AUDIOCLINIC (Q and A) (Jan. 1976)

Controlling Power Line Transients

Editor's Note--In the August, 1975 installment of "Audioclinic" I asked readers for their comments regarding he suppression of power line transients which can, and often do, cause interference in the form of clicks and pops in audio output signals. Many leaders took time to write in regarding this serious problem. Thank you, one and all, for sharing your knowledge. Just one example of this shared knowledge is in the form of a letter written by Mr. Fred Krock of San Francisco, Calif.: In answer to your question about controlling line transients, may I suggest General Electric's line of MOV Varistors? A common model is the V130LA 10A. If the offending machine is known, he varistor should be applied to that machine. Otherwise, the varistor can be applied to the a.c. input terminals of the amplifier in question. When the varistor is connected to the amplifier, it also protects voltage-sensitive components from high voltage spikes on the power line which can run as high as 2,000 volts on a 120 volt power line according to General Electric Research.

If the noise is not removed by a power-line transient suppressor of this kind, it can be eliminated in almost every case by treating it as a case of r.f. interference. This is true for every type of amplifier from a hearing aid to an electronic organ, as well as all types of high-fidelity equipment.

What people often miss when faced with a line transient problem is that the transient creates r.f. energy, which s radiated from the power line or the a.c. wiring inside the amplifier, is then picked up and detected by the audio circuitry.

Curing r.f. interference is usually a case of cut and try, involving grounding, shielding, and circuit modification. The various techniques are well known. The best circuit modifications usually involve a series resistor mounted as close as possible to the tube grid or transistor base with small bypass capacitors on each side of the resistor and ground.

In experimenting with "click" suppression, a Weller soldering gun makes an excellent transient generator. The size of the transient depends on the exact point in the power-current cycle where the switch is opened.

Triggering it a few times is bound to cause some nice, large transients.

Mr. William Blair III of Morton, Ill., refers to two articles which are definitely worth reading. The first appeared in the February, 1975 issue of Popular Science and refers to a surge protector. This particular device is designed to eliminate extremely high voltages from entering power line-operated equipment, voltages of the sort which could be generated by the near miss of a lightning bolt or a severe power line transient. In that article the appropriate surge protection device is described.

The second article appeared in the May, 1975 issue of Radio-Electronics. This article describes the basic operation of the GE MOV Varistors referred to by Mr. Krock. I note that this article refers to the GE 750 as the varistor of choice for use with home entertainment systems.

Mr. Bernard J. Van Dyke of Lewiston, Idaho, tells us of his success in curing power line transients. The simple scheme he used was simply to put the audio equipment on its own separate circuit.

Mr. C. Allen, FPO N.Y. says that MIL SPEC equipment employs capacitors wired directly across switch contact in order to absorb line transients when the contacts open. In the case of a refrigerator, the capacitor is wired between the two contacts of the thermostat.

In the case of a home heating system, where thermostat contacts operate a relay, which in turn operates a motor, capacitors might have to be placed across the thermostat contacts, and then across the relay contacts.

Because audio equipment contains power transformers and, in some cases, motors, such equipment will have inductive devices which can produce power line transients when they are turned off. Again, it is good practice to place capacitors across switch contacts.

Mr. Allen specifies values of capacitance in the order of 0.05 pi , with voltage ratings of at least 300 V d.c. (Because of the rather high voltages which can be present when the field of an inductive device collapses, I suggest the voltage rating of the capacitor should be at least 1 kV d.c.) .

More on "Stuck Stylus"

Q. Concerning Mr. R. Padilla's letter of August, 1975, I own one of the better turntables, which I recently equipped with a new cartridge. Then my troubles started. I finally sat and watched the cartridge; I saw immediately that the arm was not moving well horizontally. I got out my can of WD40 and put a squirt in the appropriate bearings. Daily the motion of tonearm improves as the lubricant gradually enters the bearings. The table was two years old when this trouble first began.

I realize that this in not a procedure to be recommended except to technicians, but it needs to be noted that trouble of this kind can occur even in the better turntables, but that it is easily remedied.

-Richard Lee Hallett, Pittsfield, Mass.

I have found in my repair experience that in some automatic turntables, the "stick" is caused by friction in the reject trigger arm which is pushed by the tonearm itself. This is a common problem which seems to be caused by the anti-skate control. It can fool the best of repair men.

-John Mears, Muskegon, Mich.

Bass in a Small Room

Q. Is it true that a loudspeaker can produce less bass output when in a small room as compared to a large room?

-Randy S. Parlee, Greenfield, Wisconsin

A. A loudspeaker will produce just as much, or more apparent bass in a small room as in a large one. Because of the nature of room acoustics, however, the actual amount of bass heard is less in a small room than in a larger one. All of this has to do with the acoustical wavelengths of low frequencies as compared to the size of the listening room. If the wavelength becomes an appreciable portion of the room size, bass will be lost from this frequency down.

AM Radio Antenna Problem

Q. The reception on my table radio was pretty good but I wanted to improve it. I connected an external, outdoor antenna to it. This improved the sensitivity but also gave poorer selectivity, bringing in "birdies" and gargles. What causes this, and is there anything I can do to cure it?

-Willard Ramsey, Newcastle, N.B., Canada.

A. You are hearing "image responses." These are other radio signals to which the set is not directly tuned, but which bear a fixed frequency relationship to the dial setting to which the receiver is tuned. The degree to which the set will produce these images depends both on the quality and number of tuned circuits in the frontend of the receiver.

Most portable and table radios have just one tuned circuit, consisting of the loop antenna and the tuning capacitor (not counting the tuned circuit of the local oscillator). The selectivity ("Q") of this circuit is amazingly high, due to the ferrite rod on which the antenna coil is wound, and because of the multi-stranded wire used in these coils (Litz wire). When an external antenna is added, it is coupled to the built-in antenna.

This reduces the selectivity of the circuit, thus lessening its ability to reject images in favor of the main signal.

Remember, this circuit is the only one responsible for keeping unwanted signals out of the receiver. Further, the added sensitivity provided by the long, external antenna tends to overload the mixer, producing additional spurious responses.

The best cure for the problem is to use the original antenna, or to keep the length of the added antenna to a minimum.

One Phono Input

Q. Some Layfayette equipment which I have has only one magnetic input. Can I use aux or tape, etc. How can I arrange this?

-James Golden, West Roxbury, MA

A. From your letter, one of your problems is that you have just one phono input and need to use two turntables. Your best approach is to ignore even the one input you do have, and obtain a mixer having at least three inputs. If your output is stereo, the mixer must have two channels, with three inputs per channel. Two of the inputs are for the two turntables, and the third one is for a mike, to be used for "patter" in this "record hop" application. The mixer would need, therefore, the capability of both mike and line inputs. If the mixer has only mike inputs, it can be used for line inputs with the appropriate voltage dividers to reduce the signal strength from the high level sources to be discussed later on. Mixers are often low impedance united, having unbalanced lines. The attenuator is nothing more than a resistor placed in series with the "hot" input. Its value is generally between 100 K and one megohm, depending on the strength of the line signal and on the overload characteristics of the mixer.

There are numerous possibilities, but this should provide you with a basis for further consideration.

Because of the lack of RIAA compensation in a mixer of the kind we have discussed, the phonograph cartridge cannot be directly fed into one of the mike inputs. What is needed is a device which has the necessary gain and equalization to enable the signal from the cartridge to be suitably processed to appear at line level, like that of a tuner. A number of manufacturers produce relatively inexpensive units which do just this task. You will need one for each turntable.

(Audio magazine, Jan. 1976, JOSEPH GIOVANELLI)

= = = =