Audioclinic (Dec. 1977)

Voltage & Turntable Speed

Q: When I turn my air conditioner or any other appliance that draws high turn-on voltage, my turntable slows for an instant causing an audible "wow." I talked to an electrician about installing a separate line for my stereo equipment but he said that it would not make any difference. Could an electronic direct-drive turntable system solve the problem?

- J.F. Garofalo Jr., Chicago, III.

A: I wonder why your electrician says that a separate line would not help to stabilize the operating voltage in your turntable system. Unless the electrical service coming into your home is so poor that it cannot supply constant voltage under heavy transient loads, an extra line run right from the main box should do the trick nicely. If you have a good a.c. VOM meter, measure the voltage at the time the air 30 conditioner goes on. Make this measurement at both the high fidelity system wall outlet, and at the main electrical feed box. If the voltage changes to the same degree at both locations, a separate line will not help.

However, if only the wall outlet changes then a separate line will help.

Even a direct-drive turntable must operate within certain voltage limits.

When the operating voltage falls below these limits, then the turntable will either slow down considerably or, perhaps, come to a sudden stop. You will have to check with the manufacturer to find the voltage limits of his specific turntable.

The only other answer to your problem is to obtain a constant voltage transformer which is expensive.

Dynamic Expansion on Disc

Q: Is there any way to increase the dynamic range of records electronically?

-Thomas M. Lockwood, Duquesne, Pa.

A: The dynamic range of a record can be electronically expanded through use of a device known as an expander. Most amplifiers are fixed gain devices, however, with a dynamic range expander, as the volume of sound increases, the gain in the amplifier also increases. This causes an increase in dynamic contrast beyond what was originally present on the disc, although it will probably not duplicate the original dynamics present at the time the original master tape was produced.

Phono Motor A.C. Leakage

Q: With the platter and belt removed from my turntable I can detect a small vibration when I gently rotate the motor spindle with my fingers, indicating a slight current affecting the motor. While a DPST switch would eliminate this, is it necessary? Does this "problem" harm the turntable motor in any way?

-Dave Watts, Glen Ellyn, III.

A: It appears from your description that there is nothing wrong with your turntable. You are feeling either the individual magnetic poles as the armature passes them or, perhaps, some slight leakage from the power line. If it is the latter condition, unplugging the turntable from the wall will eliminate this condition.

However, if it does turn out to be power line leakage, measure between the prongs of the wall plug and the motor case with an ohm-meter to see if there is a low resistance path. Measure this with the motor switch turned on. If the resistance is below one megohm, this can mean excessive powerline leakage which may indicate either a defective line bypass capacitor or a slight short between the motor windings and the case. In the latter case, the turntable motor would have to be replaced.

Filter Capacitors

Q. What is the use or of what value are the filter capacitors in the amplifier power supplies?

-Rex Mills, Columbus, Oh.

A. A power amplifier must operate from a source of pure d.c. voltage, otherwise hum will be heard in the loudspeaker. The amount of hum will depend upon the purity of the d.c. voltage, in other words there must be no variation in the amplitude of the d.c. voltage.

The d.c. current is derived from the 117 V a.c. power line by means of a device known as a "rectifier." The voltage is d.c. but it varies in amplitude up and down many times a second and the purpose of the capacitor is to remove these rapid fluctuations in voltage.

Because of the large amount of current drawn by solid-state amplifiers, a considerable amount of capacitance is required to smooth out variations in the rectified d.c. voltage. In an effort to use less capacitance and maintain a constant voltage over a longer period of time, some manufacturers use special regulator circuits consisting of Zener diodes and transistors. In such circuits only a small amount of actual filtering, in the form of filters, is required because of the nature of regulator circuits.

dB and DynamicRange

Q: What is meant by a dynamic range of 120 dB?

-Thomas M. Lockwood, Duquesne, Pa.

A: A dynamic range of 120 dB means that there is a 120 dB difference be tween the lowest and highest volume produced by the performers. This would correspond to a sound range of where the softest components would just be audible and the loudest ones would be on the threshold of pain.

Sound System & Cracking Plaster

Q: I have recently gotten into high fidelity equipment and have already en countered a big problem. I knew, when choosing my equipment, not to get a powerful system for my small bed room, but even though this system has been installed for just a short while, I have already noticed cracks in two out side walls of the bedroom, although the two inside walls have no cracks. Is there anything I can do to prevent further cracking?

-Jeff McBride, Hickory, N.C.

A: I really doubt that your sound system is powerful enough to crack the outside walls of your bedroom. The cracking is more likely the result of such factors as settling of the house, old plaster work, dampness, or a combination of any or all of these factors.

(Audio magazine, Dec. 1977; Joseph Giovanelli)

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