AUDIOCLINIC (Q and A) Sept. 1985

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Motor Noise in Portable Recorders

Q. I recently purchased a portable AM/FM radio and cassette recorder/ player combination. My problem with this unit is that the motor produces output from the loudspeakers. I have operated the equipment on batteries to see if what I was hearing was really 60Hz interference. It was not, so I must conclude that it is coming from the motor. How can I filter or otherwise remove this unwanted sound?

A. Motor noise is a problem with many small machines like yours because of the common power supply for the motor and the electronics. In some instances, one needs to filter the motor to a greater degree than what may have been provided by the maker.

We often find the motor connected across the power supply, with a filter capacitor connected across the windings of the motor. Thus, there are no time constants in the form of either series resistance or inductance, which, if present, could have isolated the motor from the rest of the package.

(These motors have brushes and commutators that produce interruptions in current flow, which may then create instantaneous voltage changes that cause the noise you hear in the loudspeakers.) I suggest that you disconnect the "high" motor lead, the one not connected to ground, even where this high lead is negative with respect to ground. Connect a resistor between the motor lead and the original point of connection to the rest of the system.

You may also want to connect a capacitor between the junction of the motor winding and the added resistor and ground, observing proper polarity. The value of the capacitor should be the same as that of the main bypass across the supply. As for the value of the resistor, keep it as low as possible, perhaps 5 ohms. A significant voltage drop across this resistor may adversely affect motor speed.

Filtering is not always the problem. Some sophisticated motors use servos to control their operating speed. If the motor contains a strange pair of wires other than those needed to actually turn the motor, these are the servo leads; there may well be a voltage present on them. An a.c. servo's signal generator can he considered an audio-frequency generator much like those used in many Hammond organs, so if these leads run too close to the playback circuitry or head leads, the servo signal may be reproduced in the loudspeakers. It is also possible that the servo control board itself is located so close to the playback components that the signal tone is induced. You may have to install a metal shield between the servo board and the rest of the circuitry. This shield must be grounded, and it must not be permitted to short out other circuit elements.

Blowing Fuses

Q. I have a mono power amplifier. I made the mistake of connecting a light organ to its output terminals, which caused the fuse in the power transformer's primary circuit to blow. I replaced the fuse, but it blows with each replacement and subsequent trial. I checked the power transistors; they were fine. I connected an ohmmeter to the power cord and measured 0.03 ohm d.c. resistance, which seems too low to me. The rest of the power supply was checked, and it appeared all right. It there something I have overlooked?

A. Because of the high power output of today's amplifiers, their power transformers must deliver sizable amounts of peak current. This means that the d.c. resistance of their primary windings is often very low. Even so, 0.03 ohm is low enough to be easily accounted for by just the resistance of the line cord plus that of the test leads which connect it to the ohmmeter. In any event, you cannot equate the d.c. resistance in the primary of a power transformer with that primary's impedance at 60 Hz. The latter governs the current to be drawn when a.c. voltage is applied to the transformer.

You cannot rule out the possibility of a short circuit in the primary of the transformer. To determine whether one exists, disconnect the secondary windings. Once again plug the amplifier into the wall socket and turn it on. If the fuse blows, this may be an indication that the primary is shorted or that there is a short in the secondary.

Before giving up and replacing the power transformer, physically check all of the electrical connections. Remember to unplug the light organ before making your checks, and don't forget any convenience outlets. It just could be that your trouble is a short in the light organ, which, under normal circumstances, should be capable of operating just as you had intended.

Output Transformers and Speaker Voice-Coil Protection

Q. Some amplifiers use output transformers. Does their use have a bearing on voice-coil protection in the event of a shorted output transistor? Also along these lines, fuses help to protect a loudspeaker from damage, but what about loss of damping be cause of their series resistance? Is this significant?

A. Output transformers used in some solid-state amplifiers will, in the event of output-transistor failure, bear the brunt of the short-circuit current resulting from a defective output transistor, thus sparing the loudspeakers.

Fuses will reduce the effective damping. (In my opinion, this loss of damping will not produce noticeable audio-quality degradation.) This small sacrifice is more than offset by the prevention of possible voice-coil damage in the event of a shorted output transistor.

RMS Power

Q. What is meant by the rms power output of an amplifier and the rms power-handling capacity of a loudspeaker system? Also, if a particular driver is rated at 30 watts music power, is this the same as 30 watts peak?

A. An "rms" power or power-handling rating refers to average, continuous power. (Technically, the letters stand for "root mean square," one way of calculating such averages.) An amplifier may be capable of producing a higher output for short times (peak, or music power); a loudspeaker may be able to withstand higher-than-average power for an instant (which is peak power, again).

(Source: Audio magazine, Sept. 1985, JOSEPH GIOVANELLI)

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