AUDIOCLINIC (Q and A) Aug. 1985

Usable Volume-ControlRange

Q. I recently purchased a clock radio/tape recorder. Its volume control is the slide type, with positions numbered from 1 to 10. I find that if I advance the control to 1, the volume is as loud as I would ever want. Thus, my effective use of the control is very narrow, making it difficult to adjust volume.

Can I make this control less sensitive by placing a resistor in series with it, or perhaps placing a resistor in series with the loudspeaker in order to lower the speaker's efficiency?

-Scott A.; Kracen, Sycamore, Ill.

A. To solve this problem, first mea sure the resistance of the volume control with an ohmmeter. (Disconnect the "high" side of the control and lower the setting to zero. Otherwise, you may obtain an erroneous measurement.) Next, place a resistor, about equal to the measured resistance of the control, in series with the high side and its original connection.

Unfortunately, many controls of this kind are mounted on the printed circuit board containing the rest of the circuit ry. If this is so in your case, you must cut the foil associated with this terminal and bridge the added resistor across the break.

I do not recommend that you add a resistor in series with the loudspeaker because you may then have insufficient volume available from the loud speaker before distortion sets in. I have noticed that some of these clock radios have more audio gain than can be handled by their output stages. There fore, overdriving the output stage is a real possibility when you decrease the efficiency of the loudspeaker circuit.

Insufficient Phonograph Volume

Q. I have just connected a new turn table to my old Electrophonic system, hoping to upgrade my system's performance. I tried playing a record with the volume control set to a low level, and I heard nothing from the loudspeakers.

It turns out that for me to hear music at all, I have to turn the volume control to maximum. Please give me your professional opinion as to the nature of my problem.

-Mark E. Clement, Greensburg, Pa.

A. Your low signal problem is not hard to track down. Your player's amplifier was designed to accept what is known as a ceramic cartridge; your new turntable, however, contains a magnetic cartridge.

Ceramic cartridges produce much larger signals than magnetic cartridges, so the latter need more amplification. Magnetic cartridges also need special equalization in order to deliver a flat frequency response; this is not the case with a ceramic cartridge.

All is not lost! There are some phono preamplifiers made to do just what your sound system calls for. They both amplify and equalize the tiny signals produced by magnetic phonograph cartridges, bringing sufficient signal to the input of a sound system like yours so that proper performance is re stored. Thus, the upgrade you had hoped for is really effected. (These units are manufactured by Shure Brothers, Stanton, Radio Shack, and others.)

Installation is simple. Merely connect the unit between the turntable and the input of the Electrophonic unit normally used to accept signals from the original cartridge.

Tonearm Cueing Problem

Q. I have a problem with my new turntable. I was careful to set it up according to instructions for balancing, stylus overhang, etc. As a last step, I used a very accurate bubble level to level the turntable front/back and left/right.

The problem is that the cueing does not work properly. When the "up" control is pressed, the arm rises and then moves inward approximately 1/16 of an inch. This means that the arm will not be at the same point on the disc when I lower it again.

Following the maker's instructions, I adjusted the level until the cueing problem was solved, but the level was off by a few degrees! Which setup is correct, and is something wrong with my turntable?

-Peter DeMeo, San Bernardino, Cal.

A. It sounds as though the lever which actually lifts and lowers the tonearm may be bent, not absolutely parallel to the turntable. Thus, if it tends to dip toward the center of the record, it may cause the arm to slip in that direction. If you are mechanically oriented and willing to mess with it, bend the lever so that it is now parallel to the record surface. Be sure to level the table properly.

It also occurs to me that if you have your anti-skating compensation set in correctly or if it is mis-calibrated (which amounts to the same thing), the tonearm may have a natural tendency to move inward, even with the turntable correctly leveled.

Of course, you did solve your problem to some extent by just knocking your system out of level. If this adjustment does not result in excessive anti-skating force, it probably does not make much difference that you have a slightly misadjusted level. Chances are, if you continue to operate the turn table off-level, you will need to reduce your anti-skating force.

Multiple Tweeters With a Crossover Network

Q. I am designing my own loud speaker systems, using four 8-ohm tweeters. Two are wired in series, and the next two are also wired in series; these two groups are wired in parallel to produce 8 ohms of impedance. By wiring several tweeters in this way, will the performance of the crossover net work be affected? Will there be a danger to my tweeters?

- Loren Gade, Virginia Beach, Va.

A. The crossover network will not care how the 8-ohm impedance which it expects to "see" is obtained. You have done everything correctly.

Improving AM Reception

Q. Is there any way for me to improve my AM reception? I live in a rural area in Wisconsin, but I enjoy a Chicago AM station--WGN-50,000 watts, clear channel. This station, located 368 miles southeast of my home, fades at about 7:00 in the morning.

I have a 62-foot television tower and a 10-foot satellite dish. Is there a way to use either of these for radio reception?

-R. G. Yancy, Bruce, Wisc.

A. Aside from using your TV tower as an anchor point for an AM antenna, your tower and dish won't be too helpful. But before answering your question fully, I need to know more about the fading you mention. If the signal disappears completely, little can be done to salvage the situation. If the signal is still present, but is much weaker, then you may be able to do something to make it more usable.

Connect a length of wire to your tuner's AM antenna input terminal. Run this antenna so it is clear of surrounding objects, and attach it, using some kind of insulator, to a tree or other object on your property as far as possible from the house, so your wire can be as long as possible-about 50 feet, mini mum. Your TV tower might well serve as an anchor point if it is far enough from the house.

I cannot guarantee perfect reception once you add this wire. TV sets in the neighborhood can cause problems; someone vacuuming a carpet can cause problems. Summer storm static can be troublesome, too, and if there are any local broadcasts nearby, they may introduce sufficient signal to over load your receiver.

Usually a station operating at the lower portion of the broadcast band will not tend to fade during daylight hours, once its minimum signal level has been reached. You can definitely expect a constant fading in and fading out of the signal at night, with changes in audio quality along with the variations in signal strength. This phenomenon, known as selective fading, is a fact of life with long-distance AM reception.

The cure sometimes employed by commercial receiving sites is to use two separate antennas, spaced per haps one mile apart, with each antenna connected to its own receivers. The receivers are interconnected to what is called a voting system. This simply means that the receiver having the strongest signal at any given time is the one allowed to transmit its audio to the main system. This arrangement is known as dual diversity reception.

Headphone Efficiency Problem

Q. My power amplifier has two stereo headphone jacks. I own two pairs of phones, each from a different maker, and use them simultaneously.

Should I have a resistor in series with each pair of headphones so that one does not rob volume from the other? One pair has a bit more volume than the other.

-Name withheld

A. Resistors can help, but not for the reasons you have suggested. You see, it is not a matter of one pair of phones stealing signal from the other. It is simply that one pair yields more acoustical output than the other from a given amount of input power applied.

Yes, add resistors (whose values must be determined experimentally), but only in series with the more efficient pair of headphones. With properly chosen resistance values, both pairs of phones will be equally loud for a given input signal.

(Source: Audio magazine, Aug. 1985, JOSEPH GIOVANELLI)

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