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Oak Unipivot Tonearm - "The Stick"

Whilst experimenting with my 401 I've become increasingly impressed with its performance. I was using my Ekos arm but I was reading more and more on the net about the superior musical performance of unipivot arms, such as the Hadcocks. I had been advised that the classic partner for the 401 is a Rega RB250-derivative gimbal arm, so the unipivot idea seemed an interesting alternative.

James D on the WD forum championed simple DIY unipivots, such as the Altmann arm. Looking at the photographs of the Altmann arm I found it hard to believe that something with such a rustic appearance could make a decent sound, let alone be "hi fi". But as what amounts to little more than a stick balanced on a knitting needle point is such a simple and cheap device, how could I not give it a go?

Design

Just as I was starting to seriously think about making a DIY unipivot arm Richard Higgins posted a photo of his own stunning unipivot arm on the WD forum. Richard subsequently brought the arm to Eggfest 3 mounted on his Lenco GL75 idler drive deck. Here was evidence that a DIY unipivot could perform at a high level. Talking to Richard he felt that his arm retrieved more information from a record than his Ekos…

Chatting with both Richard and James was really helpful. Thinking about my own arm I knew I didn’t have Richard’s skill and turning a wand from my nice piece of oak would be disappointing. So I decided to keep it as simple as possible. My dad has a table saw which makes cutting wood accurately very simple. So making the arm section rectangular was an easy choice.

I had a think about the stresses that might be induced in the arm and quickly came to the conclusion that the physical constraints of constructing the arm should mean it had more than sufficient strength. Believing that less mass is better (up to a certain point) as there would be less inertia, the arm ended up fairly arbitrarily at 11mm wide x 13mm deep.

Lengthwise, I intended to use the arm with a Denon 103 cartridge. James recommended a 12" arm as a good match for the 103 due to the mass of the arm, so after a quick look at 12" arms in the tonearm database at Vinyl Engine I chose the dims for the SME 3012 mk2 fairly randomly. Realising that I was limited in accuracy by using wood and reasonable crude tools I didn’t get too hung up on geometry, choosing an effective length of 305mm and offset of 17degrees.

I was more concerned with the interface of the headshell and the wand, and friction at the pivot. The simplest solution for the headshell seemed to be to cut it from the same piece of wood as the wand.

I struggled a little to come up with a pivot arrangement. For the point I settled on a 2.75mm diameter knitting needle mounted in a piece of turned oak with a screw as a clamp. Not sure why 2.75mm, it just seemed a decent place to start. I also bought 2mm diameter knitting needles but they seemed rather more flexible than the 2.75mm. Whether this would make a difference is another matter entirely…

For the mating pivot surface I drilled a 2mm diameter hole to the middle of the arm using a pillar drill, then overdrilled to the majority of the hole depth to make sure the needle wouldn’t foul the arm.

I was a little more uncertain of the counterweight arrangement. I wanted to keep it as close to the pivot point as possible, both horizontally and vertically, again to minimise inertia. So I sandwiched 4 layers of lead glued together with pva between oak cheeks. The lead makes the size smaller than if only oak was used, thus minimising the lever arm, and doesn’t reflect vibrations back up the arm in he same way steel, for example, might.

Construction

Starting with a nice piece of oak.

First cut: a thickness of 13mm, the depth of the arm.

After marking out the "headshell" the wand is cut. Two holes for the cartridge mounting screws were drilled before the cut to form the headshell was made.

It's easier to see in a close-up.

Some alternative views.

The scrap wood is removed, unfortunately...

...too much to the wand.

Now the counterweight.

Gluing up the counterweight.

Turning the remaining piece of the oak for the base. The rest is for another project.

The finished base with knitting needle point and screw for a clamp.

Setting up the arm

Attaching the counterweight to the arm was a lash-up. A length of solid core pvc screened wire was wrapped around the two and the ends twisted together to tension the wire. This was decidedly rough and ready, but meant I could quickly have a listen to the arm…

The final stage was to attach 4 lengths of tonearm wire from Satcure and solder some phono plugs. Cotton thread tied around the arm attached the wires to the underside of the arm.

Setting up the arm was simple but a little fiddly owing to the minimalist nature of the arm. Setting tracking weight was done by trial and error and checked with a digital scale bought from ebay. These scales are remarkably inexpensive and obviously intended for more "relaxing" pastimes. They claim to be non-magnetic but I found there was a small pull so I used a piece of folded card to lift the cartridge away from the magnetic effect of the scales. The lean of the arm due to the offset cartridge was corrected by moving the counterweight laterally.

Listening Impressions

So how did it perform? Rather well, I was quite surprised. It seemed to have everything the Ekos has, but at a fraction of the cost. Good detail and focus, big bass, nice tunes. It was certainly a smidge more musical, less mechanical perhaps. I was very pleased, however, if I did have a criticism it was that the bass was perhaps not quite as controlled as the Ekos. But nevertheless, for less than £15 the performance was remarkable.

James advised that the counterweight fixing simply wasn’t rigid enough and I couldn’t help but agree. So I cut two strips of oak and used them with two 3.5mm diameter 75mm long machine screws from B&Q to clamp the wand and counterweight together. Dead simple, if a little "industrial".

Sure enough, this tightened up the bass. But what surprised me was that the whole frequency range tightened up too, from top to bottom. The whole sound was more focussed and detailed, instruments were clearer and cleaner. A significant improvement.

But the biggest improvement came when I mounted the 401 in a high mass birch plinth. I had a theory that as I have a concrete floor and the plinth was coupled to the concrete with spikes and ball bearings that the plinth mass would be fairly irrelevant. How wrong I was. With the big plinth the sound improvement was dramatic. Instruments sounded incredibly realistic, and ones I had never heard before appeared from nowhere. Yes, the plinth was instrumental in this, but the arm (and the 103!) was good enough to resolve the detail. This combination is by far the best I have used.

Improvements

The first thing I must do is repeatedly treat the arm with some finishing oil. This fills the pores in the wood with the finishing oil and damps resonances which apparently improves the performance of the arm.

If I was constructing the arm again then I would make a few amendments. First of all I would "measure twice and cut once" when I was cutting the headshell, then I wouldn’t remove more material in the wand than I had intended! This should make the headshell more rigid.

The second mod would be to the pivot. I don’t have facilities to fabricate anything fancy in metal so anything I use would have to be available ready-to-go. I think I would try synthetic sapphires with a stainless steel vee point jewel pivot.

The third mod would be a closer look at the geometry of the arm, and possibly use something a little more tailored.

Finally, and more of a cosmetic issue, I would cut a rebate on the underside of the arm so the wires could be glued and would be hidden. With the vta of the arm set low to suit the 103 the bottom of the arm very nearly fouls the record so there's not too much margin here.

But whether I will build another arm like this is unlikely as I'm tempted by a DIY Schroeder type arm. Reputedly better than the "stick" type arm...

The write-up for the stick first appeared on Steve Cresswell's excellent website.

 

 

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