AUDIOCLINIC (Q and A) (Sept. 1976)

Phonograph Cartridges and Cable Capacitance

Q. Some cartridges appear to be particularly sensitive to the amount of shunt capacitance into which they are connected. I am using a Shure V-15 III, which requires "400-500 pF total capacitance per channel" for optimum results. I understand that this total is made up of the capacitance with in the arm and interconnecting cables, as well as that in the preamp circuitry. I know that the value of capacitance for my AR arm and connecting cables is 145 pF per channel.

Is there a way by which I can deter mine the shunt capacitance my preamplifier presents to the cartridge utilizing either a VOM or a VTVM? I have no other test equipment.

-Dr. Leonard Drasin, Liberty, N. Y.

A. With the test equipment you have on hand, there is no way I can think of whereby you can determine the amount of capacitance in the phonograph input section of your preamplifier. This capacitance value will be only a few pF at most, how ever, and, for your purposes, can be ignored. You can consider the capacitance of the leads within your tonearm and that of the interconnecting cables to be the total amount of capacitance your cartridge "sees."

The additional capacitance needed in your case--using your figures--will be about 250 pF per channel. This will provide you with 400 pF. This additional amount of capacitance can be added by obtaining the correct capacitors and soldering them into the preamplifier. One capacitor will be required for each channel. It is wired between the "hot" and ground terminals of the phono input connector.

An alternative solution is to obtain a small metal box. Mount appropriate input and output connectors on it, so arranged that the box can be connected between the preamplifier in put and the phono cables. Such a de vice will eliminate the need to make internal modifications to your equipment.

The input and output connectors on the box are wired "straight-through," so that when the box is added, the phonograph will operate as before. The next step is to wire the appropriate capacitor values between the "hot" and ground terminals of the input connector--just as was done for the preamplifier.

The use of this little box will probably require the use of some additional cable, which will have capacitance of its own and which must be taken into account when calculating the values to be wired into the box.

Tuning Accuracy of FM Receivers

Q. A tuner whose frequency is determined by a crystal-controlled oscillator is precisely tuned to the de sired frequency. This is fine, but is it not possible that the desired station is not broadcasting precisely on its as signed frequency? Can an FM station control its broadcast frequency accurately enough to match the frequencies tuned in by a crystal-controlled oscillator? If there is an error in the broadcast station's frequency, it follows that a receiver tuned with more conventional means but equipped with an accurate tuning meter can be adjusted more accurately than a crystal-controlled tuner. Is this correct?

-Dr. Leonard Drasin, Liberty, N.Y.

A. An FM station must broadcast on its assigned frequency with an extremely high degree of accuracy.

Therefore, any crystal-controlled tuner whose crystals are properly trimmed and whose i.f. and detector systems are properly centered, must be properly tuned. In any case, slight inaccuracies of tuning cannot be normally detected, especially with today's wide-band detectors.

Even a tuning meter will have a certain amount of offset, in that it is not a completely accurate device--right down to the cycle. Fortunately, such accuracy is not required.

(Audio magazine, Sept. 1976, JOSEPH GIOVANELLI)

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