Counterweight Position and Low Centre of Gravity

A common idea that should be questioned is whether a tonearm counterweight should be positioned as close as possible to the yoke for best performance. This minimises inertia and thus reduces “see / saw” effects over record warps.  A variation of this theme is that low slinging the weight drops the centre of gravity and thus stabilises the arm, reducing bearing chatter and vibrational rocking etc. We will address these two concepts in turn as the principles involved are entirely different.

 

COUNTERWEIGHT POSITION

Frequently you find that observations on the performance of particular arms, make folk jump to conclusions that miss the real causes of the performance changes perceived. Although the theory of decreasing the inertia of the counterweight seems very plausible, there is another more proven explanation. It gives peace of mind to realise that when the counterweight is positioned at the end of the stub on an Origin Live arm, you are NOT losing performance.

The notion that counterweight position affects performance has much more to do with vibration and lack of structural rigidity than inertia effects - in other words on some arms it pays to keep the counterweight close to the yoke as it decreases resonance effects from counterweight “waggle”. On these arms the counterweight causes an increasing vibrational whiplash effect as it gets further from the pivot. Origin Live have gone over and over this with their arms and established conclusively that the counterweight position hardly affects performance at all on a properly designed arm. The idea that inertia is an enemy is also based on pure ideology - the fact is that without inertia the stylus could not read the record groove at all! Some systems add weight at the headshell to INCREASE inertia as it is proven to be beneficial up to a point. 12 inch arms have much higher inertia than 9 inch ones but nobody seems to bat an eyelid!

Calculations show that an 8 gram increase in cartridge weight will increase inertia 4 times more than having your counterweight position at the rear end of your arm stub.

COUNTERWEIGHT LOW CENTRE OF GRAVITY

Another case of observations drawing questionable conclusions is the case of low slung counterweights. Low centre of gravity IS important for unipivot arms but dual pivots and gimbal arms do not “sway about” and listening tests prove that all other things being equal, it makes no audible difference to sling the weight low. As previously outlined, any beneficial observations are usually down to resonance effects on certain arms but not Origin Live arms.

Many low slung weights use varying methods of clamping themselves to the rear stub. This alone is very influential on the sound but the mistaken assumption is made that the difference is due to low slinging 

We have probably experimented with counterweight materials and attachment more than anyone. Avondale Audio was the first company to modify Rega counterweights and they sub-contracted us to produce them, since then this has become almost a bandwagon of inferior copycat ideas.

(optional read for calculation of inertia issues)

A common idea propogated in Hi Fi circles is that the
counterweight should be as close as possible to the yoke to
minimize inertia and reduce “see / saw” effects over record
warps. We believe this idea is based on certain observations
on particular arms that have then led to a totaly incorrect
diagnosis of the real causes of any performance changes
perceived. Although intuitively the theory of decreasing
the inertia of the counterweight seems very plausible, there
is another more proven explanation. The reason this is
helpfull to realise is that it gives peace of mind that when
the counterweight is positioned at the end of the stub on an
Origin Live arm, you are NOT losing performance.
The notion that counterweight position affects performance
has much more to do with vibration and lack of structural
rigidity than inertia effects - in other words on some arms it
pays to keep the counterweight close to the yoke as it decreases
resonance effects from counterweight “waggle”. On these arms
the counterweight causes an increasing vibrational whiplash
effect as it gets further from the pivot.
Origin Live have gone over and over this with their arms
and established conclusively that the counterweight position
simply does not affect performance very signifi
cantly on a
properly designed arm. The idea that inertia is an enemy is
also based on pure ideology - the fact is that without inertia
the stylus could not read the record groove at all!
Some systems add weight at the headshell to INCREASE
inertia as it is proven to be benefi
cial. 12 inch arms have much
higher inertia than 9 inch ones but nobody seems to bat an
eyelid!
The following calculations are given to try and give a sense of
proportion to a rather intuitive but incorrect notion.
Inertia differences are relatively insignifi
cant. To get the weight
closer to the pivot, the counterweight has to be heavier and
this adds inertia - all in all a difference of 20mm or so is not a
big deal.
To illustrate this using laws of physics - the moment of inertia
is m x r squared where m is the mass and r is the distance to its
centre of rotation
Say that a 130gram counterweight is 45mm away from the
pivot - moment of inertia is 0.13 x 0.045 squared = 0.26 x10
to the minus 3
Equivalent downforce can be achieved with a 235g weight
at 25mm from pivot so - moment of inertia is 0.235 x 0.025
squared = 0.146 x 10 to the minus 3
This is a difference of 0.000114
Now compare this with the MUCH HIGHER increase
in moment of inertia cuased by a 16gram cartridge in
comparison to the average 8grams
16 gram cartidge inertia is 0.016 x 0.220 squared = 0.774 x 10
to the minus 3 (0.220 is pivot to cartridge distance in m)
8 gram cartidge is 0.008 x 0.220 squared = 0.387 x 10 to the
minus 3
This is a difference of 0.000387 - nearly 4 times higher than
the inertia saving of moving the counterweight in.
Another case of observations drawing the wrong conclusions is
the case of low slung counterweights. Low centre of gravity IS
important for unipivot arms but dual pivots and gimbal arms
do not “sway about” and it makes no difference in practice
to sling the weight low. As previously outlined, any benefi
cial
observations are down to a heavier weight or different method
of attachment reducing resonance effects on certain arms but
not Origin Live arms. This is something we have tried and
tested repeatedly.

Address

Origin Live Ltd. Unit 5, 362B Spring Road Sholing, Southampton SO19 2PB, UK