Audioclinic (Jun. 1979)

Turntable Hum

Q. I'm having difficulty in eliminating a loud hum from my turntable which is connected to the receiver. My receiver is grounded to the wall outlet and the ground lead on the turntable is connected to the receiver ground. I have a timer plugged into one outlet, and my receiver is also plugged into a clock. What can be done to remedy this hum?

- O.B. Madenwald, Hines, Or.

A. The interconnecting cable between the turntable and the receiver must be short and direct and not run near a.c. lines. If you have harnessed this line among many other cables, including a.c. lines, hum is sure to occur.

The location of the turntable itself is somewhat critical. Components having coils or power transformers must be kept well away from the phono cartridge. Is the receiver or tape deck mounted directly above or below the turntable? If so, you will likely have hum. The hum may vary as the phono cartridge is moved over the surface of the disc. When this occurs, you will know that the cartridge is picking up stray magnetic fields, and it must be moved away from the presence of such fields.

Your timer also has a motor which must be kept away from the turntable as it is not shielded and is certain to radiate stray hum fields.

Turntable Drive Systems

Q. Much is mentioned in audio literature about the design parameters used in turntables. Manufacturers are always boasting about either their belt- or direct-drive systems. Can you elaborate, specifically, on the drive system/rotor assembly?

- Thomas Maguire, Bronx, N.Y.

A. Motor rotors must be kept well balanced if vibration is to be kept to a minimum. This is true regardless of other considerations such as belt drive or direct drive. There is always vibration from a motor, especially when that motor must turn at a relatively high speed, as with a belt-driven motor. However, if the motor is properly shock-mounted, then the belt it self serves as a good isolator between the motor shaft and the turntable. The shaft must be as "true" as possible, otherwise it will cause the belt to vibrate and this vibration will be transmitted to the turntable and then into the phono cartridge as rumble.

The flywheel action of the platter will prevent these shaft eccentricities from producing "flutter." Properly designed and manufactured, belt-driven turn tables are excellent.

Direct-drive turntables are more complicated in their design, as the motors used with them must turn at the same speed as the platter. In this design, precautions must be taken to avoid having the motor hesitate be tween successive magnetic poles. This hesitation, if transmitted to the platter, takes the form of flutter and rumble.

Because of the slow rotational speed of such motors, vibration is very low; and this is essential because of the direct coupling between the motor and the platter. When properly constructed, these turntables function very well indeed.

Pre-Preamp Uses

Q. Do pre-preamplifiers and/or step-up transformers have other uses besides their applications with moving-coil cartridges?

-S. Campanile, Pleasant Hills, Cal.

A. The only other possible use for either a pre-preamplifier or a step-up transformer is to boost the signal on experimental ribbon microphones, whose impedance is also very low.

Outdoor Antenna Noise

Q. In the metropolitan area where I live there are many FM stations and nearby hills cause multipath interference on some stations and block reception on others. I am using a directional outdoor FM antenna mounted 25 feet above ground level connected to my receiver with a coaxial cable.

However, when using the outdoor antenna, I hear a crackling noise on medium strength and weak stations. Switching to a simple, indoor dipole usually eliminates the crackling, but increases background hiss. What causes the crackling noise? What kind of lead-in cable is best, coaxial or shielded twin-lead? Would raising the antenna higher help?

-Bill Hatchell; Hayward, Cal.

A. Some improvement can be obtained by stacking antennas. You may be able to use two or perhaps four directional antennas to make the directional pattern sharper, minimizing the pickup of reflections from directions other than those from the de sired signal.

It sometimes works out that by pointing the antenna in a direction other than that of the desired signal, you can pick up a reflection of the de sired signal better and with less multi-path distortion, rather than pointing the antenna directly at the transmit ting tower.

In severe multipath problems the use of a coaxial cable is better than a shielded twin-lead. The shielding of the coaxial cable is more effective, thereby eliminating signal pickup by the cable itself.

There is a chance that the noise you hear is the result of your antenna being located too close to a power line or some industrial plant. The indoor antenna is, perhaps, further away from the power line, hence, less likely to pick up interference from it.

Raising the antenna might allow it to clear power lines, thus lessening the crackling noise and possibly helping to reduce multipath distortion. Also, moving the antenna to a new location can help. Just a small change in antenna position can often produce a situation where reflections are less, with an attendant reduction in multipath distortion.

Low-Impedance Loads

Q. My amplifier is rated at 30 W per channel, and my two pairs of speakers (one rated at 8 ohms and the other at 4 ohms) are connected to the main and remote speaker terminals. The sound, this way, is better than with either pair alone. I figure that the total impedance is 2.5 ohms. Does this constitute a danger to the amplifier?

-Albert Sadler, San Diego, Cal.

A. If the amplifier manufacturer states that you can run it with loads of less than 4 ohms, then there will be no danger to your amplifier. However, if the instructions do not state this, then you run the danger of damaging your amplifier.

(Source: Audio magazine, Jun. 1979; Joseph Giovanelli )

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