Audioclinic (Oct. 1983)

Muting Switch

Q. What is the purpose of the -20 dB muting switch that can be found on many preamplifiers?

-Scott Van Note, APO, N.Y.

A. The -20 dB muting found on some preamplifiers is designed to en able you to quiet the music system during phone calls and to restore the sound to the volume you were using before the call was received.

Assuming that you did not have such a switch, you would have to turn the volume control down and, when appropriate, raise it to its former position.

Tuner With a Receiver

Q. Can I operate a separate tuner through my receiver (possibly via the tape loop), or must I acquire an amplifier as well?

-Jeffrey R. Myers, Langhorne, Pa.

A. You can use any tuner with your present receiver. The tuner contained in the receiver will never know about the new tuner and it won't be jealous! Use any appropriate, high-level in put-tape loop, AUX or what have you. Everything will work as you hope.

Solid-State Rectifier Substitution

Q. Please comment on the replacement of a tube-type rectifier by a solid state rectifier. My intention here is to improve low-frequency response from an audio amplifier.

-Franklin F. Coperich, Silver Spring, Md.

A. I do not think that this change will improve the low-frequency response. If the amplifier is deficient in bass, it may be that your output stage is unbalanced because of weak output tubes, unbalanced drive from the previous stage, or misadjusted bias on the output-tube grids.

It could also be that the output trans former lacks sufficient inductance to transmit bass to the speakers. If so, replace the output transformer with one having a heavier core (assuming that there is a physical way to mount the larger, replacement transformer).

The replacement must also have the correct, specified plate impedance and the appropriate screen grid taps.

And if you change the transformer, you will probably need to make adjustments in the feedback circuit.

From the nature of the question, I suspect that this amplifier is quite old.

If it once possessed better low-frequency response than it does today, this is probably due to the failure of capacitors somewhere in the circuit.

The simplest way to cure that is to replace all electrolytic capacitors and proceed from there, rather than do extensive tests and troubleshooting.

Replacing a tube rectifier with a solid-state one can cause problems, too.

With a solid-state rectifier, there will be a surge at the moment the equipment is turned on; a rectifier tube's gradual warm-up would prevent this. This surge may hasten failure of the power-

supply filter capacitors, if they are not rated conservatively but are operated near their voltage breakdown point.

Operating voltages run higher with solid-state rectifiers too, due to the rectifiers' lower internal resistance. If this voltage comes close to the filter capacitors' rated voltage, you'll need to lower that voltage to its original value, by inserting a series resistor between the rectifier and the rest of the circuit. But if the filters and other components can withstand the higher operating voltage, omit the resistor. The amplifier will then produce somewhat more out put power.

Because the solid-state rectifier has no heaters, the system will also run more efficiently, producing less heat.

Cartridges and Sonic Definitions

Q. My question concerns phono cartridges. I want a very good, very compliant cartridge to take advantage of my turntable's tonearm characteristics. I have been to many audio salons. Their salesmen's descriptions of various cartridges use the following terms--"flat," "silky," "smooth," "musical." What does all this mean?

-Lee R. Antokal, Central Islip, N.Y.

A. The various terms used by audiophiles, dealers and others describe tonal qualities and are attempts to di vine what their ears hear. Because sound quality cannot be described in absolute terms, people try to use words which describe their subjective feelings and experiences.

You may find it difficult to tell much difference between one cartridge and another, as they are all surprisingly similar in sound quality. The only way you will know any of this for sure is to take some of your favorite (and hence familiar) recordings with you to a dealer who stocks at least most of the phono cartridges in which you are interested. Use your ears and see what you come up with by way of a cartridge which suits your case.

There are some points for which you can, and should, listen. If there are bells on some discs or perhaps percussion with a considerable amount of high-frequency energy, listen for distortion, especially at the inner bands of a disc. Be sure to bring discs which have been difficult for your present system to track cleanly. I tend to give the highest marks to a cartridge which handles all of these difficult passages properly. Let us understand, however, that there are some discs which will never sound good because they have been cut at too high a level. No stylus can trace these grooves properly.

You should also listen for "smoothness." Smoothness and silkiness are related-here we go with those words again! Listen to strings and hear whether they appear to "scream" at you or are pleasing to your ears.

How is the bass? Is it "boomy," where some notes tend to stand out more than they should? See how such bass passages sound with various cartridges. If all cartridges sound alike in this regard, the bass problems you are hearing have more to do with the listening room or to the loudspeakers than they gave to do with the sound of the cartridges. You should then ignore that problem. Believe me when I say that noting differences from one cartridge to the next is not easy. You well may find that there is little or no difference for you.

Under such circumstances, I suggest that you buy the cheapest of those that you like. Chances are that you will be very happy with your choice.

Burglary Protection

Q. I am a student in a university, enrolled in a course titled "Industrial Research." Presently, I am trying to determine what type of electrical alarm circuit would best protect a multi-component home stereo system.

-Paul S. Howard, San Francisco, Cal.

A. Burglary is rampant, and stereophonic equipment is a prime target for burglars. The problem is not how to protect the equipment itself; the first line of defense is to deter the would-be burglar from entering the premises in the first place. Should he gain access, there is a second line of defense, but more on that later.

My thoughts, to put it succinctly, are simply that you must keep the thief out of the house. If a professional burglar is really determined to steal your gear, there is little you can do to stop him.

Fortunately, however, most burglaries are committed by amateurs. They do not want to hang around, make noise, and otherwise attract attention. If there are bars in front of the windows and a solid, wooden door with a good lock, these will make the would-be thief think twice about entering your home. The noise, plus the time required to gain access, will dissuade many thieves from even trying to break in.

A light that's always on when no one is present is not necessarily good protection. The light in itself might give just the opposite impression from that which you wish to create. After all, if a person is really at home, there would be times when a light is not lit.

A timer used to control lights is not always helpful, either. Timers are excellent at turning lights on and off at precise times, but today's burglars often watch the homes of their intended victims, just to determine the regularity of such events.

There is at least one class of timer which helps overcome this. Rather than turning lights on and off at specific times, this equipment controls lights in a more or less random way, centered around the actual on and off times chosen by the user of the equipment. In addition to lights, these de vices can control appliances such as radios or TV sets. The sound of these devices, heard by the would-be thief, gives one more illusion that there is someone home. The BSR X-10 and its Radio Shack and Sears equivalents are examples of this. Not only can they control gear plugged into outlets, but they can also control outside lights via wall-switch modules which substitute for wall switches.

Control is achieved by radio-frequency signals sent over the power line. Because of the frequencies on which they operate and because of the lack of frequency discrimination in the receivers, these systems are subject to "jamming" from devices such as wire less intercoms (which also transmit over the power line). Also, when lights are turned on, a considerable amount of r.f. "hash" is generated, causing interference to AM radio reception. (I have heard that one manufacturer offers a module which eliminates this interference. I do not have the name of this company nor have I had the opportunity to check as to whether it truly does this.) Radio Shack offers random on/off timers that can be substituted for wall switches and are completely self-contained, are not controlled by external signals, and do not produce radio "noise." They cannot, therefore, be jammed in the manner just described.

Another piece of equipment which can be useful is one or another of the various alarm systems now widely sold. Some are "hard-wired" into the home, which makes them inappropriate for temporary housing, such as school dormitories. A wireless system is probably best here. Some systems include very loud sirens which can scare almost anyone, including the owner of the alarm. They will attract attention! Many wireless systems are de signed to "listen" for the sounds of breaking and entering-breaking of glass, splintering of wood, etc. Their sensors are, to a degree, frequency-discriminating, and will therefore tend to overlook sounds which are normal to the environment.

Some of the more recent entrants on the market can also act like hard-wired systems, sensing the state of magnetic switches, pressure switches and the like. These systems operate from the power line; some will continue to operate in the event of a power loss and some will not. These systems can control recorded alarms. When activated, they can dial one phone number (or perhaps several in sequence), transmitting a "spoken" message that help is required at your address. One of the numbers generally included is the 911 police emergency number. While all this can be helpful, so many of these systems are set off accidentally that the police often do not respond to them as high-priority matters. You can see, therefore, that help may arrive too late to catch the burglar in the act.

If, after all of your attempts at prevention, the burglar does gain entry, it may help to have a second line of defense-a motion detector. No matter what its operating principle may be, it will sense motion in the vicinity of the equipment you want protected. It can then activate a silent, recorded alarm or set off an audible or visible alarm (or any combination).

In short, anything you can do to slow the thief down will help deter him, but not necessarily stop him. The information used in this answer was obtained in part through some years of association with a police precinct community council, and in part by my having installed and/or worked with a number of the systems mentioned. Many police departments offer free security checks and advice. I strongly suggest that you take advantage of these services, if they are available to you.

(adapted from Audio magazine, Oct. 1983; JOSEPH GIOVANELLI)

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