the difference is that i want to know if it can be made with stuff you can buy then assemble into a 'good sounding' arm with a few hand tools that lots of folks have in the shed .
the arm tube will be 12mm carbon fibre of the 3k weave type with a 1mm wall , the headshell will be aluminium or carbon fibre depending on what can be had cheapest ,
the pivot .
this is a critical part of any tonearm , with a unipivot it can be constructed easily but, it must offer minimal friction , be robust enough to take all the load it is likely to see in its life and act as a mechanical drain .
point up or down?
IE should the pivot point be in the arm or on the support pillar , also we could use a ball in cup pivot or a honed point in a 'V' cup , there is also the possibility to use gem stone points and cups .
the ball and cup method is tempting as it is so easy and cheap to make , the issues with it are the lack of true fixed point mounting , a ball and cup arrangement has the potential for the ball to move in the cup as the record pulls on the stylus , not good, we can get round it by using a very small ball and cup to minimise the potential for this movement , we could also use a ring of balls ( ball race) to act as a cup and rest the balls on another single ball in the centre of the race , by doing this we could control how deep the single ball sits in the ball race which would eliminate any possible unwanted back and forth movement . it would add complexity and the potential for ball rattle , no one wants their balls to rattle ! :-D.
V cup and honed point looks like the best option to me , it will certainly be the easiest and cheapest to try the idea with. it will also be the quietest in operation and easiest to maintain .
we can in fact use a simple bit of steel rod with a V drilled in the top end for the arm pillar and a set or grub screw as the point ,
the arm shroud or bell , the part which the armtube and counterweight stub attach too . for my arm i will be using a readily available item i can get from many places in a myriad of styles and finishes ,
so, what is it ?
a simple £10 gear knob , they tend to be made of aluminium alloy and are the prefect shape for the job , drill a few holes run a tap into them and et viola we have the a perfect and cheap mounting.
i will post up the full build and all materials needed with costs , it will take time , things like the counter weight need consideration as a unipivot doesnt have the same mechanical grounding as a multipivot type arm so we must be careful or we can end up with all that vibrational energy simply bouncing back and forth along the arm and thus we get parasitic input to the music signal as the arm resonates at certain points across the audio bandwidth . compliant mounting of certain aspects of the arm is one option but even this can have deleterious effects on the end 'sound' of the arm .
material choice can be utilized to great effect in this respect , aluminium and carbon fibre connected together can offer superb 'energy losses ' with even handed broad band damping but if we over do it we can over damp the arm which can manifest a rather uninspired end sound , it can work well with cartridges that dump a lot of energy into the arm but well controlled carts sound duff in such structures .
the pivot point must be on the same horizontal plane as the stylus tip , if we ignore this simple alignment the stylus will move forward and back as the arm moves up and down while traversing even slightly warped records , all fine and well with a spherical tip but if you are using a very high quality extended line contact tip this would be disastrous .
even though this arm will be cheap it will have some costs and much time invested in it so we may as well try and get it right .
damping ? here i mean movement damping , as has been done by lots of other arm makers , we can actually make the arm quite low mass and use fluid viscosity to control effective mass as seen by the cartridge , while we are at it we can kill another bird with this same stone , the bell housing of the arm will want to rock as the record groove applies energy into the cartridge which in turn tries to move the arm , this energy is of very short duration so by using fluid we can utilize its non Newtonian properties ( fluids can behave as a solid momentarily at certain frequencies ) silicone fluid is easy to obtain in all viscosities from water like to thick grease like weights , get it from any rc car hobby shop .
more soon :-)
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