Q. How much channel separation is really necessary for stereo pick-ups? I have seen glowing reviews of certain pick-ups which had little if any separation above 10 kHz.
A. Separation should not be less than 15 dB at mid-frequencies, falling at each end of the band as shown in Fig. 1.
Any improvement above say 20 dB at 1 kHz and 10 dB at 12 kHz would be difficult to hear!
Q. What is meant by "tracing distortion?"
A. This is the distortion caused by the fact that the recording cutter head is sharp, chisel-shaped, and the reproducing stylus is round or spherical. Obviously, it cannot be chisel-shaped or it would tear up the record! Figure 2 shows how the stylus path is distorted by the shape of the groove wall. Even if the stylus was small enough to fit into the bottom of the groove at B, it would still not follow the groove precisely.
Tracing distortion is not normally significant except at high recording velocities or near the record center, where the groove curvature increases (see Fig. 3).
Q. Is an elliptical stylus better than a conical?
A. It all depends.... An elliptical or, more correctly, a bi-radial stylus is certainly nearer the shape of the cutter than a conical type but tracking force has to be less and the increased mass of a bi-radial could even nullify any gains! It goes without saying that bi radial styli must be very carefully made and polished or increased record wear will occur. Bi-radial styli are made in sizes from 0.7 to 0.9 mils in the large dimension and 0.2 to 0.4 mils in the smaller. In practice, a lot depends on the record itself-whether Dynagroove or some other kind of groove compensation is used-and the particular cartridge and tonearm. Some time ago, ADC released a cartridge with three styli-a conical and two bi-radials. A quick check with four users was very interesting-two said they left the conical stylus in, two stayed with the medium bi-radial. All four said the differences between styli were quite small.
Q. Does a pick-up have to be matched to an amplifier?
A. Yes, within certain limits. Fortunately, most amplifiers now use a standard input resistance of 47 K which matches the great majority of cartridges.
The capacity of the screened input leads can adversely affect the matching by rolling-off the high frequencies. Figure 4 shows the effect of different capacitances on a typical phono cartridge (ADC XLM).
Q. What does skating force mean?
A. Skating force can be defined as the force that draws the tonearm towards the center of the record. It is neutralized in top quality tone arms by the use of tiny suspended weights or a magnetic counterforce. In the days when the effects of skating force were greatly exaggerated, many enthusiasts used to tilt the motorboards to give a compensating bias.
Q. What is a Shibata stylus?
A. This is a kind of bi-radial stylus specially developed by the Japanese Victor Company for use with their discrete four-channel records. As can be seen in Fig. 6, it makes better con tact with the record grooves and is capable of better high frequency response with lower tracing distortion.
Naturally it can be used to advantage with ordinary stereo discs and we will see and hear more about the Shibata stylus in the near future. Only snag at the moment is the relatively high cost of manufacture.
Q. How is the output of a phono cartridge rated; in other words, what does 1.4 mV per cm/sec mean?
A. There is no standard rating method but the most commonly used measurement refers the output to a 1 kHz signal which is recorded at a velocity of 1 centimeter per second. Thus, a pick-up with a rated output of 1.4 cm/sec would produce an output of 7 millivolts from a 5 cm/sec signal (5 x 1.4) and 28 millivolts from a 20 cm/sec signal. Highest velocity normally recorded is about 30 cm/sec. The CBS standard test record has a 1 kHz signal recorded at 3.54 cm/sec-which means the output figure has to be divided by 3.54 to arrive at the 1 cm figure.
Q. What does compliance mean?
A. Compliance is the opposite of "stiffness" and can be defined as the distance of movement caused by a certain force acting against a spring or mechanical resistance. In phono cartridges, the distance is expressed in centimeters and the force is measured in dynes (1 dyne equals one-thousandth of a gram). The forces involved are microscopic so compliance is stated in millionths of a centimeter (10^-6) per dyne. Thus a typical stereo cartridge might have a compliance of 15 x 10^-6 dynes/cm with a top quality model measuring 35 x 10^-6--which is near the upper limit. At one time it was thought that the higher the compliance figure, the better, but this particular numbers race soon came to an end when it was realized that other features, such as tip mass, were more important and a high compliance figure did not necessarily indicate a good pick-up. For this reason, plus the fact that the methods of measurement vary widely, compliance figures are rarely quoted these days.
Fig. 1--Showing minimum separation for a good stereo image.
Fig. 2--How tracing distortion is caused. A-A is the stylus path.
Fig. 3--Tracing distortion at different velocities, frequencies, and record
diameters. Stylus size: 0.6 mils. (after M. S. Corrington, "Tracing
Distortion in Phonograph Records," RCA Review, June, 1949.)
Fig. 4-Showing effect of lead capacity.
Fig. 5-Anti-skating device as used in the SME arm.
Fig. 6--Shibata stylus.
------- JVC 4MD-20X
(Audio magazine, Aug. 1972)
Also see:
A New Concept in Diamond Syli (Aug. 1972)
Understanding Phono Cartridges (Mar. 1979)
A Practical High-Frequency Trackability Test for Phono Pick-Ups (Aug. 1972)