Harman/Kardon Rabco ST -7 Turntable (Jan. 1977)

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The concept of tangential or straight line tracking for phono pickups has long appealed to engineers because of its many advantages, such as zero tracking error, lower tracking forces, and the fact that no anti-skating compensation is required. After all, records are cut this way, so it would seem a logical method for playback too. The Rabco straight-line turntable was introduced a number of years ago (the arm itself in 1968, if I'm not mistaken) and, although the design had merit, there were several minor problems.

Since Harman/Kardon took over the project, it is obvious that a great deal of engineering skill and money has been invested, and the present ST-7 is a tremendous improvement over the early models.

The motor of the current model is a d.c. "Hall effect," brushless type and coupled to the turntable by a urethane belt.


Above: A used H-K ST-7 can often be purchased for a reasonable price.

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MANUFACTURER'S SPECIFICATIONS

Speeds: 33-1/3 and 45 rpm.

Adjustment: ±5.5 percent.

Rumble:-68 dB (DIN B).

Wow & Flutter: 0.04 percent (NAB weighted).

Tracking Error: None.

Vertical & Lateral Friction: None.

Stylus Overhang: None.

Platter Weight: 2.4 lbs (1.1 kg).

Dimensions: 6.25 in. (17.1 cm) H x 16.5 in. (42 cm) W x 16.25 in. (41.3 cm) D w/dust cover.

Weight: 22.2 lbs. (10 kg).

Price: $430.00.

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Another belt is connected between the turntable spindle and the tracking shaft, which drives the tonearm via an ingenious roller mechanism that automatically adjusts for variations in the groove pitch (see Fig. 1). When the tonearm is tangential to the record groove, the tracking roller is biased at an angle so that the carriage travels towards the center of the platter at approximately 0.17 in. per minute, corresponding to the average stylus velocity for cutting the groove on a record master disc. As the tonearm attempts to pivot in angle to track pitch variations, the angle of the tracking roller axis changes accordingly. This change in the tracking roller direction accelerates or decelerates the motion of the carriage to track the pitch of the groove. (Pitch, in this context, relates to the groove spacing, which is automatically increased when the master is cut to allow for heavily modulated signals that would otherwise interfere with adjacent grooves.) The tonearm has a counterweight at the rear, and the tracking force is set by a small sliding weight located at the front. Calibration markings are from 0 to 3 grams and the maximum cartridge weight is stated to be 15.5 grams with the heavy counterweight supplied. Parallel with the tonearm, and just to the right of it, is a control arm which is used to manually position the tonearm itself. A small bar on the front acts as a tonearm restraint when it is in the upper position. The cue lever which raises or lowers the arm is located at the right-almost on the edge of the unit and a dashpot mechanism slows the downward movement. The tonearm with its control arm only begins to travel along the rotating bar when the stylus is in the record groove, and vertical friction is cancelled by a rolamite bearing with counter-rotating bands. Since there are no relative rotational forces between the pickup stylus and the arm, there are no friction problems to contend with here. At the end of a record, the photocell operates a switch which turns the motor off and causes the solenoid to lift the tonearm from the record.

Speed is electronically controlled, and at the left front there are two fine-speed controls with a strobe window. On the right is a touchbar using finger-operated contacts-a light touch is all that is necessary to select 33-1/3, 45 rpm, or to switch the motor off. A transistor sensing circuit is employed, and each function has a separate indicator light-blue for 45, green for 33-1/3, and red for off. The off mode applies to the motor only, and in order to switch off the electronics, it is necessary to use the manual switch underneath the front panel.

The turntable comes with mounting hardware for the phono cartridge, including several shims. Detailed instructions are given in the well-written manual which comes with a gauge to help obtain accurate alignment. The tonearm is adjustable in length, and after the cartridge is mounted and the arm length set, it can be balanced in the usual way with the tracking weight moved to the required position. As there is no anti-skating device, the tracking force can be set somewhat lower, a decided advantage.

For the initial tests, I used one of the new Goldring 900 SE cartridges which needs 1.25” to 1.5” grams with high quality arms, but it was perfectly happy with one gram on the ST-7.

The first test was for wow and flutter, and the combined weighted figure was 0.03 percent, which was excellent.

Rumble measured-63 dB using the ARLL weighting which is roughly equivalent to the-68 dB claimed with the DIN B weighting. Speed variation was ±5 percent and was not affected by voltage line variations. Full speed was reached in less than a second-a tribute to the "Hall effect" motor and drive system.

Cable capacity is 75 pF to suit CD-4 cartridges, and the instructions suggest that extra capacitors be used to make up the manufacturer's recommended values and then connected to the proper terminals, if necessary.

Listening Tests

The ST-7 was connected to my audio system (Sony 2000 and Soundcraftsmen PE 2217 preamps, a Phase Linear 400 amplifier, and a two dynamic-electrostatic hybrid speaker systems) and used over a period of several weeks. In order to confirm that the theoretical reduction in distortion was audible, it would be necessary to have two absolutely identical cartridges and two identical records, but I did make some A/B comparisons using a Shure V-15 III and an ADC XLM cartridge in SME and Thorens tonearms, changing them around with the 900 SE in the ST-7. On occasion I could detect a slightly cleaner sound on the inner grooves of the record, but where ST-7 really scored was the way it played warped records. Some of these, especially CD-4 discs used in later tests (with the Shure M24H cartridge), were virtually unplayable on ordinary turntables, yet the ST-7 played them with ease.


Fig. 1--Showing how changes in the groove pitch or spacing are compensated for automatically.

The touchbar worked like a charm, and I must add that the unit was a real pleasure to use. Styling is very much a matter of personal preference, but most people who saw the unit were impressed by the clean lines and almost clinical appearance (I should mention that the turntable is made of satin-finished aluminum with matte-black fittings). Obviously, the ST-7 deserves a really top-quality phono cartridge for the best results, and it deserves more careful handling than some other turntables.

At $430.00 [late 1976 price], the ST-7 is not at all cheap, but it is worthy of consideration by the most serious audiophiles and music lovers who want the best. I noted that the engineers and designers responsible for the turntable have their names inscribed on a plate underneath the unit-they can be proud of their achievement.

(Source: Audio magazine, Jan. 1977; George W. Tillett)

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