Audioclinic (Nov. 1983)

Range Expanders and Noise Reduction

Q. Would a good dynamic range expander provide as much reduction in high-frequency annoyances as a good noise filter? Does a range expander reduce audible rumble?

-Roger Ross; Peshastin, Wash.

A. A range expander is used for enhancing the dynamic range of suitable program sources. Because the device is single-ended (not involving the recording of the signal), it is really not designed to act as a noise reduction aid. But, as you will see, it can do so under some circumstances.

If you use the range expander and keep your loudest listening to its present level, the soft passages will be heard at a lower volume. This will mean that, along with the reduction in loudness of the softer passages, the noise will be lowered proportionately.

The problem is that most listeners want their loud music even louder, and the softer passages to remain at their present volume levels. This will not result in audible reduction of noise, including rumble. For this purpose we can consider that the rumble is a part of overall noise.

Channel Separation

Q. What is channel separation? How is it measured? What factors tend to degrade it?

-S. Untermyer; Lisle, Ill.

A. Let us assume that we are feeding a signal into only the left channel of a device. This signal will appear, of course, at the output corresponding to the left-channel input. Unfortunately, since nothing is perfect, some signal will also appear at the right-channel output. This unwanted signal is known as crosstalk. If the amount of signal appearing at the left-channel output is measured and compared to that measured at the right-channel output, we would obtain a ratio between these two signals, called separation, which we then express in dB. We try for the greatest possible channel separation. Experts have made studies, however, which tend to support the idea that 12-dB channel separation is all that is really required in order to produce flawless stereo.

What tends to degrade separation? This depends on the nature of the device we are considering. In a power amplifier, common coupling in the power supply is the likely cause of lost channel separation. In tape recorders, channel separation is limited by the degree to which the heads have been shielded against inductive and/or capacitive pickup between channels.

Phonograph systems are limited because of both the geometry and the elasticity of the grooves. Cartridges also degrade channel separation by the very nature of their mechanisms.

Both channels share a common stylus, and when it vibrates, its motion is not absolutely accurate. This will result in the translation of some energy into the "wrong" channel.

Regulated Power Supplies

Q. What are the advantages and disadvantages of a regulated power supply?

-B. A. Meichan; Toronto, Ont., Canada

A. In a power amplifier, the advantage of a regulated supply, is that the power delivered to an external load by each channel will be constant, regardless of whether one or both channels are driven. The amplifier will not have a higher output for a single channel than for that same channel with the second one driven. This means a more linear output curve can be realized, thus lowering distortion.

The question in my mind is that, once this has been achieved, can the listener determine a difference between the regulated and the unregulated system? I do not have an answer.

A disadvantage of regulated power supplies is inefficiency. In order for the regulator to operate, it must be fed by a voltage which is considerably higher than the voltage actually required. The difference between the unregulated and the regulated voltages can be used to calculate the power losses in the system. The power transformer must also be somewhat huskier for a regulated system. "Switching" power supplies eliminate such power losses and thereby improve the efficiency of the regulated power supply.

(adapted from Audio magazine, Nov. 1983; JOSEPH GIOVANELLI)

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