Overshoot or undershoot can be overcome if the mass, friction, and compliance of the cartridge are in an optimal combination for all kinds of passive linear arms. The benefit of a linear air-bearing arm is that the arm starts with 0 friction so there is a lot of room to increase the mass(heavier mass is usually better) and adjust the level of damping. I believe most mechanical linear arms may be undershot due to friction. In order to reduce undershooting, you have to reduce the mass. This is why additional damping isn't needed for mechanical linear arms. That is also why I said friction is everything. For a mechanical linear arm, the most important task is to find a structure with minimum friction.
If a pivot arm is on an active linear stage, you actually allow tracking errors due to eccentricity and the variable gaps between grooves since the linear stage can only move in one direction in incremental steps. This is why I am not fond of active arms. A well-designed passive linear arm doesn't allow tracking errors.
I bought a cheap stethoscope from Amazon and will do some experiments with the air-bearing arms. It will be nice if the stethoscope can interface with a phone app or an oscilloscope so I can see the difference in the levels. There is one that can interface with an app on a phone, but it is too expensive.
I just found this video on Youtube. It seems to me that it may work. So I can record the signal and do some analysis. Stay tuned.
Last edited:
What about a set of the in-ear headphones with an attached microphone. Stick the microphone on the arm base and see if the phone will pick anything up. I used a similar setup when I was a kid to record my heart beat for a school project.
Eccentricity creates tracking errors on ALL types of tonearms. Also what's the point of making an arm that can follow eccentricity? I thought an active arm advancing one direction is an advantage. You do NOT want a servo arm to wander around to follow eccentricity. That would create more problem than necessary.
A well-built linear arm should follow eccentricity without displacing the cantilever too much. It doesn't make sense to me if a linear arm can't do that. Here is a video of one of my air-bearing arms. Please look at the top of the screen. You can see the air-bearing moves with eccentricity. So is the body of the cartridge.
For an active linear arm, it is difficult to do that. First, the active linear arm needs a sensor system to detect the position of the groove. Secondly, its servo system needs to act very fast to trace the groove. Unfortunately, it gets very complicated. Therefore, almost all the active linear arms can't do that except one. This is the reason I dislike active linear arms. The only one, as I know, that can do it is ELP laser turntable. The ELP laser turntable uses a laser sensor to detect the position of the groove, then, the servo drives the laser head following the groove. The laser head reads the information embedded in the groove. I would prefer passive linear arms until active linear arms can operate as ELP laser turntables.
There is another problem associated with a pivot arm on an active linear stage. Where is the starting point? The pivot arm needs a guiding mechanism to ensure the cantilever is tangent to the groove at the beginning. If not, there will be a tracking error across the surface of the record. I am sure that warrjon has the capacity to solve such a problem and am very interested to see how he does it.
When tracking eccentricity all good arms should be able to do so with minimal tracking errors due to cantilever deflection. This includes both linear and pivoted types. What neither type of arm can do is correct for the varying angle of the groove as the record moves forwards and backwards. The only way to correct for this is by centering the record using something like the expensive gadget DD posted a couple of months back. Of course the pivoted arm also has geometry working against it.
Mechanical linear trackers can also achieve very low cantilever deflection. This video is of a high compliance cartridge on a massive eccentricity and zoomed in pretty tight.
Niffy
Mechanical linear trackers can also achieve very low cantilever deflection. This video is of a high compliance cartridge on a massive eccentricity and zoomed in pretty tight.
Niffy
I also use a contact microphone to record different parts of the arm/plinth as well as the stethoscope to ***** various modifications. I have a recording with the mic on the bearing housing where the music is clearly audible.
IMO there way too much emphasis put on tracking error. How many have actually measured how far off Zenith their diamond is in the CL. I haven't measured Zenith but I have measured Azimuth in my Dynavector XX2 mk2, Stanton 881s and Technics EPC-205 and all of them off by at least 0.5deg or more. So there is little chance that diamond Zenith is better, so even if the tonearm is perfectly tangent the diamond is not.
In both those videos there is slight CL deflection visible which is not caused by friction, it's caused by the mass of the carriage taking xtime to come to rest when the stylus stops and heads back the other direction. I know a pivoting arm also deflects the stylus by skating force as well as inertia, but worse it scrubs back and forth creating FM distortion.
Keep in mind tracking the music signal stylus displacement is not visible. A 0.1micron deflection of the stylus will generate a -50dB signal (source Pierre Lurne) so any visible deflection will generate an audible signal. Now this signal might be very low frequency but it will modulate every note in the music signal.
My goal here is not to beat up linear tracking arms, my hope is to foster discussion to cement ideas I have in my mind. I got very little discussion on the thread I started for my servo LTA. I understand every single TA is a product of compromises and I hope from this discussion I can build a much better LTA.
IMO there way too much emphasis put on tracking error. How many have actually measured how far off Zenith their diamond is in the CL. I haven't measured Zenith but I have measured Azimuth in my Dynavector XX2 mk2, Stanton 881s and Technics EPC-205 and all of them off by at least 0.5deg or more. So there is little chance that diamond Zenith is better, so even if the tonearm is perfectly tangent the diamond is not.
In both those videos there is slight CL deflection visible which is not caused by friction, it's caused by the mass of the carriage taking xtime to come to rest when the stylus stops and heads back the other direction. I know a pivoting arm also deflects the stylus by skating force as well as inertia, but worse it scrubs back and forth creating FM distortion.
Keep in mind tracking the music signal stylus displacement is not visible. A 0.1micron deflection of the stylus will generate a -50dB signal (source Pierre Lurne) so any visible deflection will generate an audible signal. Now this signal might be very low frequency but it will modulate every note in the music signal.
My goal here is not to beat up linear tracking arms, my hope is to foster discussion to cement ideas I have in my mind. I got very little discussion on the thread I started for my servo LTA. I understand every single TA is a product of compromises and I hope from this discussion I can build a much better LTA.
I'd be interested in the results of the noise test on the air bearing/rail. I measured the noise in an SME bearing and it was 20dB noisier compared to my DIY pin pivot tonearm.
Hi warrjon, was this just by moving the arm around or was there a more scientific stimulus for the measurement please?
M
I have had a stethoscope for a while and until now found it useful but not particularly revealing.
Prompted by this discussion i just did some experiments.
Oh dear!
So, my TA, an RTA using radial rails, demonstrated the following with a record playing but no amplification.
Just to give you an idea, there is little needle talk with this set up, so hearing music through the stethoscope is quite surprising!
Stethoscope on the rails, near the mount, I can hear the music plainly and in detail
Down through the rail mount block, across the pivot joint and down to the arm board base is all similar level.
Seems well coupled through there.
The lower plinth is quieter.
The shelf below the sand bed, i can still hear it and
Even the wall shelf shows up some quiet notes!
I imagined that even in the wall i could hear some notes occasionally, its amazing what this micro-vibration does.
Thats all quite as expected in terms of the gradual decay, but if the music can cross the sand bed, it's not doing its job well at all!
Meanwhile the most significant thing is that the music is roughly 50% louder on the rails on the inside of the carriage nearer the record centre than on the rails that are nearer the arm mount.
So, gents, i need a diagnosis and cure please.
By the way, i cannot dare put the stethoscope on the carriage itself to see how well coupled that may be, possibly i could put a small contact mic on there if small ones are available?
M
Hi Warren,
In the video of my arm and ortofon cartridge the hole has been enlarged to give an eccentricity of 1.6mm causing the arm to move back and forth by 3.2mm. In this extreme case about 60% of the cantilever deflection IS due to inertia. The total tracking error is still only about 0.15° at its maximum.
With any normal record bearing friction will dominate. On even the worst unadulterated record total tracking error will be no more than 0.085° and much less on most. This is with a high compliance cartridge. With a medium or low compliance the maximum error would be lower still.
The effect of inertia and friction is cantilever deflection. With either my arm or Jim's the induced tracking error is insignificant. You also mentioned that the deflection will cause scrubbing that will modulate speed causing FM distortion. With my example with the worst normal record will cause wow at 0.55hz of about 4x10^-6%. Completely inaudible. The FM modulation caused will be just just as inaudible. Really nothing to worry about.
Niffy
Sorry. Fat fingered my calculator.
My calculations for the amount of wow just didn't look right.
The correct amount of wow would be 0.0085%. Quite a bit more than the previous post but still too low to be an issue.
Niffy
My calculations for the amount of wow just didn't look right.
The correct amount of wow would be 0.0085%. Quite a bit more than the previous post but still too low to be an issue.
Niffy
I think the lack of discussion on servo arm, including the Rabco thread by diyrayk (Ray Kilmanas), is due to the fact that many diyers and observers, myself included, are intimidated by servo and logic circuitry. A traditional tonearm is just a mechanical device that most people can grasp visually and intuitively. I'm not against servo arms at all, even owned a Rabco and several other linear servo turntables like Yamaha PX-3, and I think they can be great performers. The incentive are to keep the footprint smaller and circuit simpler. Keep those discussions coming in that thread though and they might encourage others to dive in.
Hi Mike,
I've dug-out my stethoscope and tried the same tests.
I can clearly hear the music on the rail. In my case it is loudest close to the carriage and get quieter the further away I test. It is a bit louder at the left end of the rail compared to the right when the carriage is in the middle. I would expect this as the right hand end is more solidly connected to the arm pillar. The left being cantilevered, though in my case there is a massive beam connected to the left hand end specifically designed to support it.
I can hear the music from the arm pillar and the rest of the arm structure. Like your result it is pretty similar in level regardless of where I test.
The level is reduced on the sub-chassis but is similar in level regardless of where I test. My sub-chassis has an acrylic upper piece mounted on an aluminium-acrylic sandwich structure. Surprisingly the music is slightly more audible on the sandwich structure. As expected it is louder again on the main bearing mounting plate showing that the platter is also being coupled nicely.
Slung underneath my sub-chassis I have a heavy, highly damped slab of laminated aluminium and lead. This is designed to act as a sink for the vibrations coming through from the sub-chassis. The music is only just audible here. Looks like my coupling and channeling of the energy from the stylus-groove interface is working as intended.
Testing on the main plinth, that is separated from the sub-chassis with the suspension, I could hear nothing. Good news as it shows that the suspension is isolating well.
It seems strange that you are hearing sound all the way through your support system. Are you sure that you are not just hearing the stylus talk transmitted through the air?
I also checked the deck with the stylus not in the groove to check for bearing and motor noise. Very happy to report that I couldn't hear anything even with the stethoscope against the bearing mounting plate. I have my motor mounted on the sub-chassis with silicone isolation. Although I could clearly hear motor noise when testing on the motor body directly I could hear nothing anywhere on the sub-chassis. I also use a couple of idler wheels the belts run around and again the bearings for these are still running silent. Good news here as I went through at least half a dozen different designs trying to get them silent.
Niffy
Last edited:
CL deflection causing tracking error is not the issue and immaterial IMO. The issue is CL deflection causes the cartridge to output a signal. FM and AM distortion is not inaudible. It's audible as smearing to a note caused by sidebands on the note. Below are plots from my Technics EPA-100 tonearm that show AM sidebands caused by the CW (Counter Weight) dynamic damping, the second plot is with a new CW and stub that rigidly locks the CW to the stub. This change was audible as improved detail and clarity. I did have these plots for my LTA but unfortunately lost them with a PC crash. The LTA was superior to the stock EPA-100 but was not better than the modded EPA.
Mike's stethoscope test shows his LTA is well grounded. So the energy generated by the cartridge has a good path to ground reducing reflected energy back to the stylus. A phono cartridge is a VERY sensitive transducer and is capable of picking up a conversation 20meters away if the stylus is placed on a stationary LP, I discovered this when developing a POM platter for my SP10. So any low level vibration will be picked up and turned into unwanted signal.
Here are a few things I've learnt from building LT and pivoting arms.
My second LTA built similar to @niffy arm was extremely quiet, ie no clicks and pops on very scratched LP's. Great bass reproduction I suspect from high lateral mass preventing the stylus waggling the arm back and forth, remember under 200Hz on an LP is horizontal modulation. These were the 2 standout qualities.
Modifying the EPA-100 replacing the CW and stub to lock CW rigidly, new far more rigid mounting base/post and viscous damping. These mods improved the EPA-100 to where it was significantly better than the LTA, except for click/pop suppression.
From the EPA-100 modification experience I designed and built the pivoting arm in the last pic. This arm originally started life to be mounted to a linear carriage driven by a motor, I'll go into why I parked the project later. This arm is designed and built to be massively rigid and neutrally balanced, careful attention was paid to mass distribution. It's an incredibly detailed tonearm, the only area the LTA outperforms it is in click/pop suppression. Even though this arm looks massive its minimum effective mass is 12g. So the learning from this is mass distribution and rigidity are 2 key elements to making a great tonearm, @Mike56 has used these principles to construct his LTA and it certainly has worked for him.
You'll all have to forgive me as I'm going to talk about servo linear arms to bounce ideas in order to refine my design.
I parked the servo LTA because of rail noise, I couldn't eliminate rail noise as the carriage moved. I think I have this somewhat fixed with a different rail and some damping.
From the above learning a pivoting arm on a servo driven carriage has a lot of advantages in that horizontal effective mass is not a product of carriage mass. Viscous damping can be implemented off the LP to prevent possible leakage. The downside is complexity and possible motor/rail noise induced into the cartridge.
A short rigid arm rings far less than a longer wand type evident in the click/pop performance of a short rigid carriage. One possible solution is a Dynavector style split vertical/horizontal arm setup, the down side is non-uniform ellipsoid of inertia, which can cause erroneous movement ie as the stylus tracks a warp it's possible for the arm to move sideways and not straight up/down. Implementation of vertical viscous damping requires trough over LP. This makes me think a very rigid as short as possible pivoting arm wand could be the best compromise.
Mike's stethoscope test shows his LTA is well grounded. So the energy generated by the cartridge has a good path to ground reducing reflected energy back to the stylus. A phono cartridge is a VERY sensitive transducer and is capable of picking up a conversation 20meters away if the stylus is placed on a stationary LP, I discovered this when developing a POM platter for my SP10. So any low level vibration will be picked up and turned into unwanted signal.
Here are a few things I've learnt from building LT and pivoting arms.
My second LTA built similar to @niffy arm was extremely quiet, ie no clicks and pops on very scratched LP's. Great bass reproduction I suspect from high lateral mass preventing the stylus waggling the arm back and forth, remember under 200Hz on an LP is horizontal modulation. These were the 2 standout qualities.
Modifying the EPA-100 replacing the CW and stub to lock CW rigidly, new far more rigid mounting base/post and viscous damping. These mods improved the EPA-100 to where it was significantly better than the LTA, except for click/pop suppression.
From the EPA-100 modification experience I designed and built the pivoting arm in the last pic. This arm originally started life to be mounted to a linear carriage driven by a motor, I'll go into why I parked the project later. This arm is designed and built to be massively rigid and neutrally balanced, careful attention was paid to mass distribution. It's an incredibly detailed tonearm, the only area the LTA outperforms it is in click/pop suppression. Even though this arm looks massive its minimum effective mass is 12g. So the learning from this is mass distribution and rigidity are 2 key elements to making a great tonearm, @Mike56 has used these principles to construct his LTA and it certainly has worked for him.
You'll all have to forgive me as I'm going to talk about servo linear arms to bounce ideas in order to refine my design.
I parked the servo LTA because of rail noise, I couldn't eliminate rail noise as the carriage moved. I think I have this somewhat fixed with a different rail and some damping.
From the above learning a pivoting arm on a servo driven carriage has a lot of advantages in that horizontal effective mass is not a product of carriage mass. Viscous damping can be implemented off the LP to prevent possible leakage. The downside is complexity and possible motor/rail noise induced into the cartridge.
A short rigid arm rings far less than a longer wand type evident in the click/pop performance of a short rigid carriage. One possible solution is a Dynavector style split vertical/horizontal arm setup, the down side is non-uniform ellipsoid of inertia, which can cause erroneous movement ie as the stylus tracks a warp it's possible for the arm to move sideways and not straight up/down. Implementation of vertical viscous damping requires trough over LP. This makes me think a very rigid as short as possible pivoting arm wand could be the best compromise.
Attachments
Yes moving the arm horizontally with the stethoscope on the bottom of the bearing post the bearings were audible. My DIY pivoting arm the horizontal pivot housing has to be spun fast to detect any noise in the bearing, moving it back and forth it's inaudible.
Really useful update and feedback thanks Niffy,
I have learnt some more.
It is staggering that the tiny stylus movements generate enough energy to reach as far as they do.
I shall look at further stiffening the rails to see how the relative levels next to the arm post and away from it are affected as it would seem likely that performance would be better if that were not the case.
Which leads me to damping, and a feeling I have from experimentation.
Over time i have damped the carriage parts and rails on the basis that "its obviously going to be better."
Often i spent quite a lot of effort on the tiny parts doing this and mostly I was disappointed.
I often measured poorer results and the sound was then relatively dull.
Now i imagine that damping these vibrations relatively close to the cartridge is reducing the transmission of the energy away from the cartridge, leaving it close by and able to modulate the sound.
Thats not what we are trying to achieve, so I question the use of damping anywhere before the base plinth and feel that stiffness and transmission is the better answer.
Warrjon comments later on those same facts/opinions.
Concerning your question Niffy, sadly it is definitely heard from the stethoscope when I hear sound I don't want to, I have hardly any audible needle talk anyway, good news and bad news, bad news it's not as good as it might be, good news I have some clues on what to work on next!
I can foresee some winter experiments coming up!
M
Warrjon, I hadn't read this when I wrote my other recent note.
Damping, what is really happening there, is the unwanted vibration energy converted to heat in the damping medium and so satisfactorily dissipated or is it left in the cartridge end of the arrangement as a result, ready to modulate the original signal. It seems to me that damping works against the idea of encouraging the flow away of vibration/energy, and my experiments and measurements convince me it hasn't improved the sound.
After that i am lost in the complexities again!
I will be doing some damping against stiffness comparisons on the rails this winter.
M
Mike I think stiffness is the key, as stiffness increases so does the resonant frequency. If the Fr can be pushed high enough then the harmonics are also pushed higher and the 3rd is usually the biggest offender. If the 3rd can be pushed out of the audio band you are doing very well. Then a very small amount of damping can be applied.
One thing with damping is what are you damping. Ideally the damping should target the problem. When I was testing my DIY 4 point I performed resonance tests on the arm wand with different damping material until I found the best result, then the CW stub was bolted on and re-tested to ensure the performance was still ok. Small changes like adding the CW stub can drastically alter vibration profile of the structure. What I was looking for was a reduction in level of the fundamental resonance and harmonics without adding other peaks. What I found was incorrect damping added other resonant peaks.
The plots in order.
Bare arm wand, arm wand with CW stub, Silicone tube inserted into the arm wand, last is 3 small earplugs inserted into the wand.
What we.re looking for is ideally a flat plot or as close as possible to flat.
One thing with damping is what are you damping. Ideally the damping should target the problem. When I was testing my DIY 4 point I performed resonance tests on the arm wand with different damping material until I found the best result, then the CW stub was bolted on and re-tested to ensure the performance was still ok. Small changes like adding the CW stub can drastically alter vibration profile of the structure. What I was looking for was a reduction in level of the fundamental resonance and harmonics without adding other peaks. What I found was incorrect damping added other resonant peaks.
The plots in order.
Bare arm wand, arm wand with CW stub, Silicone tube inserted into the arm wand, last is 3 small earplugs inserted into the wand.
What we.re looking for is ideally a flat plot or as close as possible to flat.
