AUDIOCLINIC (Oct. 1987)

Concealed Speaker Wiring

Q. I am in the process of re-modeling my den. As part of the project, I plan to install speaker wiring in the walls. What is the best wiring for me to install? My system is 150 watts per channel and I wish to accommodate any reasonable future upgrades. The longest wiring run between amplifier and loudspeaker is about 50 feet. Also, I wish to install some type of jacks on a wall plate. What type should I choose? Where can I purchase them?

-David Michael Hudson; Dallas, Tex.

A. I would use 12-gauge wire, which can be of solid copper, the kind used for house wiring. (I would not recommend this wire if a wall plate were not to be used.) As for the jacks, I would use Hubbell twist sockets, whose mating plugs would be used to attach the speakers to the new wiring. These twist-socket/ plug sets are similar to the three-prong plugs used for common household wiring, except that the prongs are all curved rather than flat, making it impossible to plug a loudspeaker into the power line. The contact surfaces between socket and plug are very large, minimizing d.c. resistance and consequent loss of damping.

The boxes on which the sockets are mounted could be standard electrical boxes recessed into the walls. If appearance is not a problem, it is perhaps easier to use a surface-mounted box. Be sure to obtain cover plates that will fit the boxes you select and accommodate the sockets you use.

All of the materials I have discussed here can be obtained from a good electrical supply house.

This arrangement is great for loudspeakers wired in parallel. A series hookup, however, could present a challenge as to how to arrange the wiring. (Such a hookup would be required only if you planned to use multiple loudspeakers for each channel and if the loudspeakers would represent too low an impedance when connected in parallel.) No matter what you plan, there is always the likelihood that you will want to make changes. Because of the semi-permanent nature of concealed wiring, be sure to plan now for several sockets located in various parts of the room, with a similar number allocated for each channel. Use colored wires in order to keep track of phasing. To make addition or retrieval of wires easier, you might wish to consider using a conduit of some sort which can guide the wires. If you use no conduit, be sure to leave your "messenger" (the string or wire used to pull your wiring along) in the wall for later use. Leave the ends in a couple of the boxes.

Amp/Preamp Mismatch

Q. My preamplifier is designed to feed a minimum load of 47 kilohms.

The impedance of my power amplifier is 22 kilohms. Will this mismatch affect the performance of the overall system? If so, can I compensate for the problem?

-Jeff Samich; Malverne, N.Y.

A. The only possible problem that I can see is a slight loss of the extreme low bass. If this does not take place or isn't noticeable, don't worry about the mismatch.

If you wish to correct the situation, double the value of each of the output coupling capacitors in the preamplifier. If there are input coupling capacitors in the power amplifier, you may wish to jump them out of the circuit. All should be fine. If your power amplifier is solid-state, be sure not to short the input to ground once you have bypassed the capacitors.

CDs and Stepped Waveforms

Q. I know that CDs can have signals 90 dB below zero level. Such a signal is made up of just four bits and has a step-like look to it. Even with noise added, signals at this level still don't look like sine waves. How can a CD produce good sound with such step like waveforms? What is the lowest signal level which would produce good sound?

-Name withheld

A. All digital recording/reproducing systems that I can imagine consist of stepped waveforms, but those steps are smoothed by the filter which removes the reproducer's sampling frequency from the analog output waveform. It's true that, since fewer bits are used to encode low-amplitude signals, the smoothed-out waveform will be a less accurate model of the original when the signal amplitude is low than when it's high and more bits are available. This is why noise and distortion, our main measures of signal inaccuracy, are higher in digital systems at low signal amplitudes.

If you could listen to a sine wave that's been recorded onto CD at a very low level, the playback would not quite sound like the original. In order to notice this difference, however, one would have to advance the volume control to a point where the loud passages would be deafening. The only way I discovered this was to listen, at rather high volume, to a test tone meant for virtual inaudibility. Certainly at most signal levels, the departure from a sine wave would not be audible.

Even at its worst, I don't believe the departure from linearity on CD is any worse than on some open-reel tape machines I have checked. I have not heard any instances in which the perceived distortion increased with decreasing signal level, especially with real program material.

Novel Approach to Record Cleaning

Here is a suggestion that your readers might appreciate: Use Teflon tape to clean your records! This tape is sold at plumbing supply outlets and at do-it-yourself stores. Teflon is inert and extrudes; thus it is an ideal pipe-thread lube and sealer. It is also slippery.

Rubbed on the surface of a spinning phonograph record, the tape apparently leaves a trace of material in the grooves which acts as a lubricant. It works very well; there is a definite, audible difference in noise before and after cleaning with this tape.

A fresh piece of tape will clean both sides of a record. With the turntable moving at 45 rpm and the tape wound onto a stiff, stubby brush, I slowly move the tape across the record and then, just as slowly, move it back. Mild to slightly firm pressure is sufficient.

That's all there is to it. Although I find the use of a brush best, I have had some success with the tape wrapped around a finger. Of course, I don't permit the finger itself to touch the surface of the record.

-O. O. Callaway; Carlsbad, Cal.

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(Source: Audio magazine, Oct. 1987, JOSEPH GIOVANELLI)

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