC external cable. When I asked Johnnie about the use of the popular Cardas incognito wire, he pointed out that given the number of connections made within the supporting electronics, a single connection adjoining the internal wire to the external is of no significance. And besides, the internal cable is comprised of hair-thin stranded cable which is not only higher in resistance than a thicker wire, but is also considerably more fragile, more susceptible to RF interference hum than a properly shielded external lead, and the wire can act as a microphone, transmitting airborne feedback into the arm. The SOFC external cable has an RF shield and non microphonic layer and is coated in a soft, flexible silicon cover which makes it particularly good for use in a suspended turntable.

Also specified were upgraded horizontal and vertical bearings. These precision bearings are hand selected from a batch of a hundred to fit the individual bearing shafts and bearing housing. The modifications were topped off with a new stainless steel rear weight and end stub, and a foam fill to help dampen the 600Hz ringing in the arm tube.

The arm was bolted to the arm board using 30 mm M3 Allen bolts, which will facilitate the addition of VTA shims should they ever become necessary. I personally believe that Linn’s use of a P clip has more to do with using the arm cable to prevent lateral movement of the sub-chassis than it does to stop the cable interfering with the ‘bounce’ of the suspension. I therefore drilled a 40 mm hole beneath the arm and dropped the cable straight through.

With the deck assembled I balanced the suspension. I found that when using a thick but lightweight felt mat, the platter barely had sufficient mass to compress the springs given the length of the bolts. A Linn platter is a kilogram heavier so this comes as no surprise. Fortunately bonding a plate of stainless steel to the top of the Thorens platter will bring it to almost exactly the same mass as a Linn platter and is a mod I will make soon. In the meantime it is still possible to achieve a perfectly good bounce when the weight of a record is in place.

This concludes Part Three. In Part Four I’ll cover some general notes accumulated over the build of both turntables and sum up with some listening impressions. It is also highly likely that we’ll perform future modifications to the AAP12 in future postings, links to which can be found in the Series Overview.