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 stabilis
es the arm, reducing bearing chatter and vibrational rocking. We will address these two concepts in turn as the principles involved are entirely different.
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. 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.
Calculations show that an 8g increase in cartridge weight will increase inertia four 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, all other things being equal, that it makes no audible difference to sling the weight low. As previously outlined, any beneficial observations are usually 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.
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 a bandwagon of inferior copycat ideas.
Calculation of Inertia
The following calculations are given to try and give a sense of proportion to a rather intuitive but incorrect notion. Inertia differences are relatively insignificant. 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 16g cartridge in comparison to the average 8g 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 cartridge 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.