Audioclinic (Nov. 1977)

Volume Expanders

Q. I recently purchased a dynamic range processor which can expand the program source from 6 to 14 dB. At the 14 dB gain, the processor not only expands the signal level to a level of 14 dB, but also boosts the volume to where it seems twice as loud as normal. With this large gain in volume, is my amplifier delivering more power at a lower volume setting than it would without the processor?

-Steve Roginski, Hazlet, N.J.

A. Whether your amp is delivering more power with the expander engaged than with it out depends on the particular circuitry of the expander. Some expanders work downwards, making low level signals even lower, while others only work upwards, expanding only above a certain sensitivity. If yours is one of the latter, then your amp will deliver more power. If it's one of the former, then it depends on the setting of the unit's sensitivity control, that is whether it's been set to the middle of the range of incoming signal voltages.

Incidentally, 14 dB more signal coming into an amp is quite a bit more. If the amp has been running close to its limits previously and a hot spike comes along, the amp will be overdriven, resulting in poor sound quality and possible damage to the amp and/or speakers.

Passive and Active Networks

Q. What is the meaning of a "passive crossover" and an "active crossover" as applied to loudspeakers?

-Eli Sammett, Fresh Meadows, N.Y.

A passive network is a set of components that do not contain devices to amplify the signal. Components that might be found in such networks would be inductors, capacitors, resistors, and diodes. In the case of a crossover network used in a loudspeaker system, usually one finds inductors and capacitors, with resistors only used occasionally.

An active network may contain any or all of the above components, and in addition there may also be vacuum tubes, transistors, and/or integrated circuits which are capable of amplifying signals. This also, implies that feedback can be used, and the nature of the resulting response can be shaped by the passive components used in conjunction with the active ones.

Speaker Hum

Q. I own a pair of electrostatic loudspeakers which are placed in a small room, and the 60 Hz hum I hear while the speakers are plugged into a.c. is quite obnoxious. Is there anything that can be done short of disconnecting the speakers? The manufacturer recommends that they remain plugged in at all times. Is this the normal situation with electrostatic speakers? Name withheld by request.

A. The fact that you hear hum in your electrostatic loudspeakers doesn't convey a complete set of information. Is the hum present even when the amplifier is turned off? If the hum is still present, then we know that it has something to do with the speaker system.

Could it be that the hum is mechanical in nature, caused by vibrations of the power transformer laminations in one or both speakers? This condition might be made less audible by shock mounting the speakers so they cannot use the floor oh which they sit as a sounding board.

The polarizing voltage which makes the electrostatics operate must be filtered, just as any other d.c. voltage must be filtered. These filters deteriorate with age, and this causes some hum in the speaker system.

If the hum isn't present except when tile amplifier is turned on, then the hum is not coming from the speakers themselves, but is associated with problems with the amplifier which can be the result from defective filtering (again), hum associated with grounding, or other problems.

If you have a problem or question on audio, write to Mr. Joseph Giovanelli, at AUDIO, 401 North Broad Street, Philadelphia, Pa. 19108. All letters are answered. Please enclose a stamped, self-addressed envelope.

(Source: Audio magazine, Nov. 1977; Joseph Giovanelli)

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