AUDIOCLINIC (Jul. 1992)

Balanced and Unbalanced Circuits

Q. Please explain balanced and unbalanced inputs and outputs used with amplifiers and preamplifiers. Are there sonic advantages of one over the other?

-Lorenzo Brice, Brooklyn, N.Y.

A. The main advantage balanced circuits have over unbalanced circuits is that balanced circuits tend to reject hum pickup via interconnecting cables. Most of the time, hum is not a problem, even with unbalanced circuits; thus, any additional hum reduction offered by balanced connections is not usually important.

You're already familiar with unbalanced circuits, the kind typically found in home audio equipment. Unbalanced systems have two wires, one of which is "hot" while the other, the shield, is grounded. Balanced systems require three wires: Two signal leads, neither of which is at ground potential, plus a ground. The two signal-carrying conductors are twisted around each other and contained within the shield. Any hum voltage that penetrates the shield will be introduced equally into both signal-carrying leads. Because these leads are of opposite signal polarity and the signal is introduced at the same polarity, the hum is cancelled, or nulled out.

I suppose a case could be made that a preamplifier that used balanced, push-pull circuitry throughout would produce less harmonic distortion than an unbalanced preamplifier would.

However, I'm personally not convinced that we could hear the difference.

Video Cables for Audio

Q. I have a question about interconnecting cables used between different audio devices. I notice that some cables used for video gear have phono plugs on each end-just like those used in home audio gear. The cable is RG59. Are these cables as good as those we usually use? A video cable must carry a wider frequency band than is needed for audio, so it seems to me that video cables should be fine. I tried some video cables, and I can't hear any difference when using them.

-Richard Costa, Houston, Tex.

A. If the video cable has less capacitance per foot than your present audio cable, it will work fine, even giving you less high-frequency loss on long cable runs. There are many varieties of RG59 cable, with capacitances ranging from about 16.5 to over 21 pF per foot, depending on the brand, size, and type of insulation.

In a video setup, the impedance seen by each end of the cable will be 75 ohms. In an audio setup, the impedance at one end of the cable will be rather low, while the impedance at the other end will be high. At the frequency where the capacitive reactance of your cable becomes equal to the output impedance of the audio component feeding it, output will be reduced by 3 dB, and higher frequencies will be attenuated even further. However, as long as your video cable has less capacitance than the audio cable you've been using, the results should be at least as good as you are getting now.

One caution, however: RG59 cables frequently have solid center conductors. If these cables are flexed or moved a lot, that conductor could break. In setups where the cable will not be moved, it will work well.

Intermittent Noise From a Sound System

Q. My amplifier sends sudden, loud, bursts of noise to my loudspeakers.

They seem to be triggered by vibration or by the touch of my hand. I have taken steps that have reduced but not eliminated the problem.

The noise is produced only after the system has been on for a few minutes.

If I strike the floor with my fist just hard enough to make it vibrate a little, the noise is heard. Even a heavy footfall in the room causes the sound.

I don't think the problem resides in either of my program sources--CD player or cassette recorder. Even with these devices turned off and with the volume fully counterclockwise, the noise can still be triggered.

I have experimented with grounding and with tightening up all connections, to no avail. The only change I made that has helped reduce the noise is to plug the equipment directly into the wall outlet rather than into the voltage step-up transformer. (I live in Japan, where house voltage is 100 V a.c., 50 Hz.) The system now must be on for a longer period of time before the noise is produced and the loudness of it is decreased.

Do you suspect, as I do, that the a.c. power is at the root of the problem? Will operating my gear on 100 V rather than its recommended 120 V cause damage? What do I do now?

-Stuart T. Foster, FPO Seattle, Wash.

A. No! The transformer or your a.c. wall supply is not the root of the problem, but it does, as we will see, point us to the culprit.

It is not clear as to whether you have an integrated amplifier or have a separate power amplifier and preamplifier.

This may matter. You have already done some good troubleshooting.

Let's see how to use what you have written to make a final determination of the location of the problem: We already know that when the volume control is set to its lowest point, the noise can still be triggered even though no signal is reaching the circuits after the volume control. Thus, you are correct in assuming that your program sources are not responsible.

If you have a separate power amplifier, it could be the culprit. To prove it, disconnect the input cables but leave the loudspeaker cables connected.

Turn the amp on and wait an appropriate amount of time and see if the noise can be triggered. If it can't, we know that the problem is in the preamplifier.

If it can, you know the power amplifier needs some work.

I said that the power-line voltage and frequency were not the root causes.

Why, then, did removing the transformer help? The equipment now operates on a lower voltage. This, in turn, means that less heat builds up inside the faulty unit. Somewhere in the unit there is probably a poorly soldered connection which, when heated, becomes less secure and makes the electrical connections intermittent when they are vibrated. There could be a slight crack in a circuit foil, which opens slightly with applied heat, making the electrical path intermittent at this junction. The problem could be really simple. I recently solved a problem like yours when I tightened a screw that held the p.c. board to chassis ground.

(adapted from S Audio magazine, JOSEPH GIOVANELLI, Jun. 1992)

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