AUDIOCLINIC (Nov. 1986)

Radio Reception in a Phonograph

Q. I was about to play a record. When I touched the cueing lever, I heard something from the speakers, so I turned up the volume to hear it more clearly. To my surprise, I was picking up a local radio station--and my tuner was not even connected. I let go of the lever, and the signal disappeared. I touched other parts of the turntable and got no signal. Is this common? Why does it happen?

-Carol Hilton, Milwaukie, Ore.

A. What you have described is a common occurrence (if I can judge from the mail I receive). Although this unwanted radio reception can occur in various parts of the audio system, the phonograph input circuit is the most common part. In order to raise the tiny voltage from a phonograph cartridge to a level suitable to drive a loudspeaker, an enormous amount of amplification must take place. Couple this to the fact that solid-state devices are capable of rectifying a.c. signals, and you have an elementary "detector." (You may have constructed a "crystal set" at one time or another, or at least read about solid-state-rectifier radios.) In the case of a phonograph circuit, a small amount of radio-frequency energy enters it and is rectified. The pulsating d.c. is amplified, and you hear the radio signal.

By touching the cue lever, you act as the receiving antenna. It is probable that this lever (in your system, at any rate) is not at ground potential. Thus, the signals introduced to this part of the phonograph can be capacitively coupled into the phono preamplifier.

You are probably quite close to a broadcast station. If this were not true, I do not think you would have experienced this phenomenon.

Damaging a Ribbon Microphone

Q. My ribbon microphone has a quarter-inch phone plug. I inadvertently plugged it into the headphone jack of my cassette machine while intending to plug in headphones. I started to record, and heard a high-pitched noise coming from the microphone.

Not realizing what was happening, I stopped and started the recorder a couple more times.

In order to determine if the mike was still working, I plugged it, along with a non-abused one, into the mike inputs of the recorder and recorded the sound of a kitchen radio. I watched the meter needles, and observed that the mike I suspected was damaged caused its associated meter needle to deflect more slowly and possibly not as far over as the needle on the other channel. I then reversed the mikes and found that the slower needle moved to the opposite channel. I cannot hear any difference in audio quality from one mike to the other. Of course, I only recorded voice. Did I damage my mike?

-Terry F. C. Jang, Vancouver, B.C., Canada

A. The fact that the mike works at all should be some consolation, and is perhaps a small miracle. I know of three ways to wreck a ribbon micro phone. The first is to drop it; the shock, together with the air rushing against the ribbon, will cause it to break. The second is to blow directly into it (as all too many poorly trained sound men do in an effort to hear "noise" from the loudspeaker); it's better to tap lightly on the mike's grille. The third way is to drive it as one would drive a loud speaker or headphones, since this deflects the ribbon beyond its normal limits. The ribbon stretches, unfolding some of its corrugations to the point where they cannot refold again. If this has happened, the ribbon's rest position will no longer be centered in the area of maximum magnetic flux. This means that when it is again used as a microphone (and not a tweeter), it will produce less output than it would have if the ribbon were located in its proper part of the gap. Additionally, because the ribbon is very thin (perhaps one ten-thousandth of an inch), it may tear partway. This can significantly change its impedance, causing changes in frequency response.

If the deflection is great enough or continued for a long time, the ribbon will actually break; no usable output can then be expected from it. Fortunately, you didn't get that far. The only way you will know if your mike is dam aged to the point where it cannot pro duce satisfactory recordings is to make one, using suitable "live" sources, and compare it to a recording made with a microphone that you know is not damaged.

Remember that ribbon microphones don't produce a great deal of signal to begin with. Reducing the output any further will degrade your system's noise figure by making noise from your microphone preamplifiers relatively more of a problem.

Dirty Contacts in a Dry Climate

Q. I live in a dry, desert environment in southern California. All of my stereophonic equipment develops a common and quite annoying problem: Rotary controls (such as volume) become "scratchy," and switches often fail to make proper contact.

In the case of both rotary controls and switches, the problem appears to be the result of some sort of buildup on the contacts; repeated rotation of the controls or activation and deactivation of the switches seems to temporarily solve the problem. The same condition crops up again, however, after a period of non-use.

The problem seems to be common in the equipment of my friends living in this same climate. Interestingly, I do not experience the same problems with equipment in my second home in Florida, where it's more humid.

I have tried commercial electrical switch/control spray cleaners; all have had only a temporary effect. Are such conditions to be expected in a dry and dusty climate? Could it be corrosion? Does anything short of periodic and expensive replacement of all affected switches and controls offer possible elimination of the problem?

-David A. Coup, Calexico, Cal.

A. You already know that you can clean your controls (temporarily) with contact cleaner, rather than having to replace them. There are many good contact cleaners available. I use one which is a bit off the beaten path, but it has an advantage over some cleaners in that, in addition to cleaning the contacts, it covers them with a thin film which appears to keep oxidation low. I refer to WD40, often used as a lubricant for light machinery such as typewriters. I use this to clean TV tuner contacts, where the wrong cleaner can cause serious misadjustment of the oscillator.

I am not a chemist, but I naturally tend to think of corrosion in terms of water combining with oxygen. I wonder if, in desert climates, there is an alkaline content to the air. If so, can this be in sufficient strength to corrode contacts? Perhaps some readers who are more qualified than I am will take time to share their experiences with us.

Transient Clicks

In response to the item titled "Transient 'Click' on Turnoff" in the July 1986 "Audioclinic":

I used to have the same problem with my amplifier. One day I turned the unit off; it produced an irritating click followed by some buzzing. At first I thought there was a fault in the amp itself. While disconnecting it from the rest of the system, I noticed that a ground had slipped off. I reconnected it, and, presto, the problem was remedied. Apparently my amplifier had no place to discard "extra" electricity when I turned it off.

A simple but effective ground that I have used is the center screw of a wall outlet's face plate.

-Mark Townsley, Huntsville, Ala.

I don't pretend to fathom how the loss of a ground could cause transient clicks. Many systems suffer from such clicks; perhaps publishing this letter will elicit more observations.

Power Failure While Recording

Q. What can be done if the power fails while you're making a recording? This has happened to me numerous times.

-Steve Dusik, Franconia, N.H.

A. One solution is to use a portable recorder; a number of models are capable of making fine recordings. I do not know if any will automatically transfer to battery power should the a.c. power be interrupted-in many, I know, attaching the line cord disconnects the batteries, rendering such a transfer impossible. If that is the case, you might wish to make the whole re cording under battery power whenever power outages seem likely (during storms, for example).

You might be able to solve your problem by using an "uninterruptible power supply" (UPS), a device made to enable computers to continue functioning during power interruptions not longer than perhaps 30 minutes. Typically, these devices are made to come on line within a few microseconds after a power failure has occurred. Batteries inside the device provide the backup power. Of course, voltage supplied by batteries is d.c. (and only 24 V), but inverter circuitry inside the UPS turns that into the 117-V a.c. that home audio gear requires.

I am using a UPS as a backup as I write this on my word processor. My reservation is that the waveform produced by many of these units when operating on their internal batteries is not a good sine wave. Also, with the equipment I use, AM reception is interfered with when the UPS supplies a.c. from the power line rather than from internal batteries. I think that most or perhaps all such interference could be eliminated by plugging a good LC power-line filter into the outlet on the UPS. The line cord between such a filter and the UPS outlet should be kept short so that the cord will not itself become a radiating antenna.

UPS devices are available in sizes to handle various wattages; order one which can handle the power that your equipment draws from the power line.

The one I happen to use is made by Lumitrol, 110 Bi-County Blvd., Farmingdale, N.Y. 11735. Check with a good computer dealer.

Another Look at Phase Inversion

Q. In the November 1985 issue, you gave a non-answer to a question about phase-inverting preamps.

You do not hook both amp cables to the "inverted phase" outputs. Instead, you hook one channel to its "inverted phase" and the other to "normal phase" output. Then you reverse the speaker wires for one channel.

This setup is explained in the Yamaha C-70 owner's manual. "The left and right channels of the power amp are operated out-of-phase, greatly reducing the demands of low-frequency signals on the power supply. This can dramatically improve bass reproduction on many power amps." I have used this arrangement with three different power amplifiers. In each case, the bass was more natural and effortless with the inverted-phase hookup than when connected in the normal way. This hookup also made for a sweeter sounding high end.

One caution, though. This setup won't work with Polk SDA speakers.

You don't get wraparound sound from both channels!

-John McCutcheon, Walloon Lake, Mich.

A. Whether my earlier reply was an answer or non-answer depends on whether you're considering absolute or relative phase inversion.

Your helpful information concerns the somewhat special case of preamplifiers which can invert one channel relative to the other, in order to squeeze a bit more bass power out of an amplifier. There are few such pre amplifiers.

My original answer concerned the more general case of preamplifiers which invert the phase of both channels: The two stereo channels emerge at the output in phase with one another, but opposite in phase (or, more properly, opposite in polarity) to the way they came in, inverting the signals' absolute phase. There are many such preamps and amps, because any component with an odd number of circuit stages will behave this way (amps or preamps with even numbers of circuit stages won't).

There is a body of opinion which holds that it is important for the sound waves reaching the listener to have the same absolute phase as those reaching the microphone that was used for recording, in which case it would be important to know which components in the system did or did not invert. If the preamp does invert phase in this manner, one can compensate by using a power amplifier which also inverts, or by reversing both sets of speaker leads. However, since there's no way to know how many circuit stages a signal went through between the microphone and master disc, there's no way to tell which way to set your system's polarity to duplicate that of the original sound. If the recording was made (as most popular recordings are) from a multi-track master tape, different portions of the signal may have different polarities in the final mix-down. Phase-inversion switches are available as accessories, and some components have them built in so that one can see how a given record sounds with either polarity.

(Audio magazine, Nov. 1986, JOSEPH GIOVANELLI)

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