Grooving on a Tangent - Harman Kardon Rabco ST-7 Turntable

After a week of my car being in the shop, and work getting in the way, I was finally able to get down to Point Roberts to pick up my most recent turntable acquisition.

This one is the most exotic yet, and will hopefully hold top spot for quite some time to come. Considering it cost me more than any table I have purchased recently I hope it does. Of course it seems for now to need nothing in the way of upgrades or maintenance as was the case for many of the other pieces of audio gear I've recently acquired. The fellow I bought it from on AudioKarma did a bunch of work on it already so there is little for me to do, except perhaps polishing some blemishes out of the lid which I am not only getting pretty good but it costs me mostly just time.



The turntable in question is a Harman Kardon Rabco ST-7 and has, as the owners manual says, a "straight line tracking system." Otherwise known as linear or tangential tracking this method of tonearm design essentially means the tonearm does not travel through an arc, but always stays at a tangent to the groove. In theory at least this should sound better with less potential for distortion.



The principal reason this should be better is that the stylus is always positioned in the groove in the same way that the record cutting lathe created the groove on the master discs. Doing this ensures the stylus reproduces the sound accurately. When a tonearm describes an arc as the vast majority do there is simply no way to have the stylus always at the correct angle to the groove. The best you can achieve is a compromise, though that can be a very good compromise and for most is more than adequate for good sound reproduction.

I've always been intrigued by linear tracking turntables but have never seen one I would either want to own or could afford. They only represent a very small percentage of the turntables made and in the shops I go to usually the only ones I ever see are the lowest end versions and usually in very bad shape.



I honestly am not sure how much better this type of turntable can sound for me. It eliminates what is know as "inner groove distortion" which is concern for conventional tonearms. In the inner grooves the difference between the angle of the stylus/tonearm to the groove is the greatest so there may be more audible effects or differences from the way the source material was added to the disc. I am not really that critical a listener, rarely sit in the sweet spot and really listen, and don't have the ultimate is system quality so...

In other words the potential differences are really quite subtle to most mortals. However even if I can't hear as much of a difference as some folks, the engineering behind this design also fascinates me.



There are various ways that the tonearm follows the groove. In the case of the Rabco ST-7 this is done by a secondary belt driving the tracking shaft. Riding on that shaft is a rubber tracking roller and based on the easily adjustable tracking angle the speed at which the cartridge moves across the record can be changed. Because there are varying lengths of music on record sides, there are not always the same number of spirals so the rate at which the stylus reaches the centre of the record will also change. Being able to adjust this angle can compensate for this variance.

From a maintenance standpoint this arm and drive system should need little work. The tracking roller is know to wear out or disintegrate but relatively easily found and modified O rings can be used in its place. The belts are also supposedly quite normal in size.



Installation and alignment of the cartridge can get tricky. Harman Kardon did sell these with a gauge that would come in very handy, as would another tonearm wand for easy swap of cartridges, so I will be keeping my eye open for those.

This one is well set up right now, so hopefully these issues will be sometime down the road. For the time being I will simply try to enjoy the sound and the Bauhaus like design.