In my opinion, in order to make a great passive linear arm, you need to
1. Simple structure. Get rid of any unnecessary structure which may disturb the natural state of cartidge body
2. Friction. Friction. Friction. It is very thing
1. Simple structure. Get rid of any unnecessary structure which may disturb the natural state of cartidge body
2. Friction. Friction. Friction. It is very thing
Hi all,
The bearings of a tonearm have several important criteria to fulfill. Having low friction is one of these. When I was designing the bearings for my arm I found it useful to look at friction in terms of tracking error. I built a test rig to measure the actual amount of force required to overcome bearing friction. If you know the compliance of your cartridge and the force applied to the stylus you can calculate the amount that the stylus will be displaced relative to the cartridge body. If you measure the length of your cantilever then you can easily calculate the angular tracking error that the applied force will produce. (This isn't 100% accurate as most cartridges have falling compliance with frequency but will give a close enough estimation). You can do a similar calculation for the inertia due to the mass/effective mass of the arm.
If you can see the stylus moving relative to the cartridge with the naked eye it's probably moving by at least 0.2mm. With a 5mm long cantilever this equates to a tracking error swinging from 1° to -1° constantly. Most cartridges will mistrack by over 3° before skipping.
Tracking error due to bearing friction doesn't just cause the stylus to be misaligned with the groove but also to misaligns the generator in the cartridge. With small alignment errors this isn't much of a problem but with larger errors the coils/magnet can move out of the gap making the cartridge nonlinear with high levels of distortion. If you can see the stylus moving it's moving too much.
It is of course desirable to minimise friction in order to avoid tracking errors. The level of tracking error should be low but chasing it too far can be expensive and not give much more return on the investment. For instance, The tracking error due to eccentricity of an average record is about 1/3° for the inner grooves and even a well aligned cartridge is probably still the best part of half a degree out.
With the exception of air bearings, the types of bearings used for tonearms will have a linear relationship between the amount of friction and the applied load/weight. This relationship is defined by the coefficient of friction of the bearings. You can halve the friction simply by halving the mass of the carriage. This may seem like an attractive option. However bearing friction is only a tiny part of the tonearm as a whole. Reducing the mass may also reduce the rigidity and cost a lot more than the gains made in reducing friction.
My carriage is about 55g. I investigated reducing bearing friction by reducing carriage mass and built a lightweight carriage of about 28g. I put a lot of work into this carriage in order to make it as good as I could. If I had built this carriage first I would have been very pleased with the results, it sounded very good. My more massive and rigid carriage still beat it in every aspect of sound quality, and by a comfortable margin.
Niffy
The bearings of a tonearm have several important criteria to fulfill. Having low friction is one of these. When I was designing the bearings for my arm I found it useful to look at friction in terms of tracking error. I built a test rig to measure the actual amount of force required to overcome bearing friction. If you know the compliance of your cartridge and the force applied to the stylus you can calculate the amount that the stylus will be displaced relative to the cartridge body. If you measure the length of your cantilever then you can easily calculate the angular tracking error that the applied force will produce. (This isn't 100% accurate as most cartridges have falling compliance with frequency but will give a close enough estimation). You can do a similar calculation for the inertia due to the mass/effective mass of the arm.
If you can see the stylus moving relative to the cartridge with the naked eye it's probably moving by at least 0.2mm. With a 5mm long cantilever this equates to a tracking error swinging from 1° to -1° constantly. Most cartridges will mistrack by over 3° before skipping.
Tracking error due to bearing friction doesn't just cause the stylus to be misaligned with the groove but also to misaligns the generator in the cartridge. With small alignment errors this isn't much of a problem but with larger errors the coils/magnet can move out of the gap making the cartridge nonlinear with high levels of distortion. If you can see the stylus moving it's moving too much.
It is of course desirable to minimise friction in order to avoid tracking errors. The level of tracking error should be low but chasing it too far can be expensive and not give much more return on the investment. For instance, The tracking error due to eccentricity of an average record is about 1/3° for the inner grooves and even a well aligned cartridge is probably still the best part of half a degree out.
With the exception of air bearings, the types of bearings used for tonearms will have a linear relationship between the amount of friction and the applied load/weight. This relationship is defined by the coefficient of friction of the bearings. You can halve the friction simply by halving the mass of the carriage. This may seem like an attractive option. However bearing friction is only a tiny part of the tonearm as a whole. Reducing the mass may also reduce the rigidity and cost a lot more than the gains made in reducing friction.
My carriage is about 55g. I investigated reducing bearing friction by reducing carriage mass and built a lightweight carriage of about 28g. I put a lot of work into this carriage in order to make it as good as I could. If I had built this carriage first I would have been very pleased with the results, it sounded very good. My more massive and rigid carriage still beat it in every aspect of sound quality, and by a comfortable margin.
Niffy
I agree. In any cases, low friction is the best starting point. If the friction is low, you have a lot of freedom to design carriage. I found that heavy mass has better sounding than low mass in most cases, especially in bass area. Due to high friction, it is not possible to use heavy carriage for mechanical linear arms. This is the main reason I love air bearing and this is why I said friction is everything.
Jim
Last edited:
Air bearings do tick most of the boxes.
A very important job of the bearing is dimensional stability. It has to hold the cartridge in the correct place. In this respect air bearings are superb as they typically have play of less than a micron. (I'm not talking about the low pressure aluminium angle design, I'm talking about the high pressure enclosed type)
Air bearings are also low noise if well designed, the hissing of escaping air is a bit of an urban myth. Bearing chatter is nonexistent.
Of course the main thing is low friction. With mechanical arms a bit of friction can be beneficial as it damps the motion of the arm. Air bearings have no self damping so damping has to be applied externally. This is probably for the best as the damping can be more easily fine tuned. A small amount of damping is important to control the cantilever resonance. The air in the gap can also act as a spring which in combination with the mass of the carriage will have a resonance. This is quite a big issue with low pressure systems. If using a good high pressure bearing this resonance will be at a high frequency and easily damped.
One area that the air bearing doesn't do so well on is mechanical grounding. As there is no solid contact there is no pathway for energy within the carriage to escape via. Damping unfortunately doesn't help here.
For me the main drawback of air bearings is the pump. Well, to be honest the main drawback is my house as it doesn't easily accommodate the pumps requirements.
Which is better? An air bearing design like Jim's or a mechanical jewelled bearing design like mine? Does Jim's lower friction count for more than my solid grounding? Who knows?
Niffy
A very important job of the bearing is dimensional stability. It has to hold the cartridge in the correct place. In this respect air bearings are superb as they typically have play of less than a micron. (I'm not talking about the low pressure aluminium angle design, I'm talking about the high pressure enclosed type)
Air bearings are also low noise if well designed, the hissing of escaping air is a bit of an urban myth. Bearing chatter is nonexistent.
Of course the main thing is low friction. With mechanical arms a bit of friction can be beneficial as it damps the motion of the arm. Air bearings have no self damping so damping has to be applied externally. This is probably for the best as the damping can be more easily fine tuned. A small amount of damping is important to control the cantilever resonance. The air in the gap can also act as a spring which in combination with the mass of the carriage will have a resonance. This is quite a big issue with low pressure systems. If using a good high pressure bearing this resonance will be at a high frequency and easily damped.
One area that the air bearing doesn't do so well on is mechanical grounding. As there is no solid contact there is no pathway for energy within the carriage to escape via. Damping unfortunately doesn't help here.
For me the main drawback of air bearings is the pump. Well, to be honest the main drawback is my house as it doesn't easily accommodate the pumps requirements.
Which is better? An air bearing design like Jim's or a mechanical jewelled bearing design like mine? Does Jim's lower friction count for more than my solid grounding? Who knows?
Niffy
Niffy,
A thin film of air is an excellent damping mechanism just as jewel type needle bearing. Needle is an excellent damping mechanism. This explains why air bearing arm is much quieter than regular ball bearing arms. I believe, although I don't have any data to support my believing, needle bearings are far better than regular ball bearings. If someone can use very high quality and noiseless ball bearing, it may be close to needle bearing. This is why I highly recommend needle type bearing.
Jim
A thin film of air is an excellent damping mechanism just as jewel type needle bearing. Needle is an excellent damping mechanism. This explains why air bearing arm is much quieter than regular ball bearing arms. I believe, although I don't have any data to support my believing, needle bearings are far better than regular ball bearings. If someone can use very high quality and noiseless ball bearing, it may be close to needle bearing. This is why I highly recommend needle type bearing.
Jim
Regardless, all forces on the carriage originate from and are born by the stylus/cantilever. It has enough work to do already just extracting the audio signal from the groove
Yes but, on a conventional arm the longer the lever arm the lower the amount of sideways force on the cantilever due to arm pivot bearing friction
At the risk of being perceived as trolling, I would like to see a closeup video of a ‘perfected’ passive linear tracking a high compliance cartridge and causing less equivalent tracking error due to friction related cantilever deflection than from geometry related tracking error on a good quality conventional pivoted arm
Yes but, I like high compliance cartridges and have owned/own several pivoted arms that don’t cause any observable cantilever deflection, and I’m sure that there are many other conventional pivoted arms extant that won’t do that either, unless they are defective or have been rewired improperly or…
I am a total fan of linear tracking tonearms. I just believe that the best way to implement them is to employ a conventional and proven pivoted design using unipivot or gimbal bearings, eliminate the offset angle, and move the pivot with a servo to maintain tangency. I sense that servo technology is an anathema on this thread and I don’t think my posting is contributing much here.
Ray K
I totally agree. I think your implementation of air bearing technology is the best I've seen on the internet. But, I consider your arm 'active' because it employs an air pump, not 'passive'. That's a complement, BTW.
Ray K
Hi Ray
A couple of years ago I undertook quite an extensive investigation into the tracking errors inherent in different arm designs. The results were based on a combination of direct measurements and mathematical analysis. I modeled 6 different arm whilst playing a record with a 0.5mm eccentricity which would result in the cartridge having to "wobble" back and forth by 1mm. I tried to take into account everything that could have an effect such as bearing friction, arm geometry, effective mass, skating force bearing play and the angle of the groove itself due to the eccentricity.
I modeled both 9" and12" pivoted arms. I also modeled a couple of low mass linear arms with different grades of ball race bearings as these were representative of the type of arm most people participating in this thread were building at that time. I modeled my arm which used jewelled bearings running on tungsten carbide rails. And finally I modeled a low mass air bearing arm. For all of these models I used a 22um/mN cartridge as that is the cartridge I am using (Ortofon 2M black)
Attached are two graphs showing the average tracking error across a record and the maximum error.
The pivoted Arms fair the worst mainly due to geometry.
The linear arm with the standard grade bearing fairs little better overall.
The arm with the least tracking error in unsurprisingly the air bearing. My arm with jewelled bearings came surprisingly close.
The dashed green line is the error due to the eccentricity itself. This represents the minimum tracking error achievable for this record.
It is worth noting that if the same models were run with a perfectly centred record the air bearing would have almost zero error. The jewelled bearing arm wouldn't be far behind. The errors for the pivoted arms and the light ball bearing linear arms would barely change.
With a good passive linear arm, even though the arm is still driven by side force acting on the stylus, the tracking errors are still much lower than for any pivoted arm.
Niffy
A couple of years ago I undertook quite an extensive investigation into the tracking errors inherent in different arm designs. The results were based on a combination of direct measurements and mathematical analysis. I modeled 6 different arm whilst playing a record with a 0.5mm eccentricity which would result in the cartridge having to "wobble" back and forth by 1mm. I tried to take into account everything that could have an effect such as bearing friction, arm geometry, effective mass, skating force bearing play and the angle of the groove itself due to the eccentricity.
I modeled both 9" and12" pivoted arms. I also modeled a couple of low mass linear arms with different grades of ball race bearings as these were representative of the type of arm most people participating in this thread were building at that time. I modeled my arm which used jewelled bearings running on tungsten carbide rails. And finally I modeled a low mass air bearing arm. For all of these models I used a 22um/mN cartridge as that is the cartridge I am using (Ortofon 2M black)
Attached are two graphs showing the average tracking error across a record and the maximum error.
The pivoted Arms fair the worst mainly due to geometry.
The linear arm with the standard grade bearing fairs little better overall.
The arm with the least tracking error in unsurprisingly the air bearing. My arm with jewelled bearings came surprisingly close.
The dashed green line is the error due to the eccentricity itself. This represents the minimum tracking error achievable for this record.
It is worth noting that if the same models were run with a perfectly centred record the air bearing would have almost zero error. The jewelled bearing arm wouldn't be far behind. The errors for the pivoted arms and the light ball bearing linear arms would barely change.
With a good passive linear arm, even though the arm is still driven by side force acting on the stylus, the tracking errors are still much lower than for any pivoted arm.
Niffy
Niffy, you alone may replace the whole research and development laboratory. May you remind me, what lateral, and what vertical effective masses would be optimum for the linear arm, which is split into two parts ( the way Dynavector pivot arms are split)? Cartridges to use will be of medium to low compliance...
Hello Ray,
I agree 100% with that statement.
An air-bearing tone arm will not function without an air compressor that has to be plugged into an AC outlet, while a servo controlled tone arm will not function unless its servo is plugged into an AC outlet. A mass produced quality air compressor costs about $1,200 the last time I checked, while the components for the servo in my tone arm cost about $50. Whether you are pumping air molecules or electrons, it's not passive.
Sincerely,
Ralf
I can hardly agree to classified air bearing arms as "Active". An active tone arm to me is its cartridge must be driven by external force. Any arms are driven by push and pull of the wall of groove IS passive. Plugging into power doesn't necessarily mean it is an active arm. Compressed air is NOT a force, it is just a medium. However, it is not that important. So, if you think it is active, it really doesn't matter too much.
Last edited:
Second, please continue to contribute, Ray K.
Third, if there's a motor it's active according to some of us
Third, if there's a motor it's active according to some of us
Ray,
Thanks for the compliment if I didn't understand you correctly! But for me, this is purely for conceptual comprehension.
Jim
Newbie question.
in typical two pulley linear tracking long wand tonearm since the motion of stylus+wand is as shown in pic (1) with red arrows. This creates perpendicular forces on rails by the pulleys as shown with blue arrows. So this may create additional friction. Suppose we shorten the tonearm almost to the point that the axis is almost in line with stylus point pic (2) so whatever horizontal movement stylus has being in the same direction of carriage movement will help the pulley(carriage) move (Shown with dotted arrow) easily with less friction. Isn't it ?
in typical two pulley linear tracking long wand tonearm since the motion of stylus+wand is as shown in pic (1) with red arrows. This creates perpendicular forces on rails by the pulleys as shown with blue arrows. So this may create additional friction. Suppose we shorten the tonearm almost to the point that the axis is almost in line with stylus point pic (2) so whatever horizontal movement stylus has being in the same direction of carriage movement will help the pulley(carriage) move (Shown with dotted arrow) easily with less friction. Isn't it ?
Attachments
This is an advantage. A shorter arm will create less torque about the bearings. A shorter arm will also be more rigid. However making the arm too short can cause problems with increased VTA error (due to warps and varying record thickness) and increased warp wow. It's all a balancing act.
Niffy
Hello super10018,
It is not my intention to put you or anyone else on the defensive. You are right when you say that it does not matter what I think because at the end of the day, you have designed and built a very nice tone arm.
Now, about the above quote, The tangential tone arm I have introduced in this forum a little while ago, fits the above description. The floating head-shell is driven purely by the grooves in the record. It is not driven by the tone arm's servo and therefore belongs in this thread. 🙂
Sincerely,
Ralf
So if you use compressed air from a canister, as for example CO2 , as I did for some time, the "active " air bearing suddenly becomes "passive" ?
This makes no sense to me , at least..😉
Let me put few words as to active-passive air bearing arms matter. We should give correct names to things in order to correctly understand them and avoid misconceptions. I would call linear air bearing tonearm: passive linear tonearm with active linear bearing.🙂