Audioclinic (Jun 1985)

Special Mixer Features

Q. On my mixer are connectors marked "Send" and "Receive." A short jumper cable connects the two. When / disconnect this cable, there is no sound from the output. What are these jacks for? I would also like to know what the "Bus In" and "Cue Out" connectors on this mixer are for.

-Tom Wick, Hunting ton Station, N.Y.

A. The "Send" and "Receive" jacks work like a preamp's tape monitor or external-processor loops, but without the front-panel switch. They allow equalizers, reverb units and other signal processors to be placed in the signal path, getting their signals from the "Send" jack and returning them to the "Receive" jack.

The "Bus" for each output channel is the common line into which all signals are mixed. If you need to mix more signals than your mixer has input channels for, you can feed the extras into the bus. The added signal sources must, of course, have output-level controls, since there will be no way of controlling these levels at the mixer. Bus jacks are also sometimes used to link two or more mixers together.

The "Cue Out" connector provides a way to listen to signals without their getting into the regular outputs of your mixer. This allows you to cue (or set a source to its start) without having that signal reach your tape recorder or the audience.

Spikes and Surges, Part II In response to an item in the December '84 "Audioclinic" on "Spike Protection," I would like to add the following information:

Power surges and spikes of varying degrees can occur at any time. These surges and spikes are fluctuations on the line that can occur from a lightning strike as far away as five miles, a power company's switching of loads, near by electric motors, a return to full power after a brownout, or various other unpredictable causes.

There are many products to protect equipment from these surges and spikes. Some are just plain surge protectors, while others combine surge protection and EMI/r.f.i. filters. These products are basically made for use with computers and their peripherals, but they also can be used to protect high-fidelity products, TV sets and VCRs from electrical damage.

Below is a rundown of products from three of the major manufacturers of surge protectors. These represent the best protection you can buy for high fidelity equipment, though you must go to computer stores to buy them. (Note: This is the opinion of my correspondent, and does not necessarily reflect my own view or that of Audio. -J.G.) Six-outlet surge protectors that plug directly into the wall include the EPD Lemon, EG System 2, and Curtis Diamond. If you want an a.c. surge protector with six accessory outlets and a 6-foot power cord, try the EPD Lime, EG System 6 or Curtis Emerald. Three outlet units which combine a.c. surge protection with EMI/r.f.i. filters, and which plug directly into the wall, include the EPD Peach, EG System 4 and Curtis Sapphire. And if you want a unit combining a.c. surge protection with EMI/r.f.i. filters, equipped with 6 foot cords and six accessory outlets each, there are the EPD Orange, EG System 8 and Curtis Ruby.

-Christopher Sullivan, Belmont, N.C.

Phonograph Motor Vibration

Q. I have a three-speed Rek-O-Kut turntable (vintage early 1950s) which has a peculiar problem. It has not been used in years, but it has been kept covered, and with the idlers disengaged.

I recently tried it out, after lubricating the motor and idlers, and heard a vibration which was never present be fore. This was easily identified as coming from the motor, because the noise was the same whether or not the turn table and motor were connected through the idlers.

-C. F. Casey, Alexandria, Va.

A. I think the motor vibration was always present, but it is now more closely coupled to the turntable probably as a result of dried-out, hardened, motor shock mounts.

I have owned various versions of your turntable. I recall that, by replacing the shock mounts, some rumble could be removed. Still more rumble can be removed by careful adjustment of the pressure the idler exerts against the motor pulley and the turntable rim.

Limit screws are provided to achieve the right pressure-which will allow for good drive and still not induce vibration into the platter.

Check that the motor is free from surrounding objects. If it comes into physical contact with the cabinet in which it is housed or with equipment within the cabinet (such as a preamplifier), vibration will then have another path to the platter.

Obtaining Good Bass Sound

Q. I am a bass player and wish to improve the sound of my instrument. Is there any inexpensive way to do it?

-G. A. Gowman, Detroit, Mich.

A. So often, the problem of obtaining good bass sound has to do with the quality of both the instrument and the pickup used. Part of the problem, too, is that most performers play their bass "straight," with no limiting.

By limiting, I refer to a scheme by which the loudest-volume peaks are suppressed. It is done by using an amplifier whose gain varies inversely with the input signal. In other words, as the strings of your bass vibrate less and less, and the output from the pick-up falls, the amplifier connected to the pickup increases in amplification to compensate for this falling output. Limiting makes the bass smoother.

Experience with my own bass has shown that its output is not uniform with frequency. The use of a limiting amplifier between the pickup and its regular amplifier will even out the tone over the instrument's entire range. The limiter also enables the instrument to "sustain" for a longer time. This is not too important when you are playing a rapid succession of notes, but it is important at such points in the music as the end of a song, or perhaps at the end of an intro, where there might be a retard or a hold. The greater the limiting, the longer the sustain time.

Limiters can be very expensive in deed. I have seen some, however, which are inexpensive and do a remarkably good job. They are made just for applications of the kind we are discussing here, and are sold by dealers specializing in electronic musical instruments and accessories. They are in the form of a small box, equipped with an on/off switch and a knob. The knob determines the amount of signal reaching the input of the device and, therefore, the amount of limiting which takes place. If the input signal is too low, it will be below the "threshold" of the unit. The effect will be as though no limiter were in the system. As the input signal increases, more and more of this limiting effect will be noticed.

Volume Increase with Equalization

Q. When I switch my graphic equalizer into my system, there is a very audible sound-level increase, obviously more than 3 dB. Please explain this.

-K. R. Stephens, Tucson, Ariz.

A. Sometimes the use of an equalizer can produce a real or perceived increase in level, for two reasons:

The first reason is that the equalizer may have more than unity gain-it may produce more signal than was fed into it. This is somewhat analogous to turning up the volume a bit.

The second possible reason for this perceived increase has to do with psychoacoustics. If you happen to add treble in the range of perhaps 2 to 5 kHz, this added frequency boost will not increase sound level significantly.

The ear is, however, very sensitive to changes in level at these frequencies, so this added coloration may make the sound seem louder.

Crackling and High B+

Q. I have a Dynaco ST70 tube-type amplifier to which I added 100-uF capacitance by paralleling a bunch of capacitors. The higher B+ produced improved performance.

Lately, however, I hear a sound sort of a static, or "tearing" sound from the speakers, about 3S after the equipment is turned off. I do not get the sound if I disconnect the capacitor bank. I'm about to jump around each capacitor to find which one is causing the sound. I'm also wondering if there may be some weak component in the ST70, which makes this sound only when extra capacitance is used. I'd appreciate your advice.

-R. W. Clifford, Lancaster, Cal.

A. I do not believe that the sound is produced by a defective unit in your bank of added capacitors. Of course, the added B+ which is produced because of the use of these capacitors can create problems with other components in the ST70.

I believe that there is a leaky interstage coupling capacitor producing the crackling you have described. Using another amplifier to listen, check the grids of successive stages until you locate the first circuit on which the noise is present. Unless feedback is involved (giving you misleading results), the capacitor feeding this grid should be replaced.

Radiation from CD Players

Q. When scanning the instruction manual for one of the first-generation CD players, I noticed a warning that improper use of the player "may result in hazardous radiation exposure." In what sense does CD technology pose a radiation risk? Do the newer, sliding-drawer-type players, in which the disc and laser are completely enclosed, pose any less of a risk to the user?

-Name withheld

A. A laser is a very intense light source. Although I have not seen a warning like the one you cite, I can see that if a person is determined to get to the innards of a CD player, he could expose his eyes to light which could cause damage.

Based on the machines I have had the chance to study, I see no way one could become exposed to laser light without going to great lengths to destroy the player.

Of the players I have seen, top-loading units would seem to pose the greatest possibility of radiation. Even so, one would have to go out of one's way in order to take a squint at the light. Naturally, the players using sliding drawers are even less likely to pose problems.

Movement and FM Reception

Q. When using any of three separate FM sets, in three different locations ( Queens, Brooklyn, and Manhattan), people moving about in the room in which the equipment is located affect reception. Why? Can anything be done about this?

-Steve Marston, Hollis, N.Y.

A. Apparently you use indoor antennas for FM reception. It is because of this that you have had the problem.

A human body is not neutral to radio frequencies; it can absorb or reflect them. Thus, if you stand in just the right place, your body can reflect signal to your indoor antenna in such phase as to reinforce the direct pickup of these signals. Moving just a few inches one way or another can create reflected signals that cancel out the direct signal. If you stand in the right place be tween the antenna and signal source, the body can absorb energy which would otherwise reach the antenna.

The obvious solution is to use an outdoor antenna. If this isn't possible, you must experiment with the location of the indoor antennas, and find a place where the effect of people moving in the room is minimized. Placing the antenna near the ceiling is a possibility; locating it in another, less-trafficked room may also help.

Poor Performance at High Power

Q. My sound system, which includes a preamplifier, power amplifier, equalizer, phonograph and loudspeakers, performs well only up to a point. I am unable to drive the amplifier past its 75-watt level, even though it is capable of much higher power, which my loudspeakers can handle. In order to eliminate the serious distortion which occurs at this level, I am forced to roll off bass significantly via the equalizer.

What is happening, and can I do any thing about it?

-Ron Bryson, Carol Stream, Ill.

A. I suggest that you disconnect the equalizer and run the system without it. See if the system now operates at the greater power output you desire.

If you are using the equalizer be tween the preamplifier and the power amplifier, the signal level from the pre amplifier could be too great, thus over driving the equalizer. This means that the voltage needed to drive your amplifier to 70 watts output represents the overload point for the equalizer.

If the power amplifier has volume controls (and if they are not advanced all the way), turn them up. This will give the equalizer less work to do and may bring about a surprising improvement in performance.

If this remedy is not available, I suggest you place the equalizer in the tape loop, where it will probably operate more satisfactorily.

(adapted from Audio magazine, Jun. 1985; JOSEPH GIOVANELLI)

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