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hobbyists's views for hobbyists
The Unipivot Experiment - 1995

 


Building a Unipivot Tonearm

Using balsa wood makes it possible to make a long arm for playing 16" records. 16" discs (40 cm) were being used by radio stations to record their broadcasts and programs. In the nineteen thirties and forties these large diameter discs were used by record companies to make master recordings - especially of pop and jazz - from which the 10" discs were cut. In the nineteen fifties and sixties the 16"/33.3 RPM record format was used to record radio broadcasts and send these to other radio stations. Famous are the discs of the transciption service that were sent to stations of the army, typically to Germany. These discs are collector's items.

The combination of balsa wood and strips of aluminum is not just for a 16 inch arm, but also suitable for 10 and 12 inch arms. In fact you can make the arm longer if your turntable has room for it, as long as you do not exceed the recommended mass for the cartridge that you are going to use.

The choice of wood can be important to get the right sound. Balsa has warmth and a good mid band but less refined highs. And the mass is low, so you will have to add some. If carefully combined with the aluminum you can create a natural, harmonic sound balance. You will get speed and of course a very good transient response. Other kinds of wood have to be experimented with as the outcome may not be exactly what you want and also may change the sound balance of the audio spectrum, hence acoustic instruments can loose their specific (natural) character.

Materials are judged by several specifications:
* the density,
* the stiffness,
* the propagation (speed) of specific frequencies,
* the damping of frequencies (vibrations).

As said: balsa and aluminum or balsa combined with another hard sort of wood like ebony can work well. The harder material (ebony, aluminum) gives stiffness to the balsa. The sound will be more precise. The percentages have to be determined by you by making prototypes and by carefully listening to the music and measuring the signal.

The use of solely balsa is not advised. Nor is it recommended to use just ebony or another kind of hard wood as it will decrease the amplitude of the low frequencies on which the mid and high frequencies are built. Ebony is very hard and may not give a well balanced sound. Using ebony for the body of a cartridge is questionable and can only be empirically determined and may also be dependent on the arm.
The same goes for carbon. It has speed, but the arm tube needs a certain mass to deliver a well balanced sound. Of course always in relation to the compliance of the cartridge.

Click on the drawing for more details.

Someone asked me why arms generally are of the nine inch type. The fact that a standard length for the tonearm was adopted was because of the size of the largest record which had to be read, so the arm's length was about 9" (22.5 cm).
The 12" arm was necessary for the 16" (40 cm) discs. If the arms were much longer, that would mean that the turntable would take up too much space. And the arm would have a too high mass.

If you make an arm very long, longer than 12", the lateral tracking error will be less. In that case you can take lighter materials as long as the arm has sufficient stiffness. And if you have room enough to accommodate a long arm, you can make the arm much longer as long as you make the arm not too heavy for the cartridge you are going to use.
The weight (mass) of the arm should be suitable for a cartridge with a specific compliance. A Denon DL 103 can have a heavier arm. A Shure V15VMR5 will need a light arm which is of course shorter.
So you can take a long, relatively heavy arm if the cartridge has a low compliance. The recommended tracking weight is an indication of the compliance. Best check the technical details supplied by the cartridge manufacturer.
Why do high end manufacturers design short tonearms? Why is the SME V -for instance- a nine inch arm? The SME V is a short arm because the geometry of the arm is well calculated to obtain minimal error (SME have experience since the nineteen fifties) and there is also a relation of the material used, the best thickness of the wall of the arm tube and the maximum mass.
If the wall of the arm tube would be thinner, than a longer arm could have been made. But the arm tube would not be as strong and would not be free from coloration. The arm would not be a high end design, sound wise.

You can make the basic unipivot contraption yourself, or find one on the Internet. The unipivot I used is from a Keith Monks tonearm which was not complete and most difficult to operate in its original configuration. The arm base has four compartments. Normally they contain quicksilver (mercury) to make contact with the four pins of the original Keith Monks arm wand. In this case the compartments are empty as you can see at left.

"This balsa arm gives the best transient I ever heard", said a friend (who is starting his own audio business) when listening to the arm that I made of balsa and aluminum. There is no overshoot, the sound is controlled and really fast at the same time. This arm does not have the looks of a sophisticated, commercial arm. The quality was achieved by listening and eventually adding more short aluminum strips to attain the necessary mass, while always checking and listening before adding another piece.

I found that not only the distribution of the mass of the total arm was important, but also that the pressure on the "spike" of the pivot, should be of a specific weight. If that weight is too heavy or not sufficient, the arm rocks or has a disturbing resonance. It is somewhat similar to the grounding theory of an amplifier or CD player as the Goldmund engineers proposed. It is all about the best coupling of an object to a heavy mass, which in this case is the turntable.

You may have noticed the lateral aluminum strip with the glued nuts.
The walls of the groove of the stereo record do contain information for the left and right channel. These information generally differ in strength, hence the amplitude engraved in one channel may differ from that in the other channel (wall). So as an example, it is possible that in the left channel there is a very detailed signal engraved while the right channel contains heavy drumming with short silences in between.

A tonearm with a gimbal bearing will keep fully control of the arm wand. Any tendency of the arm to get out of its lateral balance is suppressed.
A unipivot may give the arm wand the freedom to get out of azimuth (temporarily changing the position of the diamond tip by a few micrometers) is not justified. Yes, we are talking micro meters. The rocking tendency can be minimized by adding the lateral aluminum strip with the nuts or some other metal at both ends. It should be less easy for the cartridge to loose its perpendicular position (viewed from the front). Of course you have to do some experimentation to get the best result. You can check optically with a mirror, you can listen and do measurements with a test record.

The counterweight is a simple construction. A long screw is clamped to the balsa wood by means of an aluminum strip. The weight can be turned and moved backward and forwards to balance the arm and to adjust the down force. In this arm I used some lengths of old Keith Monks wire (as far as I could check). You may want to look for Cardas phono wire/cable. Or maybe you will find a different brand which gives good results. It is not only the arm which gives the best transient and overall balance. As the wire plays an important role too.

This page is just to give you an idea what you could do starting to make a tonearm. Maybe the advice is helpful.



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