AUDIOCLINIC (April 1985)

by JOSEPH GIOVANELLI

Vacation Recording

A. I want to make recordings from FM while I am away from home. The owner's manual for my receiver states, however, that this model should not be used with a timer because its FM station memory will be erased after a few hours if it's plugged into a timer instead of a live a.c. outlet. Is there some kind of switch I can buy or make to overcome this problem? I'd also like to know if there is a way I can make unattended recordings on one deck and then onto a second deck.

-John De Rosa, Mattapan, Mass.

A. In circuits of the kind you describe, power must be applied to the memory portion of the equipment at all times in order to keep the information in memory from being lost. Even when the power switch of your receiver is off, power needed by the memory is derived ahead of the power switch.

One solution would be to leave the receiver on during your whole vacation, either with the volume all the way down to avoid annoyance to your neighbors, or with the volume up high enough to make burglars think you're home. Of course, that will add to your electric bill. (If your receiver draws 3 amperes, or about 300 watts, it will consume about 8 kilowatt-hours per 24-hour day, or about 111 kW-h during a 14-day absence.) And there is also a very slight fire hazard in leaving receivers or other electrical devices turned on during long absences.

If you are handy, you could devise a relay circuit which employs a 117-V a.c. relay. The coil of this relay is plugged into the timer (just the way you would normally plug your receiver into it); the recorder must also be plugged into the timer in the usual manner. The normally open contacts of the relay would be shunted across the contacts of the power switch on your receiver. The receiver would be plugged into the wall outlet as usual.

In operation, the timer turns the recorder on and also energizes the relay.

The contacts close, which, in turn, puts power into the circuits of the receiver just the way the power switch normally does. Of course, in order for this system to work, the receiver is turned off during your entire trip.

Probably the most difficult part of the scheme is to provide a means for switching from one deck to the next.

One way to accomplish this is by means of a stepping switch wired in such a way as to switch from one machine to the next each time the timer supplies voltage to its convenience outlet. This circuit can be built, but now there is a better way. (I normally do not mention specific products, but in this case, I know of no product other than the one I will briefly describe.) Chrontrol is a digital clock timer which can be accurately set to the second. On/off times can be set to the second; days of the week on which no recordings are to be made can be skipped. Another fascinating aspect of this device is that there are four outlets mounted on the rear panel. The system can store and control up to 10 timing events, and each event can be set to operate whichever one of the outlets you specify when programming the unit. With a little planning, you can use three different decks and your receiver. Each time a cassette recorder is to run, you must also program the receiver to turn on.

Thus, you may have to use two programs for each event, but it is the best solution to this problem that I know of. I have used this device for several years, and it has performed perfectly.

The Chrontrol even includes battery backup in the event of power-line failure. (For more information, write to Lindburg Enterprises, 9707 Candida St., San Diego, Cal. 92126.)

Power, Hearing and dB

Q. I read that the smallest increment in sound level that the human ear can detect is 1 dB. How much power is required to raise the sound level this much? A friend says it requires twice as much power to do this.-Name withheld A. Doubling (or halving) the power would represent a change of 3 dB, not 1 dB. A 1-dB change would mean you were raising power by only about 26% or lowering it by 21%. (The two ratios are unequal because the dB is a logarithmic ratio, while percentages are linear.) Thus, if your amplifier was delivering 10 watts of power at a given moment, a 1-dB level increase would raise that power only to 12.6--not 20watts, while a 1-dB decrease would lower the power to 7.9 watts.

Your friend's confusion may stem from the fact that researchers did think at one time that we could only detect changes as small as 3 dB. But 1 dB is now accepted as about the smallest level change that can be detected as such by trained ears. Smaller level changes are also detectible, but they may be perceived as differences in sound quality rather than sound level.

Interconnecting Equipment

Q. I need information on interconnecting various kinds of equipment. Is there a manual on this subject?

-Enrico M. Esposito, Brooklyn, N.Y.

A. To my knowledge there is no manual which explains how to interconnect the many possible combinations of equipment now available. There are so many possibilities, in fact, that I do not even know how such a manual could be written.

My suggestion is that you first consider what you want your system to do, and then draw a diagram of the audio signal flow needed to bring about these results. The less gear you have "on line," the simpler it is to deal with it.

The instruction manuals for your components should help. Also, there are a few rules to follow with equalizers and noise-reduction devices: Where a noise-reduction unit is used with a recorder, you should consider the noise-reduction unit to be a part of the tape recorder. Thus, no device should be connected between the NR unit and the recorder's input, and no external device should be connected between the recorder's output and the tape-recorder input of the noise-reduction unit. The same advice applies to companders.

Time-delay or other devices to add or modify ambience should usually not be fed with equalized signals. I suggest you do any equalizing at the output of these devices.

Where there are many individual pieces of gear that will not all be used at any one time, it is better to use either a patch bay or perhaps a switching system to produce the desired set of connections only as required.

(Source: Audio magazine, April 1985 )

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