Teres number 26 upgrades - arm mounting and counterweight

For the build story before this, check out the ARCHIVE

Teres No 26 is built up on a plinth of 10mm glass and aluminium sandwich, and when these pictures were taken it was sitting on a base of air (inner tube) suspension with silicon damper tubes in each corner. It had been running in that form for about a year so it was time to try out some new ideas.

Each of these upgrades improved the sound in a similar way: definition, detail and soundstaging were greatly improved and putting them all together has raised the Teres into a completely different league.

The record clamp

The clamp was the first change to the deck since the major rebuild onto its glass/aluminium sandwich base. The clamp is made in three pieces: The main body and the handle, both turned from aluminium billett, and a cental shaft of brass which is threaded to screw down onto the record spindle. This has an "E-clip" at the bottom to hold base and handle together. The rubber o-rings are essential to stop me dropping it onto the records, but they also helped to stop the whole thing looking home-made.

The clamp is a heavy item, and normally only needs to be lightly screwed down onto the disk. With a bit of practice, I can add and remove the clamp with the platter spinning - to remove it, I just hold the centre handle and let the revolving platter unscrew it.

The effect of the clamp upgrade was to clarify detail and firm up the bass. It also seemed to reduce surface noise, A clamp is a very worthwhile mod and well worth the time spent to make one. It took me about a day and a half, far too long because I was making it up as I went along and I also spent ages polishing the thing up.

The VTA arm mount

The next upgrade was the really big job and the one I was hoping would provide a worthwhile improvement in the sound: replace the perspex arm mount with a machined aluminium one incorporating an accurate and repeatable vta mechanism to replace the crude Origin Live VTA adjuster. The arm I'm using is an RB250 with the Origin Live wiring and stub mods. This is a major improvement over the standard arm, but their vta adjuster is very basic and neither an accurate nor precision device.

So my new arm mount consists of a 6mm aluminium base plate screwed and epoxy-resin glued to the top glass. Onto this plate is screwed and dowelled a 3.5 inch aluminium cylinder for the arm base. The arm is mounted in a thick-wall aluminium tube machined from solid bar which as a slide fit into the the arm base. This gave me serious grief when I was finishing it as I found that aluminium-to-aluminium makes a bad bearing! I was aiming for a very close clearance <.001 per inch and ally expands enough when it's hot from machining to throw those measuements out. Worse, when lapping the stuff for a precise finish, the tiniest particle of metal will 'pick up' the surface and spoil the work. If I ever make another one, the inner cylinder will be brass or steel.

The arm is raised and lowered using a Starret Micrometer head, which I modified to fit a small ball race on the end of its shaft, This sits in a housing in the bottom of the arm base and gives me a robust, fine screw thread for vertical adjustment and a really accurate scale.

Machining the arm inner cylinder

1. Base plate of 6mm aluminium plate
2. Arm base. The holes are for fixing screws and the Micrometer
3. Starrett Micrometer head
4. 3mm aluminium plate links the arm and micrometer
5. Thick wall aluminium tube turned from solid fits the rega arm and slide-fits in the arm base.
6. Rega fixing nut reduced to a ring to clear the cylinder diameter
7. ball race with inner split bronze plain bearing to press-fit onto the micrometer
8. Gasket between arm base and base plate.
9. Fixing screws. I'd have liked nice cap heads, but couldn't find any long enough.
10. Vertical adjustment locking screw and plunger
11. Set up tool, an aluminium bar that's an accurate slide fit into the arm base. With a pip turned dead centre to set the arm-to-platter distance very accurately.

The new arm mount was a lot of work, but the improvement in sound was huge. Everything improved - detail, imaging and soundstaging to the greatest extent, but the most gratifying improvement was with HMV orchestral strings. I'd always found the later ASDs sounded a bit 'thin' on this turntable, but now they are simply detailed, airy and truly believable. What I think the upgrade does is to provide much more mass as a mount and also to mount the arm absolutely rigidly and accurately, which the Origin Live mount definitely didn't.

Here's the finished arm mount together with the next part of the upgrade, the arm counterweight.

The counterweight and fine adjuster

The last part of the upgrade was to make a new counterweight. There's two reasons for this: first, I wanted to follow the 'constrained layer' idea through to the weight and, secondly, I've always found the Origin live weight very hard to adjust. You just slide it up and down the arm and then do up the lock screw, but it tends to move around as you tighten the screw.

So the new weight is a fairly complex little piece. The weight itself is a sandwich: 3mm aluminium, 2mm lead, 3mm aluminium, 2mm lead,6mm brass. The brass is threaded for 4x 4BA and the other plates screwed to that. I screwed them all together first and machined them all up as one. This was a bit of a problem as the right turning speed goes from fast for a nice brass finish to dead slow backgear to avoid tearing the lead. Also, the OL Rega arm stud is an odd size - .526 inch- that doesn't seem to match any imperial or metric reamer, so I had to bore the inner. Again, I aimed for a very close slide fit, but that's difficult as the OL stub is just a rough turned finish which doesn't allow a close bearing fit. The weight has a lockscrew on the side.

At the back of the weight is the adjuster. An alloy ring is clamped to the arm stub and a captive adjusting screw is threaded into the back of the weight.

The setup procedure is this: slide the assembly onto the arm stub and roughly adjust the tracking weight. Lock the adjuster to the stub. You can then fine-adjust the tracking weight with the screw which moves the weight in repeatable amounts. Finally, fix the weight with the lock screw.

I tried to make the weight as heavy as I dared to keep the mass as close to the bearing as possible for the lowest polar moment.

Well, this part of the project was a surprise. After a bit of fettling to fit the adjuster, the fine adjustment for the weight worked perfectly and the change in sound is, frankly, astonishing.

The improvements are the most marked of any of the changes I've made to the deck and I found a really huge jump in the performance of the arm. The change is about the same as going from a standard Rega to an OL one. Huge improvement in imaging and soundstage, far better definition of instruments and a much "tighter" sound. For an empirical prototype this is a real lucky hit, and I'm guessing that it's something very complex to do with the way that this large mass receives energy from the arm and feeds it back. Anyway, I was hearing things on familiar records that I'd never noticed before, and the overall result is simply far more coherent and musical.

The counterweight was a real prototype and there's a few scratches and dings on it where I worked out how to hold and machine it as I went along. I also want to make another adjuster assembly as I think I can get at least 30 per cent off the mass wihout losing rigidity.

The turntable with new record clamp and arm mount but before fitting the counterweight

And finally.......... I'm thinking that the next change to the turntable will be a second motor. This will be a "dumb" DC motor, set to run the deck at about 25 rpm on its own. This will leave the Teres controller and the main motor with an easier job to do to lock and maintain speed. Oh, and maybe some cones at the bottom of the dampers and some more cones to hold the glass shelf up.

The second motor turned out a bad idea and the next upgrade is to ditch the equipment stand and go for an isolated, wall-mounted shelf.

jeff@audiomods.co.uk

Jeff Spall, Surrey UK - March 2003
reviewed Dec 2006

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The inner tube suspension was replaced with aluminium cones sitting on brass discs. The improvement in the sound was very noticable - better definition and less bass overhang. I'm surprised at this as I thought the sheer mass of the base was enough to ground any energy from the platter, but obviosly not.

The turntable now also sits on an isolation platform which also helps and the motor pod has a marble base with adjustable cone feet (to level it) sitting on Delrin discs. Each additional mod has improved the turntable even more.

These pictures show it running with version 3 of the rega arm

Jeff Spall, Surrey UK - March 2005

reviewed Dec 2006

The Teres story is continued in the "rega mods" page

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