AUDIOCLINIC (Jul. 1989)

SPARS Codes on CDs

Q. Is there a standard location where the SPARS code can be found on a CD?

-Wesley S. Mayeda; Oxnard, Cal.

A. There is no standard place on a CD for the SPARS code: e.g., AAD, ADD, DDD. CBS often puts it on the CD box. With PolyGram, you are likely to find it on the disc itself. I have a couple of CDs which don't even have a code. SPARS, by the way, stands for Society of Professional Audio Recording Studios, and the code to which we refer here shows whether the original recording and mastering were analog or digital.

Bypassing the Preamplifier

Q. I have a CD player and a biamped speaker system. I do not wish to use my preamplifier when playing my CDs. Rather, I want to connect the player directly to the inputs of my electronic crossover. In fact, I have tried this; it works well, and there is sufficient volume. However, it is inconvenient to switch cables to the crossover.

Should I use Y connectors to hook up the preamp and the CD player to the crossover inputs? Would it be better to use a switch between the preamp and the player?

-C. M. Elsbernd; Cincinnati, Ohio

A. As you already have discovered, you can definitely bypass the preamplifier and send your CD signal right to the crossover network. (Personally, I would be unhappy about losing most of the functions found on my preamplifier.) In any case, you must use a switch to move the crossover inputs between the outputs of the preamp and those of the player. This can be accomplished by constructing a little box to hold the switch and the various connectors which will be wired to the crossover inputs, the preamp outputs, and the CD player's outputs. You need a two-pole, double-throw switch. One pole is for the left channel, the other for the right. The wipers connect to the crossover inputs, and the appropriate switch contacts are wired to the appropriate equipment outputs. Use a "break-before-make" switch. Place a 1-megohm resistor (wattage not critical) between each output and ground.

The purpose of this is to eliminate "clicks" when the switch is moved between sources.

Do not use Y connectors. It's a tempting thought, but what is likely to happen is that the preamplifier will attempt to drive the CD player or the CD player will attempt to drive the preamplifier's output, in addition to driving the crossover. At best, this will reduce the amount of signal available to drive the crossover. At worst, bass will be lost and distortion will rise. With some circuits, there is a definite possibility of damage. I refer here to output circuits which are direct-coupled-no coupling capacitors.

Leaving Your CD Player On

Q. Some of the "underground" magazines recommend leaving a Compact Disc player on at all times because it is supposed to sound better after a considerable warm-up of the machine. Because a laser has a lifespan of 2,000 to 3,000 hours, does this on-time decrease the life of the laser or is the laser off until play begins?

-Dennis R. Najuch; Medfield, Mass.

A. The laser is not operating except for the time the CD is being played or searched. Therefore, it is possible to leave your player on at all times. I have tried this, but my players, at least, sounded no better to me when I left them on all the time than they did when I listened just after turning the player on. It would be interesting to learn what you notice.

Balanced Inputs

Q. What are balanced inputs? I've often seen this term with regard to mixers.

-Tripp Davis, Memphis, Tenn.

A. Equipment which you are familiar with probably has unbalanced inputs, i.e., the signal is connected between the input and ground. When the signal is applied between the "hot" lead and ground, this signal forces the voltage on the "hot" lead to move above or below ground potential in accordance with the instantaneous polarity of that signal.

The balanced input is so arranged that neither signal lead is connected to ground. When this input is driven, one input terminal swings above ground while the opposite terminal swings below ground. When the polarity of the signal reverses, the terminal whose polarity swung below ground now swings above ground, and vice versa for the other signal terminal. It is called a "balanced" input because the potential differences between each lead and ground are always equal, though opposite in polarity.

The cable used consists of two conductors and a shield. The shield is connected to ground as you would expect, but it does not carry the signal because ground is only a reference.

Carrying the signal is the job of the two center conductors. The shield serves only to keep hum from being induced into the two "hot" conductors.

Meanwhile, the two conductors are wrapped around one another. This serves an important purpose: No shield is absolutely perfect. If the cable we're discussing is in the presence of strong hum fields, some hum voltage will find its way into the two conductors. This hum voltage will be introduced into both conductors more or less equally. It will, therefore, try to swing both conductors either above or below ground at the same time. Remember that, in order for a signal to feed a balanced input, the conductors must act in opposite directions. Induced hum won't act this way, but the desired signal will. (I ran 50 feet of unshielded mike cable, using a balanced input. Even without shielding, there was virtually no hum!) Because of their hum-cancelling ability, balanced circuits are often used where amplifier signals are low and where cable runs are long.

FM Overload

Q. Recently, I connected my FM receiver to a combined FM/TV outdoor antenna. My tuner's signal-strength meter reads full scale. Rather than improving performance, however, using this antenna has produced more distortion. Is it possible that there is too much signal for my receiver to cope with?

-Bill Dermer; Brooklyn, N.Y.

A. Based on your description, the most logical conclusion I can reach is that your receiver's front-end is overloaded by too much signal. The signal strength meter reads full scale, so it is impossible to tell how much more signal is being received than is shown on the meter.

You didn't mention it, but another indication of front-end overload is the reception of the same signal at places on the dial other than its correct location. If you have this problem, you have proof that the front-end is overloaded.

Chances are that you can disconnect your antenna and still receive at least one signal, possibly with a full-scale meter reading. This would be another indication that you have frontend overload.

Based on the likelihood of overload, place an attenuator between the antenna lead-in cable and the receiver's antenna input terminals. These attenuators are made for the purpose of reducing the amount of signal arriving at the antenna inputs of TV sets, tuners, or receivers. They are available with various amounts of signal reduction; you will probably require 10 or 20 dB of signal reduction, or attenuation.

If you do not receive at least one signal at an incorrect dial marking, or if signals are totally lost with no antenna connected, it may be that your real problem is multipath distortion. I note that you live in Brooklyn, which means that your antenna may be surrounded by very tall structures. If some of these are higher than the antenna, the FM signals which are distorted are being received more than once. The first time the signal is received, it arrives directly from the transmitter--just as it is supposed to. However, that same signal may bounce off a nearby structure and be reflected back to your antenna. This second signal will arrive slightly later than the direct signal, producing phase discrepancies which can only lead to distortion. To complicate the matter, the signal may be reflected from yet another structure and be received by your antenna at an even later time.

In the event that your problem is the result of multipath reception, you should move your antenna to a different location, if this is practical. Sometimes, just moving the antenna a few inches causes the undesired signal reflections to disappear or become so weak compared to the strength of the direct signal that the sound from the receiver is restored to normal.

==============

(adapted from Audio magazine, Jul. 1989, JOSEPH GIOVANELLI)

= = = =