I think you have already made up your mind. Besides there aren't a lot fans about the JMW 9 (the 9 inch one in particular.) I spent a lot of money chasing down a linear trackin table because a friend had a modded Technics that sounded very good. In the end these real cheapo arms just didn't sound that great. A good linear tracking arm has been implemented in some of the worlds top turntables. If were you I would get the Rabco and try it out. The resale shouldn't be terribly hard. Sate your curiosity and then move on or stay with it if you love the Rabco sound and reliability. |
If the Rabco was better then most of us would have one. Besides there would be probable MK II, MK III, etc. versions as is the case with Graham and Triplanar MK VII. |
I owned one in the early seventies running on a Techniques SP10 turntable. It was a fun toy, but the mechanism made so much noise that even at 15ft. from the arm, I could her the chain drive motor when it was activated to move the arm and the bead chain noise. it uses the same type of bead chain as you have on your house keys (real high tech).
Also there is a mechanical switch which has to make contact in order for the arm motor to advance. This obviously presents a lateral force against the needle in the cartridge since this switch has to make contact, thus defeating the reason to have a linear tracking arm. Because of this, it has a small amount of distortion as the arm is pressed against this switch before it's closed, but the needle is trying to move the arm without success. Also, there's always a small amount of tracking angle offset because the arm does not move continuously. It moved only when the angle of the arm is sufficient to activate and close the motor switch. This, according to the manual, is "only" about 1/6th degree of arm movement.
Aside from these issues, the actual arm is just a U shaped bent piece of metal which I can imagine has a resonant frequency well within the audio range. This arm is ompletely un-elegant in design and funtionality.
Rube-Goldberg would have been proud of this contraption.
BTW, you can Google the arm and find the manuals and several pictures.
Fortunately I sold it after several months of screwing with it trying to be more responsive to the mechanical actuator switch. |
The Rabco is servo controlled, operating on error correction logic, which means the cartridge crabs and wriggles sideways across the record. The cartridge tracking is like digital - a little bit out ALL of the time.
Personally I wouldn't subject my cartridge or records to this barbaric form of tracking.
If you want to go linear tracking, then find a used Eminent Technology ET2 or Transfi Terminator - they use air bearings. |
Agree in general with Nandric, Dover, and Notbananas. There is a parallel thread on Vinyl Asylum with some interesting comments from David Shreve, who apparently can significantly upgrade the performance of the Rabco, at some expense to the owner, and if he is still willing to do the work. So, maybe to your question of whether to choose the 9-inch VPI vs the stock Rabco SL8E, the best answer is "neither". |
I ran one of these for years. But before I could do it, I had to fix the servo. Once that is done, not only is it reliable, but the motor noise is reduced and the arm keeps up with the LP properly.
This is done by adding a timing constant to the servo that allows the motor to pick the right speed so its always on target.
The bigger problems are these:
* lack of precision in the track, some of which can be corrected by replacing the wheels with something more substantial.
* Mechanical resonance in the track, solved by damping compounds
* really poor bearings in the arm! They are cup and point, but not hard enough to hold up right.
* poor arm tube rigidity. I replaced mine with a carbon fibre wand...
* bearings above the plane of the LP (a common engineering bug to this day). This is solvable but would take some tinkering... |
If anyone is interested, here is a link to a detailed history of linear tracking arm designs and manufaturers, including the Rabco:
http://www.soundfountain.com/amb/ttrabco.html
.
.
.
.
. |
^^ Thanks for that!
I used a simple opamp (FET input) to solve the servo issues. As long as the contacts made a connection of less than 1 Meg ohm, it worked fine. The reduced current through the contacts got rid of contact noise as well. Later I sorted out that if the opamp had a cap in the feedback network, by being careful of the value of the cap the motor would come on slowly and turn off slowly- in effect it would find a speed of the groove cut and settle in on it, thus reducing the tracking error even more, and also eliminating the motor noise.
I ran the opamp off of a separate supply (a 12Volt wall-wart). The opamp drove a transistor that actually turned the motor on and off. Once it was all set up correctly, it was quite reliable and low noise.
I've often thought about revisiting the concept using modern materials- in particular there are motion tracks now that have zero slop in the bearings. Such parts are costly compared to the original arm but nothing compared to modern arms. |
I agree that the Rabco SL-E8 is "primitive" by modern standards, but Rube Goldbergian it is not. My Rabco (though heavily modified by Steve Katz—does anyone know where he is these days?) has operated faultlessly for 35 years! More modern designs have often suffered from reliability problems (ask any Goldmund owner). Theoretically, I agree that air-bearing arms are superior, but they have their own idiosyncrasies, such as noisy pumps and moisture buildup.
Probably the Kuzma is the closest modern analog to the old Rabco, though at a cost of $11,000 (the same current price of the Air Tanagent). By contrast, my Steve Katz-modified Rabco sold for the then outrageous price of $400 in 1977 ($1551 in current dollars). Of course, that inflation is characteristic of most high-end audio products these days.
RonLev
Philly |
Nandric says: If the Rabco was better, then most of us would have one. Besides there would be probable MK II, MK III, etc. versions as is the case with Graham and Triplanar MK VII.
Well, Nandric, the earliest Rabco was the SL-8 and the mk2 was the SL-8E. There was no Mk3 and Mk4, etc., because Rabco as an enterprise ceased to exist before a Mk3, etc. was designed. On the request of Harman Kardon they designed arms to be integrated in Harman Kardon turntables. Did you try a Goldmund Mk7?
Sorry for my late reply. |
