Audioclinic (Q and A) (Dec. 1973)

by Joseph Giovanelli

Phono Cable Capacitance

Q. How can one know if he has the proper phono input capacitance? My turntable has four-foot cables.-Name withheld.

A. Unless you can measure the capacitance of the cables or can measure the frequency response of the cartridge/ tonearm combination, you cannot know the capacitance of your phonograph.

Even though you know the length of cable involved, this information is not sufficient. Shielded cable is rated as having a certain amount of capacitance per foot.

Assuming that you cannot make the necessary electrical measurements, write to the manufacturer of your phonograph or tonearm and ask for the capacitance of the interconnecting cables. Also write to the manufacturer of your cartridge for his recommendations as to the optimum cable capacitance which should be used with that cartridge.

Static from an Amplifier

Q. I have a problem with my receiver and speakers. I get loud static from one speaker after the set has been operating for about five minutes. I interchanged the speakers at the receiver end of the cables. This change resulted in the static going to the other speaker. I changed speaker connectors and I also hooked the speakers up to each of the other two speaker outputs with no success.

Now. I have added a set of small, cheap, bookcase speakers. I hooked them up to the second set of speaker terminals. When playing both speaker systems at the same time, the static virtually disappears. When I play either of the speaker systems by itself however, static returns.

Any suggestions?

-Leonard J. Vieira, MSGT, USAF, Jacksonville, Arkansas

A. From the tests you have performed on your equipment, it is obvious that the "static" problem lies with your amplifier. The only conclusion I have reached is that the feedback loop is involved. When more than one speaker is loading down the output, feedback is reduced. Hence, the "static" is reduced. Perhaps there is too much feedback, causing the amplifier to be unstable.

In addition to this problem the capacitance involved with high frequency roll-off may be defective.

Microphone Phasing

Q. Is there a standard phasing of microphones as regards pins 2 and 3 of a Cannon XLR connector?

-Harvey W. Kunz, Old Tappan, New Jersey

A. There is a proposed standard for mic phasing, but not all companies use it. Pin 2 of a Cannon XLR connector will be positive when the diaphragm is compressed. There is no way for you, however, to compress a mic diaphragm in order to check this out. If the instruction manual accompanying your microphone states that wiring your mic as described therein results in adherence to the "standard," you can then phase all of your other mics to that one. If you have a mixer, the "standard" mic can be connected to one input, and a mic having unknown phase connected to another input of the mixer. Raise the gain of the "standard" mic to a point where you receive a significant reading on a VU meter when speaking into it. The mic whose phasing is not known is held next to the "standard" one. You now raise the gain of the "unknown" and see if the output tends to fall off as you raise the gain. If it does, this mic you are checking is out of phase with respect to the "standard" mic. If, instead, the signal constantly increases, the two mics are in phase.

It is more important to have all of your mics in phase than to have them adhere to a standard. Therefore, if you do not have a mic known to conform to the "standard," take any mic at random and consider it to be the "known" one. Then check phase from this mic to all others. If you have a number of mics, make phase comparisons at random. If you find that most of your mics are in phase, but a few are not, consider that the group of mics having the greatest number in phase will be YOUR standard. The few which are of the opposite phase can then be wired to conform to the majority. As you buy more mics, check them out for phasing.

Be sure that all of your extension cables are correctly wired, or you could well have misleading results. An Ohmmeter check will determine the correctness of these cables.

Light and Heavy Tracking Force

Q. Can you tell me the advantages or disadvantages of higher or lower stylus force?

-Jim Spellmeyer, Affton, Missouri

A. The lower the tracking force the better, up to a point. The more force which is required, the greater the amount of wear on a record. The record must move the stylus back and forth rapidly. As tracking force increases, the disc must do more work in order to produce this stylus motion.

Further, the friction of the stylus against the groove walls will increase. This friction will add to the wear problem.

There are many factors which can influence the minimum recommended tracking force of a cartridge. It would be difficult to specify all of them. The stylus assembly must be flexible, readily movable by the undulations in the grooves. At the same time the assembly must be rigid enough to withstand reasonably rough handling.

These two factors are mutually exclusive. The mass of the tonearm is also involved. If the arm is too massive, more force will be required to push it along, with the result that more tracking force must be used. The friction of the bearings of the tonearm must be taken into account. To some degree, the amount of modulation impressed on the disc determines the minimum force which can be used. Thus, while there is an advantage in tracking a disc at its minimum practical force, it is possible to track a disc too lightly.

When tracking too lightly, contact between the stylus and groove walls will be intermittent. The stylus will slam into the walls. Wear will be added over what would occur when using heavier tracking force, once the force has fallen below this minimum.

It is not difficult to tell when tracking is too light. The distortion is obvious, especially when listening to loud musical passages.

There are various aids to help you track your discs as lightly as possible, but still be above the minimum which will keep groove contact from becoming erratic. The manufacturer of your tone arm and of your cartridge will provide some general guide lines. Force gauges are available which will allow you to measure the force. There are some test discs which are designed to let you know when you are tracking too lightly.

Note I just want to take this opportunity to wish each of you the most joyous of Holiday Greetings.

(Audio magazine, Dec. 1973)

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