Why Side Bias (Anti-skate) Design Matters
December 2016, Origin Live won “Turntable of the Year Award” in Hi Fi+ magazine. One reason we continue to win awards is that we pay enormous attention to details few others consider worthy of investigation. For example there is not much written about the design of side bias (anti-skate) devices and even less about how they compare in terms of sound quality.
If you care about sound quality, the type of Side Bias design is surprisingly influential. Not just for its function in keeping the cartridge central in the groove but for the degradation it causes to performance.
We’ve thoroughly investigated this area and listened to the effect of the various methods used to apply side bias.
The results verify that producing a good side bias device deserves a great deal of attention. Certainly more than some who dismiss devices such as a falling weight on a thread as “cheap”, whilst praising inferior magnetic or spring loaded devices with nicely engraved dials.
Brilliantly engineered precision dials that control side bias within a second to a fraction of a gram, impress you with their sophistication and ease of use. They look great and breathe precision engineering. There is only one unanswered tick box – are they really accurate and what’s the effect on sound quality? Many discover too late that great engineering is never a guarantee of sound design.
Without a proper respect for the inconceivable effects of micro-vibration it’s easy to dismiss this type of research as obsessive and insignificant. However it’s not difficult to prove the clearly audible artifacts that whimsical side bias designs impose on sound quality. More on this later.
So before the allure of attractive dials and the illusion of precision tempts us, let’s just step back and examine first principles in more depth.
Principles of side bias
It’s commonly expected that as a tonearm tracks through the grooves it experiences an outwards force. Centrifugal force or differential friction acting on inner and outer sides of the stylus are intuitive reasons for this misconception.
In reality, frictional force acts tangentially to the groove. This “pull” on the arm is not in line with the arm pivot but offset and thus produces an inwards (not outwards) side force - see diagram below.
Things are far from simple but without going into detail it’s safe to summarize that the inward drag induced force is fairly consistent across the record decreasing by about 18% towards the inner tracks.
Side force is detrimental for the following reasons:
The unequal force on the inner walls of the record groove affects soundstage image slightly.
It causes uneven record wear.
The cartridge suspension is skewed out of alignment from it’s central sweet spot in the motor mechanism. This significantly affects performance.
A good side bias neutralizes these effects by counteracting the induced inwards force with an equal and opposite outwards force. In so doing, the stylus tracks the groove centrally to produce a notably more solid and focused soundstage. When setting side bias by ear, this is the quality to listen to.
It saves a of of fretting to realize that precision on a side bias device will be compromised regardless of how well it’s engineered or set up. To understand why, you can read the extensive research carried out on the subject years ago – see link http://www.audiomods.co.uk/papers/kogen_skatingforce.PDF
The bottom line is that even super accurate precision and optimized settings are at best a happy average on most records and tracks. Far more important and overlooked by most is the vibrational character of the device.
Requirements of a well designed side bias device
An effective side bias design should exactly counteract the varying side forces generated by friction from the outer tracks to the inner tracks.
It should be adjustable and accurate – Side bias setting is best determined by listening through trial and error so actual figures on dials are only a guide at best. The internal cable in the arm exerts a significant level of non linear twist in the wrong direction as an arm nears the inner tracks. This is often not considered in side bias designs and inaccuracy is common.
Low resonant design - avoids audible colouration and performance degradation.
Effective decoupling - aids isolation and prevents external vibration feeding into the arm.
Low friction - aids consistency and avoids mis-tracking problems.
Side bias device types.
Magnetic – This is a favoured method for some as it has the advantage of low resonance. The downside is a damping effect which sounds detrimental by giving the music a slightly “sat on quality, lacking speed and life”. Magnets are also non-linear and thus challenging to provide an even, consistent force across the whole record.
Spiral Spring – This is probably the most popular method and usually more linear than magnets. However this method introduces damaging resonance and coupling to the environment. It also exerts increasing force towards the inner tracks (the exact opposite of what you want). To overcome this issue, a cam and thread may be joined to the end of the spring but this additional complexity has it's own set of problems.
The detrimental effect of springs in tonearms have been long known to Rega owners who have followed our advice and disengaged the spring downforce setting in RB300 (303) arms and above. The more fanatical have found remarkable improvement in ripping out the spring entirely but this is not something we recommend as it devalues the arm at resale. Disengaging some of the detrimental effects of the spring is described on the link below. Linn arm owners will also benefit from trying the procedure described. http://www.originlive.com/rega-tonearm-modifications/rega-tonaearm-tuning-vta-adjustment.html
The impact of springs on sound quality can best be described as a haze and slight loss of solidity in the music.
Levers and weights – these devices are more accurate than the aforementioned but suffer from resonance and poor isolation which feeds vibration into the arm.
Cable or wire twist – Arms with a magnetic bearing are usually held by a vertical thread. The thread twists as the arm traverses the record and thus exerts an outward force. Adjusting the amount of twist adjusts the side bias. Another variant of this method is used by VPI arms and others where the arm electrical wiring is used to exert a twist on the arm. A neat solution in some ways but the disadvantage is that side bias increases towards the inner tracks and you want the reverse to occur.
No side Bias at all - A few arms have no side bias. There are also audiophiles who disable their side bias to give a greater sense of freedom in sound quality even though imaging suffers slightly. Whether this is for you depends on your cartridge suspension. Soft suspensions tend to be much less compatible as they visibly move sideways and detrimentally displace the motor mechanism.
Falling weight on a thread - This method overcomes the challenges of side bias better than the aforementioned methods in our view. It has low friction, good isolation, low resonance and even force. The force can also be governed to decrease slightly towards the inner tracks.
The sonic superiority of this method was verified by it's inventor who devised it long ago out of dissatisfaction with traditional springs and magnets. We adopted this method for performance reasons and have optimised it as far as possible for the criteria discussed earlier “Requirements of a side bias device”.
A well designed side bias is one indicator of the quality of thought that’s present in other more critical areas of an arm.
Some of the comments in this report may be controversial but will hopefully provide some insight in choosing a tonearm.
Lastly it should be said that side bias probably only contributes around 3% to the performance of an arm - easily audible, but there are so many other critical aspects that elevate performance to undreamed of levels.