AUDIOCLINIC (June 1987)

Stylus Velocity

Q. What is meant by "stylus velocity in cm/S"?

-June E. Thomas, Wantagh, N.Y.

A. When a phonograph record is played, the stylus is in constant side-to-side motion as it traces the groove walls. Its speed (which varies with the signal being traced) is expressed in centimeters per second. This number is most often used to define a cartridge's output level, which will vary with the stylus velocity. It is sometimes also used to define the cartridge's ability to track high-velocity modulations.

Amps for Musical Instruments

Q. Which high-end amplifiers/pre amplifiers are suitable for use with electronic musical instruments for live performance? Should I use dual-mono phonic equipment, or is stereophonic equipment more suitable? I do want stereo during these performances. I would also appreciate your advice concerning the selection of loudspeaker systems.

-David Chetkin, Newport Beach, Calif.

A. Many amplifiers used in conjunction with musical instruments perform other functions besides amplification:

Reverb, vibrato, fuzz, sustain, equalization, et al. Most also offer mixing capabilities. If what you need is just amplification, any amplifier will provide it. Depending upon the size of the rooms in which you perform and upon the acoustical efficiency of the loud speakers you use, amplifier output power may have to be considerable.

If you have selected your amplifier but still require the various effects I've mentioned, you will want to use a "top" or "head," a control center analogous to a hi-fi preamp (or, sometimes, to an integrated amp) but containing the in puts and many of the control features that a musician is likely to need. Just be sure that it does include the particular features you require.

I can't see how it matters whether or not you use separate monophonic amplifiers or stereophonic amps. If the amplifiers are to supply high power but have poorly regulated power supplies, you should consider separate, mono phonic equipment or perhaps stereo equipment operated in the bridged mode (and hence in mono).

I am sure you realize that there are some very fine mixing boards around which also provide amplification, along with (at the very least) equalization and reverb. I have not seen one which directly incorporates vibrato, fuzz, or sustain (although such a board may exist). I have seen at least one with built-in rhythm synthesizers too.

Even if you do not find quite all of the features you need in either a "top" or a mixing board, you may be able to find accessory boxes to supply the features that you're missing. If you need fuzz, you can find fuzz boxes to pro duce it. If you need sustain or vibrato, there are some nice boxes which pro duce these effects. If your inputs lack sufficient voltage gain for your instrument, you might try plugging in "power boosters" (actually, 1-transistor volt age amplifiers) between your instrument and your input.

Much of the sound character of electronic musical instruments is a matter of the coloration occurring in the loud speaker systems used. Thus, loud speaker selection, although critical, must be left to you.

Speaker Switches and Protective Relays

I recently read the letter from reader J. T. Satterwhite ("Audioclinic," June 1986), wherein he describes intermit tent loss of audio output from his receiver when it is played at low volume levels. I'd like to suggest an over looked point, the protection relay. Oxidation or corrosion of the relay contacts, accelerated by "hot-switching" the relay, can lead to distortion or to dropouts in either or both channels.

Oxidation can be cleaned from the contacts by careful disassembly of the relay, followed by the use of a good contact cleaner (such as Cramolin). Corrosion of these contacts requires replacement of the relay. If one chooses to attempt cleaning the relay contacts, it cannot be said too emphatically: Be careful! When removing the plastic shield, watch the coil. In applying the contact cleaner, do not bend the contact supports. When replacing the shield, do not break any wires. If you are uncomfortable doing any of this, don't do it-you can always just replace the relays. Replacements are not too expensive and can save lots of time and frustration! Another set of contacts which is prone to failure by "hot-switching" is the speaker selector switch. Switching from "speakers off" to "A" to "B" with the volume turned up can produce the same contact wear which relay contacts are subjected to. My advice is:

Prior to turn-on, adjust volume to zero. Turn the system on and wait for the relay to close. Then turn up the volume. Prior to turn-off, turn the volume down to zero, then kill power. When switching speakers, turn volume down, switch, then turn volume up.

These procedures should keep the equipment in its owner's home rather than in the local repair shop.

-Mark D., Pagan, Laurel, Md.

Hum in TV Audio

Q. I hear a hum in my audio system when attempting to feed the output from my television set into it. Can you help?

-Pat Yacques, Dallas, Tex.

A. I can think of a number of possible reasons why hum may be heard when feeding the output from a television receiver into your sound system.

If your TV set only has an earphone output and you are using that to feed signal into your system, there is a chance of hum, especially when the volume control on the TV set is turned to a relatively low setting. This will permit the inherent background noise and hum in the audio of the TV set to assume a greater amplitude, in proportion to the desired signal, than it will if the volume control is advanced.

Care must be taken to avoid over loading your main system or at least injecting so much signal as to make the volume of the main system difficult to adjust.

Assuming that your set has a "line output." I have had problems when such sets are connected to cable TV systems. I believe the problem is a result of ground loops. This occurs be cause the true ground source for the cable signals is physically distant from my main ground point, causing a serious difference in potential between the two grounds. If I am correct, what is needed is an r.f. isolation transformer to isolate the cable ground from the rest of the audio system. I have not been able to locate such a transformer (75/300-ohm baluns are usually not isolated). Can any of you help? Another possible source of hum may be that your TV set employs "intercarrier mixing" to derive its audio i.f. Your TV set needs very good limiting in such instances to avoid video buzz and hum from being heard in the audio output. It is much better if the set maker pro vides separate sound i.f. circuitry, starting from the output of the tuner, but this approach is more expensive.

Intercarrier buzz can be distinguished from other noise problems because it changes with the video picture; it is usually most noticeable when the screen is white or full of white lettering against a colored background. If this is your problem, you may be able to improve matters by increasing the signal strength to the TV's tuner (perhaps by using an antenna booster amp) to saturate the limiter.

Ordinary ground loops are another consideration. These can occur via the signal cables, through the use of three-prong grounded plugs on both your TV set and sound system, or from other causes. Try disconnecting the signal cable's shield where it plugs into one channel of your amplifier (the ground should only be disconnected at one end of any cable). If that does not solve the problem, disconnecting the shield on the other channel may help (or may cause total loss of signal until you reconnect it).

Contact-Cleaner Testimonial

In response to the November 1986 "Audioclinic" item entitled "Dirty Contacts in a Dry Climate," I offer the following recommendation based on a similar experience.

After trying virtually every contact cleaner on the market, I have found the best result with a product which is called Cramolin, available from:

Caig Laboratories, 1175-0 Industrial Ave.

(P.O. Box J), Escondito, Cal. 92025, USA; (619) 743-7143.

I am an engineer at a sound-recording studio in Southern California. As such, I am responsible for maintaining all audio equipment (mixing consoles, tape decks, etc.), and it used to be necessary for me to clean "dirty" potentiometers and switches every month or so. After switching to Cramolin, I have found that I now only need to clean these components once a year, and in some cases one application has been sufficient. According to the label, besides cleaning, Cramolin acts as a lubricant and preservative.

-J. D., Stein, San Diego, Calif.

Buzz and Static

Q. What can I do to eliminate buzz and static, which I think comes from my amplifier?

-Eric Nelson, Landisville, Pa.

A. If you hear this buzz and static only when listening to certain signal sources, your amplifier is probably not the cause. Rather, it is more likely to be in the signal source or in the cables between the source and the amplifier.

If you hear the noise only when listening to AM or FM, a better antenna might help. If you hear it only when listening to records, then you probably have an open ground connection somewhere; most likely, the ground lead that should run from the turntable or tonearm to a grounding point on the amplifier has become detached.

Both AM and (to a lesser extent) FM tuners are also prone to picking up interference from various sources, including TV sets, home computers, and household appliances. If this is your problem, physically separating the tuner and/or the antenna as far as possible from the noise source is the only solution, but it usually works well.

It is difficult, however, to cure the "buzz" produced by a TV set. Some times good power-line filters will help.

At least they can keep these signals from getting onto the power lines, which would then act as a good antenna to radiate the undesired signals.

Where a power amplifier produces buzz, it is usually a matter of the fast switching of the rectifier. If individual diodes are used, bypass each one with a capacitor rated at about 0.02 µF, with a d.c. breakdown voltage well above that produced by the power supply. In the case of a bridge rectifier "package," about all you can do is to place a bypass capacitor across the input to the bridge and another across its output. You might also bypass each input lead to the chassis.

Signal Level from CD Players

Q. Should the output level from a Compact Disc player match the line input sensitivity of the preamplifier into which it is plugged? My player has a rated output of 2 V rms. The input sensitivity of the AUX jack of my preamplifier is 50 mV. I have seen ads for input attenuators to be used with sources having too high an output signal level;

they are claimed to lower input over load distortion. Are they a luxury or a necessity? What are the symptoms of "overload distortion"? Wouldn't an attenuator introduce distortion?

-Bernard Gilman, Brookline, Mass.

A. The need for matching a CD player's output level to the input sensitivity of the equipment into which it feeds depends on a couple of things.

Some equipment is designed so that the input signals feed directly into the volume controls. Under these conditions, high-level signals are attenuated by virtue of the volume control's set ting. No distortion results because no early stages are overloaded.

However, this leaves the possibility that turning the volume control up even slightly past its minimum setting will produce a high sound level from the loudspeakers. This leaves the listener with only a very small adjustment range between near silence and a deafening roar when listening to CDs.

It also means the volume control will have to be readjusted substantially when switching between CDs and other sources-and if the volume is not turned down before CDs are played, there can be damage to the loud speakers or even to one's hearing.

If there are gain stages between the input and the volume control, these stages can often be overdriven by signals of higher amplitude than they were designed for.

In either case, I recommend that you do use an attenuator. Because it is nothing more than a voltage divider, it will not add distortion.

It is difficult to describe symptoms of overload distortion. Perhaps the best description is that it sounds like what you hear when you put too much signal onto a tape. Naturally, the distortion will only occur on peak music amplitudes, which is one reason it might be difficult to detect by some ears and with some equipment.

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(Source: Audio magazine, June 1987, JOSEPH GIOVANELLI)

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