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As we have been discussing the pros and cons of the decoupled counterweight on the ET, I have this question on my mind (admittedly a non-technical one) for a long time!
The ET uses leaf springs to decouple the counterweight so the arm doesn’t “see” the additional weight! To my thinking, this can only be possible if the counterweight actually doesn’t move during the initial movement (milliseconds?) of the arm, due to the compliance of the leaf spring. Then, after the arm has moved for a certain range, the counterweight would need to start “rebounding” to follow the arm.
If that is true, then would it create some delayed effect that could be detrimental to the tracking?
Or is my reasoning totally false? |
It sounds better Hi Richard, I am no engineer, so has absolute no idea about the physics behind it. However, as Lloyd didn't use the decoupled counterweight (which is not hard nor costly to implement), I also believe it probably sounds better that way, at least to his, and apparently also to your, ears. Actually, I have also fabricated a fixed counterweight for my ET2.5, but unfortunately have no time to test it yet. When it happens, the ET would be compared to my favorite arm, the Rockport 6000! |
Hi Ct0517,
Yes, that Rockport Capella II is still my main turntable. When ready, the ET2.5 will be set on the brass armboard opposite to the Rockport 6000. Actually, you can see a bit of the ET at the left edge of that photo already, just that the whole counterweight assembly was not installed yet!
BTW, the Rockport 6000 can also be installed on nearly all turntables! |
What about that stiff plastic tubing on the Kuzma? It seems like it would have a huge effect on the arm's movement even if both of the fittings swiveled. Does anyone know any details about the tube's fittings or if it hinders the arm's movement? Hi Ketchup, My Rockport and my friend’s Airline shares similar arrangement in the airhost. Actually, they are not that stiff, and they can be detached at the point where they enter the bearing. So, you can twist them a bit to attain a hanging n shape for minimizing the effect on movement. I can’t say they have no effect on movement, but they certainly don’t hinder it, at least not to the point of causing mistracking. If the record is 12mm out of true, the arm travels 24mm in and out with each revolution, which takes 1.8 seconds. Hi Dover, While it may not affect you conclusion on the lateral forces of the 3 arms, a record that is 12mm out of true (24mm in and out, that is nearly 1 inch!) is probably unplayable even with a pivotal arm. I think, on average, 1mm (2mm in and out) is more like it! |
Could the tubing being in a loop at the centre of the Rockport and Kuzma arm, be acting as a type of dampener for the arms motion in both directions ? Hi Ct, I think the tubing is just a necessary evil in the Rockport / Kuzma design. As for whether it affects the tracking, when I adjusted the arm to free floating (i.e. 0 VTF), I can put the arm in any position without it going in or out. So, I suppose the affect is minimal. However, there is one interesting point that I still can’t understand. When moving the arm in and out by hand, the feeling of the ET is smoother (or I should say requires less force) than the Rockport! While I don't have the spec. of the 2 bearings, I have always assumed that the Rockport has a tighter tolerance / air gap. This can be shown when trying to move the arms without the air supply. While you can still move the ET quite easily, a lot more force is required for the Rockport in this condition. But, when air pressure is supplied, shouldn't both of them be virtually frictionless? I think if the Rockport indeed has friction, the cartridge would probably mistrack. Or could it be the horizontal mass that we have been talking about? Based only on the look, if the horizontal mass of the Airline is 100g, I would estimate the Rockport is about half to 1/3 of that, so in the 35-50g range, not that far away from that of the ET! |
Hi Richard,
Thank you for the suggestion!
As this is only for experimenting, it is a bit rough. Would certainly take your suggestion into consideration if I decide I like this arrangement and fabricate the final counterweight assembly! |
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Hi Ct,
During the testing phase, my ET2.5 will be set up with a ClearAudio Sigma. When the final comparison comes, I will be using the Ortofon A90 between the Rockport and the ET.
I am using 15psi for the moment with a compressor together with its integral surge tank, and 2 regulators before the air enters the arm. I can easily increase the pressure up to 34psi, which is what I am using for the Rockport, for testing.
About the counterweight, the aluminum square is tapped and the threaded rod just screwed into it. I “reinforced” it a bit with superglue !
Hi Richard, Dover,
I don't have the knowledge to even start participating in the discussion on the physics of moving mass, however I have some general observation.
It seems to me all the other arms that we have discussed recently (Airline, Rockport, Terminator, Walker) have much higher horizontal mass than the ET with the decoupled counterweight.
Take the Airline as an example, if its horizontal mass is 100g, then it is even higher than the 85g of Richard’s modified ET. While I don't have the Airline myself, 2 of my friends have it for a couple of years already and have no problem with cartridge damage. I have also never seen any actual negative report regarding this matter on the internet. The same applies to the other 3 arms.
Looking at the massive construction of the Airline, I couldn’t help but wonder whether Kuzma could reduce the size and weight somewhat without sacrificing rigidity. They didn’t do so appears to me that they have no concern on this high horizontal mass being detrimental to the performance, or worst yet, causing cartridge damage.
Could all these arm designers be wrong when their products are getting very positive comments from actual users, not just magazine reviews?
Of course, all these other arms, with the exception of the Terminator, are very expensive, so they most likely are being matched with highend MCs with medium to low compliance. Maybe the high horizontal mass is less of a problem in these conditions?
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Hi Ct,
I am using the Apogee Fullrange, so should be able to tell the bass difference between the different setup. However, I will need to order the extra left springs from Bruce.
As I got my original ET2 secondhand, I am missing the lead counter weight. I remember asking Bruce a couple of years back but the price was really high, due to some special reasons (high cost to machine lead as it is hazardous to health ?).
So, I would like to check with other users whether using other substitutes would cause a big difference? |
Hi Richard,
I will be using the ET2.5 in my test. My ET started out as a 2, which I later added the 2.5 bearing from Bruce.
I was asking whether replacing the lead counterweight with something else (brass/bronze ?), while still using the decoupled I-beam / leaf springs, would cause a big difference in the sound. |
Hi Dover,
Thanks for the explanation, will certainly try all different set ups and see the difference myself! |
Loosen the end cap bolt (not the leaf spring bolt we have been discussing) just enough to lower the lead weights, so that they are positioned at the bottom half of the spindle; when looking at the arm from the gooseneck side. Hi Ct, In all the photos I have seen on the Walker turntable, the counterweight set-up was below horizontal. So, I suppose that is intentional and offers some advantages, at least in that design. I must apologize that due to my schedule, it will take me a while to have the time to do a thorough comparison on the counterweight set-ups in my system. I will report it here at once when I can complete the test! |
I have measured the weight of the Rockport 6000 in the weekend, and to my surprise, the whole moving assembly weighted in at 90g (including the A90 at 8g, and a spacer at 2.5g). I am using the lightest counterweight out of the 4, and the heaviest one would add another 15g!
With the figure about, it is hard to believe the moving mass of the Airline is only around 100g considering how massive it is compared to the 6000! |
Ct,
You have sharp eyes, yes the owner of that Sirius III was complaining on Audiogon that the arm couldn’t track more than 3 cuts into the LP. There are actually 2 black hosts in that photo, one for the air, and one for the signal wire. I believe it was just set up wrongly; it should be like the photo of the 6000, with the signal wire host pull back as much as possible, and both the air-host and signal wire arranged into a hanging n shape!
Dover,
My 6000 came with 4 counterweights of different sizes to match with different cartridges!
Regarding the off-centered cantilever of the VDH, I believe, while great sounding, it is just a very fragile design, and probably not really fit for air-bearing arm including the ET2. After having it fixed, my friend has used it on both the SME V and Graham Phantom with no problem!
I will try to weight the sliding assemble of the 6000 this weekend and report back! |
Hi Richard,
Yes, the 6000, and all other Rockport arms, has damping trough. The “paddle” is fixed, so unlike the Walker, you have to adjust the amount of damping oil you put in!
However, I and a couple of my friends who had the 6000, prefer it without damping; it just sounds more lively, or more PRAT! I may revisit this topic later! |
Hi Rugyboogie,
Can the moving assembly on the Airline be taken out easily? If it can, and it is not too much trouble, I would really appreciate if you could weight it to confirm whether it is really only around 100g.
Hi Ct,
Yes, I still don't understand why the ET bearing feels smoother (to the hand) as compared to the Rockport when both are having the air-supply. I would assume both of them are virtually friction free under this condition! Maybe Rugyboogie can share with us his finding when he got his ET! |
Oh, it just dawn on me, could the "less smooth" of the Rockport bearing cause by the flex of tiny airhost? If it is, then the airhost actually provide considerable "damping"! |
TEST FOR STRUCTURE FEEDBACK ……. Start jumping up and down right beside your TT……. Hi Ct, The most extremely test of this sort that I have seen was that, while following your initial settings, instead of jumping around the TT, you kick on its stand! :-) It was performed on a Rockport Sirius III at the agent, and yes, it was pretty much silent from the speakers! That stand was actually an OEM pneumatic table by TMC, and I am using a similar one under my Capella II. Unless you kick it hard enough that the whole stand / TT assembly (roughly 400-500 lbs.) moves over the floor, you can hardly hear any thumping from the speaker indeed! Hi Dover, You have mentioned that too high a horizontal effective mass would result in a raised bass response by 6-12db. So, in what frequency range are we talking about (the Fr at below 12Hz?)? I just wonder if the high horizontal effective mass is the only consideration here, and whether other factors, such as the design of the air-bearing, would make a considerable difference in the outcome. In short, does that only apply to the ET design, or also to the Rockport and Kuzma etc? I asked because my Rockport 6000, already has a high horizontal effective mass of 80g (with the lightest counterweight, and excluding the cartridge), but yet MF found it lacking in the bass! The upgraded 7000 and Sirius III arms added even more mass to “cure” this problem! Not meant to be argumentative, just want to learn more! Thanks! |
Thank you all for the detailed explanation!
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These stresses result in subsonic low-frequency Structure-Borne feedback which passes easily into equipment racks, support stands and into the turntable plinths sitting upon these racks/stands. Hi Halcro, Thanks for the information! So, any idea what is the frequency range of these structure-borne feedback? Thanks! |
Hi Halcro,
Thanks for the information again! As I can’t use wall mount in my room, I have never experience its effectiveness!
My room has suspended concrete floor. According to the specification, the pneumatic table could isolate up to 85-90% of the vibration at 5Hz, and 97% at 10 Hz, it has worked out very well for me! I certainly could hear the improvement in background blackness with its use!
Thanks! |
Hi everyone, very interesting topic! I have obtained a used ET2 with Bruce’s upgrade 2.5 bearing, but have yet to set it up.
I am very interested in the discussion on the I-beam compliance. As far as I can tell, the arm on the Walker Proscenium turntable (which is of similar design to the ET2) has a direct couple counterweight. Since the Walker is being regarded as one of the best, I was thinking of modifying the ET2 in such manner.
I wonder what are the pros and cons to the 2 different approaches. |
The Kuzma is unusual in its choice of bearing construction………… the use of porous material appears to be unique? In his review on the Kuzma Airline, Michael Fremer also mentioned that the Airline use porous material in the airbearing, while the older, but similar designed, Rockport 6000 utilized a groove-compensated bearing, which has lateral grooves in the bearing wall. He went on to comment that due to the difference in the bearing design, the much higher air-pressure used in the Airline didn’t necessary mean its bearing was stiffer. It is also interesting to note that the arm on the Walker Proscenium turntable shares a similar design with the ET2, but using a fixed counterweight and much higher pressure at 45psi. I believe Lloyd Walker is well known as a fanatic tweaker, who would not stop to squeeze out the last bit of performance from his turntable. So, why would he not use the decouple counterweight design (if he sees any advantage in it), or is that patented by ET? |
Hi Chris, Regarding the modified ET you posted, I believe it was made by a guy in China. I read an article probably 7-8 years ago, in which he detailed the process of the modification. Unfortunately I couldn't find that article anymore, but it was written in Chinese anyway. If I remember correctly, he was using it with the Micro 8000 turntable, and the black circular arm base was actually from a Technics 1200! He sold the ET and Micro later and manufactured his own turntable and air-bearing arm. You can see some photos of them here: http://www.hdavchina.com/html/72/n-2872-2.html I am not sure, but I don't think the brand exist anymore. |
Andy Payor and Bruce Thigpen both disagree with adding mass. Hi Dover, Regarding the passage you quoted Andy Payor from Michael Fremer’s review on the Rockport 6000, while I have never read that review, but reading MF’s later reviews on the Rockport Sirius III and Kuzma Airline, gave me the impression that those comment were from MF himself rather than AP. Of course, AP might very well agree with that! Can you confirm? About AP’s disagreement with adding mass, I would like to share some information. In the later, upgraded, models of the Rockport 6000, i.e. the 7000 and the arm on the Sirius III, AP chose to use heavier armtubes. I believe all 3 arms used the same air-bearing, and their armtube clamping systems are of the same (or very similar) total weight, but the clamping system on the Sirius III are said to be 20 times stiffer due to the improved design. The heavier tapered armtubes (so the heavier moving assembly) on the later arms are said to be there for better bass performance, and minimize resonance. Granted, AP might have added the weight to help (add) more in the vertical, rather than the horizontal, moving mass, for better matching with mid/low compliance cartridges, but added weight he did! That may just be a matter of compromise! |
The 6000's relatively low vertical effective mass put its resonant frequency above the ideal 8-12Hz region with moving-coil cartridges of average weight and typically low compliance, causing the bass to begin rolling off prematurely. Andy Payor solved the problem on the System III Sirius arm—see my analogsourcereviews/review in August 2000—by adjusting the mass so that the arm's fundamental resonant frequency would be compatible with a wider range of cartridges. Hi Dover, Thank you for taking the time to check! The above is a quote of MF from his review of the Kuzma Airline. By “adjusting the mass”, I am sure he meant “adding the mass”! Both the 6000 and 7000 also used carbon-fiber armtubes, but not as sophisticated as the one on the Sirius III. But then, I agree with you that the weight increase of the armtube would definitely be less than 10g, and most likely in the 3-5g range. Actually, by looking at the photos alone, it is hard to believe the massive armtube clamping system on the Sirius III has the same weight as that on the 6000. But, I have no reason to doubt AP’s claim! Sirius III6000Hi Ct, While I don’t have any really high compliance MM cartridges, I have no problem, “sound wise”, matching the 6000 to the relatively high compliance VDH Colibri! Yes, the combo is relatively lean in the bass/mid bass, but I consider it the character of the cartridge. Now, I say “sound wise”, because while the sound was fine, I (and also my friend who had the same combo) found the Colibri’s cantilever slightly off-centered after a period of time. This I took it as the fragile nature of the Colibri’s design. MF repeatedly stated the 6000 was relatively bass shy compared to the Sirius III and Kuzma Airline; I believe a major reason was that the stock pump just couldn’t supply enough pressure. As I mentioned before, I believe the 6000, 7000 and Sirius III shared the same bearing, but the 6000’s stock pump (probably due to cost constrains) could only supply a max pressure of around 11 psi, and without any surge tank. Once I upgraded it to a June Air compressor with integral surge tank, plus additional pressure regulators, supplying 34psi to the 6000, its bass weight and definition improved considerably! As I understand from the Rockport agent, the Sirius III’s arm also uses around 32-35 psi! |
