working on a new arm , 324mm carbon fibre unipivot , "woop de doo " i hear you murmur ,
  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 :-)

been a while since i updated this blog , since the last 12" arm i have made another , this one is based on a thorens tp16 , it had a damaged armtube , i was going to put it on ebay as spares or repair , then i thought i would disassemble it and check the bearings , the bearings were all fine and of a far higher quality than i was aware of , i should have known as thorens didnt really make anything we could call 'poor quality' ,
  i set about giving it a bit of thought and decided on a stainless steel armtube with dropped weight and direct clamped headshell , the effective length is 324mm , all the other numbers were taken straight from the arm calculator on the vinyl engine , and use lofgren A alignment
eff length               324mm
mounting distance 311.4mm
over hang                12.5mm
offset                       16.7 degrees ,
the only deliberate damping that i have built into the arm is from the carbon fibre dropper plate and the expansion wedge used to mount it , see the other post in this blog to see what i mean by 'expansion wedge ' , i have also used a small amount of high density foamed rubber at the headshell end of the armtube , around 5mm long , also the delrin (acetal) as an arm board as i have found it can have a very positive effect with arm structures that are very efficient at transmitting vibrational energy , arms that have compliant mounting within the arm structure are less receptive to it though so experimentation is recommended.
 the end result is somewhat disarming (no pun intended ) , the alloy arm based on the audiomods bearing assemble was really nice , but this is in a whole other league , Melody Gardot is rendered in a stark realism that i simply didnt think a home made arm that cost about £25 and 3 hours to make could ever achieve , bass? , all the way down without any unevenness , the sound is remarkably open and uncoloured for what it is , even the at95e sounds quite stunning ,
 i have still to fabricate an arm lift and nice looking rest for it and will post up more info when its all sorted but for now ,here is a quick pic of it on the 401 test plinth . :-)

mounted in an SME 2000/12 plinth , i know the plinth has a bad reputation which in my opinion is deserved , however it has some traits i like , the mid band is nicely animated and easy going , as soon as you crank the volume it soon makes its presence known , resting a hand on it demonstrates it is highly susceptible  to vibrating strongly at certain frequencies . Rather than give up on i took the springs off and replaced them with hard rubber doorstops , improvement was quite immediate but still wasn't quite usable , then i sat and thought about it a bit and considered what else is made from  1/2" ply wood in the world of hi fi , the bbc speakers and harbeth speakers sprang to mind , then i used a stethoscope to determine where it was resonating the most , i found a line running from the corner of the garrard closest to the arm base that runs to the front right corner was the worst point  , makes sense as the garrard clamps and damps the main part of the top plate , the arm board likewise , i used a ash stave 5/8 thick and 3/4 wide  that was secured by the 401 mounting stud then ran it along the pink line shown below , it is screwed to the top plate at two other points along its length , just this simple bit of wood and two screws have transformed its abilities ,

 prior to this the sme brought terribly uneven bass , after adding this bit of ash the bass is now as even handed as I have heard , i will try stiffening the carcase too as bad as its reputation is , with a little careful modding i think it may be a go'er , its main advantage is low mass which seems to bring out the best of the 401 , the mid range is more akin to a Thorens td124 , still with the Garrard grip but with a delicate velvet glove on .
 i will add more on this after some investigation and trying a few simple tweaks to sort the carcase .
more soon ;-)